Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Ceramic Society of Japan  

DOI： 10.2109/jcersj2.20138

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Association of Mineralogical Sciences  

DOI： 10.2465/jmps.191122b

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Authorship：Lead author,　Corresponding author  

DOI： 10.2465/jmps.190414

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Authorship：Lead author,　Corresponding author  

DOI： 10.2465/jmps.180912

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.2465/jmps.180409

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Authorship：Lead author,　Corresponding author   Language：English   Publisher：Tohoku University  

Crystal structures of the cubic and tetragonal spinel phases of Zn2GeO4 and the modified spinel phase of Zn2SiO4 were refined by Rietveld analysis of synchrotron powder X-ray diffraction data. The Zn2GeO4 cubic spinel phase was found to have an inverse spinel configuration. The Zn2WGeO4 tetragonal spinel phase is isostructural to Zn2TiO4, where half of Zn occupies the tetrahedral site, and the remaining Zn and Ge are ordered in two octahedral sites. In the modified spinel phase of Zn2SiO4, Zn occupies the octahedral sites. Independent individual cation-oxygen distances in the Zn2GeO4 tetragonal spinel and Zn2SiO4 modified spinel phases were calculated using the bond valence repartition method, and are in reasonable agreement with the refined structures.

DOI： 10.2465/jmps.170617

Scopus

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.2465/jmps.171219

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Knowledge of the dissolution mechanisms of carbon dioxide in silicate melts/glasses is indispensable for understanding its effects on physical and thermodynamic properties. Carbon dioxide is generally known to dissolve as molecular CO2 and CO32- species, with the latter dominant for depolymerized compositions. However, less is agreed upon about how the CO32- groups are incorporated, especially for depolymerized silicate melt compositions relevant to natural mafic and ultramafic magmas. Here we report C-13 MAS and static NMR results on a series of (CO2)-C-13-bearing glasses (quenched from melts) of diverse silicate compositions, including nominally fully polymerized sodium aluminosilicate and calcium aluminosilicate, depolymerized sodium silicate and sodium aluminosilicate, and depolymerized calcium-magnesium silicate and calcium aluminosilicate compositions (with varying degrees of polymerization), as well as ab initio calculations, to provide new constraints on the speciation of carbonates in silicate melts/glasses as a function of composition.The ab initio calculation revealed that both vibrational frequencies and C-13 chemical shift tensor are sensitive to the local environments of carbonates. The splittings of the asymmetric stretching doublets (Delta v(3)) for CO32- groups bonded to one or two tetrahedral Si/Al via two oxygens (network carbonates) are all relatively large (around 180-480 cm(-1)), contrary to previous speculations. In comparison, experimental data for CO(3)(2-)groups bonded only to metal cations (free carbonates) in minerals show zero to moderate Delta v(3) (up to similar to 100 cm(-1)). Our calculations also showed that network carbonates bonded to one or two tetrahedral Si/Al both show C-13 chemical shift tensor parameters (especially skew and isotropic chemical shift) that are distinctly different from those of free carbonates.Our C-13 MAS and static NMR data, as well as infrared spectroscopic data (moderate Delta v(3) of 60-100 cm(-1)) from the literature, for depolymerized silicate and aluminosilicate glasses are all indicative of free carbonates as the dominant species. Data for nominally fully polymerized aluminosilicate compositions, on the other hand, are consistent with carbonate groups bonded to two Si/Al via two oxygens (network carbonate) as the dominant species. The quantitative C-13 MAS NMR data also revealed the coexistence of a small amount of the other type of carbonate species, especially for Ca aluminosilicate glasses. These new structural insights should be valuable in helping better understand physical properties (e.g. viscosity) of CO2-bearing silicate melts of diverse compositions.

DOI： 10.1016/j.chemgeo.2018.01.005

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

Enstatite (MgSiO3) sample was prepared by conventional solid state reaction at 1500 degrees C, followed by cooling to ambient temperature with different cooling rates (0.04-500 degrees C/s). Quantitative analysis of enstatite phases in the samples by Rietveld refinement and Si-29 MAS NMR revealed that a large amount of metastable protoenstatite (30-40 mol%) along with clinoenstatite was preserved in all samples. The presence of protoenstatite was also confirmed by micro-Raman spectroscopy. The crystal structures of protoenstatite and clinoenstatite at room temperature were refined using the Rietveld method, and were essentially the same as those reported by previous studies. As the solid state reaction method is commonly used to prepare synthetic clinoenstatite, the present result implies that the 'clinoenstatite' starting materials used in previous phase equilibrium studies of MgSiO3 pyroxene might contain overlooked protoenstatite, which could affect the interpretation for phase transition and thus those results should be reexamined.

DOI： 10.2465/jmps.170616

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The crystal structures of Ca(Al0.5Si0.5)O-2.75 and Ca(Al0.4Si0.6)O-2.8 ordered oxygen-deficient perovskite phases are synthesized at 7 and 11 GPa, respectively, and 1500 A degrees C, and were studied using NMR and synchrotron powder X-ray diffraction. Si-29 MAS NMR, Al-27 MAS and 3Q MAS NMR measurements revealed a single tetrahedral Si and single octahedral Al peak for the Ca(Al0.5Si0.5)O-2.75 phase, and a tetrahedral and an octahedral Si peak and a single octahedral Al peak for the Ca(Al0.4Si0.6)O-2.8 phase. Using this structural information as constraints, the crystal structures were solved from synchrotron X-ray diffraction data by the structure determination from powder diffraction (SDPD) technique. To double-check the structures, first-principles calculations of NMR parameters (chemical shifts and electric field gradients) were also conducted after relaxing the obtained structures. The calculated NMR parameters of both phases are consistent with the observed NMR spectra. The crystal structures of both phases consist of a perovskite-like layer of (Al,Si)O-6 octahedra and a double-layer of SiO4 tetrahedra that are stacked alternatively in the [111] direction of ideal cubic perovskite. The perovskite-like layer is made of a double-layer of Al octahedra for the Ca(Al0.5Si0.5)O-2.75 phase, and a triple-layer with a Si octahedral layer sandwiched between two Al octahedral layers for the Ca(Al0.4Si0.6)O-2.8 phase. A unique feature common to both structures is that each SiO4 tetrahedron has one terminal oxygen that is not shared by other Si or Al. Homologous relation among these phases and merwinite (Ca3MgSi2O8) in terms of different numbers (1-3) of octahedral layers within the perovskite-like layer is noted.

DOI： 10.1007/s00269-017-0896-z

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOC AMER  

The presence of water (hydrogen) in nominally anhydrous mantle minerals may have profound effects on their physical properties (e.g., electrical conductivity, diffusivity, rheology), and these effects are expected to depend on how water is incorporated in the crystal structure. For olivine, the most abundant upper mantle mineral, despite extensive studies, mostly using vibrational spectroscopy, the interpretations are still not well constrained. To provide better understanding on this issue, we carried out a comprehensive H-1 and Si-29 NMR study on an Mg2SiO4 forsterite sample containing about 0.5 wt% H2O synthesized at 12 GPa and 1200 degrees C, complemented by Raman measurement and first-principles calculation of the geometry, stability, and NMR parameters of model structures. The Raman spectra contain relatively sharp O-H stretching bands near 3612, 3579, and 3567 cm(-1) and a broader band near 3547 cm(-1), similar to previous reports. The H-1 static and MAS NMR data revealed that there are two main populations of protons in the hydrous forsterite structure, one experiencing strong (H-1H)-H-1 homonuclear dipolar couplings and contributing to a broad peak near 2.4 ppm, and another with weaker dipolar couplings and contributing to a narrower peak near 1.2 ppm in the MAS NMR spectrum at 30 kHz. Two-dimensional H-1 CRAMPS-MAS NMR measurements confirmed that the two proton components belong to the same phase and the contrast in MAS NMR peak width is largely due to difference in the strength of H-1-H-1 homonuclear dipolar couplings. In addition, there is also a very weak, narrow H-1 MAS NMR peak near 7.3 ppm (contributing to < 0.1% of the total intensity) due to protons that are more remote from the two main components. First-principles calculation confirmed that the two main proton components can be attributed to the hydrogarnet-like substitution mechanism of four H ions for one Si [(4H) Si] in a tetrahedral site of olivine, but unlike hydrogarnet with one of the protons pointing away from the tetrahedral center and located in an adjacent interstitial site, thus experiencing weaker dipolar couplings than those in the vicinity of the vacant tetrahedron; the very weak narrow peak near 7.3 ppm can be attributed to the substitution mechanism of two H ions for one Mg in an M1 site [(2H)M-1] of forsterite. The H-1-Si-29 CP-MAS NMR spectra revealed both a broad peak encompassing the position for OH defect-free forsterite (-61.7 ppm) and a narrower peak at higher frequency (-60.9 ppm). First-principles calculation indicates that these peaks are accountable by the same models as for the H-1 NMR data. Thus, this study has provided unambiguous evidence supporting that hydrogen is incorporated in forsterite at relatively high-pressure dominantly as (4H) Si defects, with (2H) M1 defects playing only a very minor role. The much larger H-1 chemical shift for protons associated with the latter (than the former) is correlated with stronger hydrogen bonding for the latter, which in turn reflects difference in bonding environments of the OH groups (with the latter bonded to a Si, and the former only bonded to Mg). Similar correlation applies to the O-H stretching frequency.
The (4H) Si defects are responsible for the observed high-frequency O-H stretching bands (> 3450 cm(-1)), and the (2H) M1 defects give lower frequencies (undetected here due to low abundance, but most likely near 3160-3220 cm(-1) as previously reported) in vibrational spectra. These results can serve as a guide for (re-) interpretation of infrared and Raman spectroscopic data on hydrous olivine produced under different pressure and silica activity conditions, and require reconsideration of any models for the effects of water on physical properties of olivine based on different interpretations of such data. This study also demonstrated the usefulness of the combined solid-state NMR and first-principles calculation approach in unraveling the hydrogen incorporation mechanisms in nominally anhydrous minerals.

DOI： 10.2138/am-2017-5878

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

In synthetic hydroxyl-chondrodite and hydroxyl-clinohumite, two hydrogen sites (H1 and H2) are known. However, hydrogen occupying only H1 site has been reported for natural F- and Ti-rich chondrodite and clinohumite. In order to understand hydrogen distribution in these sites, stability of hydrogens at these sites in chondrodite is studied by first-principles density functional theory calculation. For hydroxyl-chondrodite, lowest energy was found when both H1 and H2 sites are half occupied by hydrogens. In this configuration, strong H1-H1 repulsion is avoided, and nearly straight hydrogen bond of O-H1 center dot center dot center dot O-H2 is realized. When a half of OH- is replaced with F- or replaced with O2- in the case of Ti-containing systems, hydrogen occupying H1 site is always favored over hydrogen in H2 site so as to form nearly straight hydrogen bond of O-H1 center dot center dot center dot F- (or O2-). Our calculations are fully consistent with the observed structures in both synthetic and natural chondrodite, and revealed that hydrogen distribution in humite group minerals is mostly governed by hydrogen bonding at H1 site.

DOI： 10.2465/jmps.160717

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

In order to clarify cation distributions in the M1 and M2 sites of (Mg,Zn)(2)SiO4 olivine solid solution, Si-29 MAS NMR spectroscopic measurement and first-principles NMR parameter calculation were conducted. The Si-29 MAS NMR spectra of (Mg0.95Zn0.05)(2)SiO4 and (Mg0.90Zn0.10)(2)SiO4 olivine samples reveled three new peaks at relative chemical shift differences of 0.2, 1.1 and 2.3 ppm from the main peak of forsterite (-61.8 ppm). These shifts can be attributed to changes in the second-nearest-neighbors of Si due to substitution of Mg by Zn. Based on first-principles calculations, these peaks can be assigned to the following three groups of local Si environments in the order of increasing shift from the main peak of forsterite: i) Si tetrahedra with one corner-shared Zn in M1 or M2 octahedron, ii) Si tetrahedra with one edge-shared Zn in M2 octahedron, and iii) Si tetrahedra with one edge-shared Zn in M1 octahedron. Since the last two peaks are well separated from the others, the relative abundances of Zn in the M1 and M2 sites can be quantified using these peaks. Preference of Zn for M1 site over M2 site was inferred from the observed peak intensities. The present study demonstrated the usefulness of 29Si MAS NMR spectroscopy for quantitatively studying cation distributions in solid solutions.

DOI： 10.2465/jmps.151104a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Diopside, CaMgSi2O6, is an important analogue for depolymerized silicate melts. We have used the complimentary spectroscopies, X-ray Raman Scattering (XRS), X-ray absorption near edge structure (XANES) and Raman, to investigate diopside glass in situ to 20 GPa. We observe a stark change in behavior of CaMgSi2O6 near 4 GPa that corresponds to a major change in the rate of inter-tetrahedral angle (/Si-O-Si) closure. Below 4 GPa, the /Si-O-Si closes rapidly at 2-3 degrees/GPa whereas above 4 GPa it decreases by similar to 1 degrees/GPa. A distinct shift to higher wavenumbers of the 870 and 905 cm(-1) Raman bands are observed at 1.3 GPa suggesting that Q(0) species have been completely converted to Q(1) or higher Q(n) species.
XRS measurements at the O K-edge suggest that Si-[5] is formed by 3 GPa. This formation is accompanied by a rapid decrease in the /Si-O-Si and a decrease in Q(0) species. A linear increase in the geometric mean of the high frequency envelope, the chi(b) value, from 999 to 1018 cm(-1) suggests that the conversion of NBO to BO is continuous up to 14 GPa. Above 14 GPa, the Raman spectra show an obvious negative shift in both, the high frequency peak maximum, and the chi(b) position. Simultaneously, the low frequency envelope looses its asymmetry at 14 GPa. This may be explained by either a loss of a vibrational mode in the range 1000-1200 cm(-1) and/or the formation of Si-[6]. The structural evolution of CaMgSi2O6 correlates well with a major change in the compressibility and diffusivity around 5 GPa. (C) 2016 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.gca.2016.01.020

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J-GLOBAL

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Language：Japanese   Publisher：Japan Association of Mineralogical Sciences  

The 4H=1Si substitution is well known for hydrogrossular, and has also been suggested for other mantle minerals, such as olivine. The inference was mostly based on infrared spectra, but the interpretation is not unambiguous. Here we apply NMR spectroscopy, in combination with first-principles calculation to shed better light on this issue. We compare the proton distributions in hydrogrossular and forsterite, demonstrating that they have similar, but not identical configurations.  <br>

DOI： 10.14824/jakoka.2016.0_68

CiNii Article

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We calculated the phase diagram of up to 15 GPa and 2,000 K and investigated the thermodynamic properties of the high-pressure phases. The investigated phases include the berlinite, moganite-like, , and phases . The computational methods used include density functional theory, density functional perturbation theory, and the quasiharmonic approximation. The investigated thermodynamic properties include the thermal equation of state, isothermal bulk modulus, thermal expansivity, and heat capacity. With increasing pressure, the ambient phase berlinite transforms to the moganite-like phase, and then to the and phases, and further to the phase. The stability fields of the and phases are similar in pressure but different in temperature, as the phase is stable at low temperatures, whereas the phase is stable at high temperatures. All of the phase relationships agree well with those obtained by quench experiments, and they support the stabilities of the moganite-like, , and phases, which were not observed in room-temperature compression experiments.

DOI： 10.1007/s00269-014-0695-8

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Language：Japanese   Publisher：Japan Association of Mineralogical Sciences  

We have applied ²⁹Si MAS NMR together with first-principles calculation to gain new insights into the cation (Mg, Zn) distribution in (Mg,Zn)₂SiO₄ olivine solid solution, as well as the correlation between ²⁹Si chemical shift and local structures associated with cation distributions. It was demonstrated that ²⁹Si MAS NMR is capable of probing/quantifying the occupancy of cations in the M1 and M2 octahedral sites, and Zn has a preference for M1 sites. This study also revealed that cation substitutions in neighboring edge-shared octahedral sites have greater influence on the ²⁹Si chemical shift as compared to corner-shared sites for olivine.

DOI： 10.14824/jakoka.2015.0_77

CiNii Article

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We report the crystal structures determined under ambient condition for two Zn2SiO4 polymorphs synthesized at 6.5 GPa and 1,273 K (phase III) and 8 GPa and 1,273 K (phase IV) and also compare their Si-29 MAS NMR spectroscopic characteristics with those of other Zn2SiO4 polymorphs (phases I, II and V). Electron microprobe analysis revealed that both of phases III and IV are stoichiometric like the lower-pressure polymorphs (phases I and II), contrary to previous report. The crystal structures were solved using an ab initio structure determination technique from synchrotron powder X-ray diffraction data utilizing local structural information from Si-29 MAS NMR as constraints and were further refined with the Rietveld technique. Phase III is orthorhombic (Pnma) with a = 10.2897(5), b = 6.6711(3), c = 5.0691(2) a"&lt;&lt;. It is isostructural with the high-temperature (Zn1.1Li0.6Si0.3)SiO4 phase and may be regarded as a 'tetrahedral olivine' type that resembles the 'octahedral olivine' structure in the (approximately hexagonally close packed) oxygen arrangement and tetrahedral Si positions, but has Zn in tetrahedral, rather than octahedral coordination. Phase IV is orthorhombic (Pbca) with a = 10.9179(4), b = 9.6728(4), c = 6.1184(2) a"&lt;&lt;. It also consists of tetrahedrally coordinated Zn and Si and features unique edge-shared Zn2O6 dimers. The volumes per formula under ambient condition for phases III and IV are both somewhat larger than that of the lower-pressure polymorph, phase II, suggesting that the two phases may have undergone structural changes during temperature quench and/or pressure release.

DOI： 10.1007/s00269-013-0584-6

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Language：English   Publishing type：Part of collection (book)   Publisher：JOHN WILEY & SONS INC  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Density distribution around Vickers indentation on commercial glasses, such as aluminoborosilicate glass and soda-lime silicate glass, was investigated by using micro Raman spectroscopy. Samples of the glasses were subject to high hydrostatic pressure by means of a multi-anvil apparatus, to prepare densified samples. Raman band shift of the densified sample was found to have a relationship with the hydrostatic pressure or density. By Raman spectrum mapping, the density distribution around the indentation for each glass was identified. It was found that there is a densified zone around the indentation in both two glasses. Compared with the soda-lime silicate glass, the aluminoborosilicate glass showed the broader region with higher densification around the indentation. (c) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnoncrysol.2012.04.035

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATL ACAD SCIENCES  

Subduction-zone magmatism is triggered by the addition of H2O-rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical endpoint. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot subduction zones. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in subduction zone magma chemistry.

DOI： 10.1073/pnas.1207687109

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Structural characterization of a new high-pressure AlPO4 phase synthesized at 5 GPa and 1500 degrees C is reported. The phase is monoclinic (P2/a) with a = 8.7437(1) angstrom, b = 4.8584(1) angstrom, c = 10.8600(2) angstrom, beta = 90.124(1)degrees (Z = 6). P-31 MAS NMR and two-dimensional (2D) Al-27 triple-quantum (3Q) MAS NMR revealed that it contains two tetrahedral P sites of 1:2 abundance ratio, and two tetrahedral Al sites with 1:2 ratio. 2D P-31 dipolar-recoupled double-quantum (DQ) MAS NMR and Al-27 -&gt; P-31 dipolar-based (through-space) and J coupling-based (through-bond) 3Q-heteronuclear correlation (HETCOR) experiments provided direct information on the linkages of these sites. The crystal structure was solved and refined from synchrotron powder X-ray diffraction data utilizing the information from NMR. The phase is isostructural to moganite, a rare SiO2 polymorph, and its structure can be derived from the latter via an ordered replacement of tetrahedral Si sites by Al and P. The NMR parameters of the phase were also calculated by first-principles method, which are consistent with those observed. Contrary to the other moganite phases known to date (i.e., SiO2 and PON), moganite-AlPO4 has a higher-pressure stability field than the corresponding quartz phase. This is the first moganite-type phase found in the ABX(4) system.

DOI： 10.1021/ic300167k

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Si-29 MAS NMR is a well-established tool for quantitative characterization of Si-Al distribution in aluminosilicates. In framework aluminosilicates, all tetrahedrally coordinated Si/Al (T) are linked via bridging oxygens to four other Si/Al (Q(4)). Each crystallographically unique T site is commonly assumed to give at most five Si-29 NMR peaks with chemical shift spacing around 5 ppm, corresponding to five Si units each linked to nSi and (4 - n)Al (n = 0-4), abbreviated as Si(nSi) hereafter. Here we report a detailed one- and two-dimensional (2D) Si-29 NMR study on K-cymrite (KAlSi3O8 center dot H2O), which possesses a double-layered structure with all Si/Al distributed in one crystallographically unique T site (Q(4)). Contrary to general belief, more than five Si-29 MAS NMR peaks were resolved. These peaks are attributable, via 2D J-coupling mediated experiments, to Si(nSi)(n = 0-4), with two peaks each (similar to 2 ppm spacing) for n = 1-3. The latter corresponds to different permutations of the nSi(4 - n)Al over neighboring T sites of two distinct Si-O-T angles (139 and 180 degrees). This suggests for the first time that the Si-29 chemical shift varies with Si-O-Si and Si-O-Al angles differently. Si-Al exchange over neighboring T sites of unequal Si-O-T angles is another possible cause for multiple Si-29 NMR peaks.

DOI： 10.1021/jp3008859

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

To facilitate identification of high-pressure K-cymrite (KAlSi3O8 center dot H2O) phase and its anhydrous form (kokchetavite) in natural rocks, we have synthesized both phases and have characterized them by micro-Raman and NMR spectroscopy. K-cymrite was synthesized at 5 GPa and 800 degrees C. Kokchetavite was obtained by dehydrating K-cymrite at ambient pressure and 550 degrees C. The H-1 MAS and H-1-Si-29 CP MAS NMR spectra of K-cymrite are consistent with the reported crystal structure that contains H2O molecules and has a disordered Si-Al distribution. The Raman spectra obtained under ambient conditions for K-cymrite (and kokchetavite) contain major peaks at 114.0 (109.1), 380.2 (390.0) and 832.5 (835.8) cm(-1). For K-cymrite, OH stretching vibration is also observed at 3541 cm(-1) with a shoulder at 3623 cm(-1). The Raman spectrum for kokchetavite is consistent with that previously reported for a natural sample found as inclusions in clinopyroxenes and garnets in a garnet-pyroxene rock. However, the data for K-cymrite are inconsistent with the Raman features of a previously reported "relict K-cymrite in K-feldspar" from an eclogite. Pressure- and temperature-dependencies of the Raman shifts for the strongest peak of both phases are also reported.

DOI： 10.2465/jmps.111020i

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MAIK NAUKA/INTERPERIODICA/SPRINGER  

It is known that a number of compressed melts undergo structural phase transitions. Data on the structural changes at high pressures in chalcogenides (AsS, As2S3) and oxide (B2O3) melts with a network structure have been reviewed. Viscosity is one of the fundamental physical properties of a liquid. For various melts, it varies in a very wide range. Structural transformations in melts induce the corresponding changes in all physical properties, in particular viscosity. The measurements of the viscosity of a number of melts at high pressures and temperatures by the radiographic method have been reported. Changes in the viscosity by several orders of magnitude have been detected when the pressure is varied by several gigapascals. The diffusion mechanism in network-structure melts at various pressures has been analyzed. The prediction of the behavior of the viscosity of various melts at superhigh pressures is of high importance for the physics of glass transition, geophysics, and materials science.

DOI： 10.1134/S0021364011140050

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

The crystal structures of two new high-pressure AlPO4 phases are reported. One phase synthesized at 6 GPa and 1523 K is triclinic (P (1) over bar) whilst the other phase synthesized at 7 GPa and 1773 K is monoclinic (P2(1)/c). P-31 MAS (magic-angle spinning) NMR suggests three tetrahedral P sites with equal abundance in both phases. Al-27 3Q MAS NMR spectra provided evidence for two octahedral sites and one five-coordinated Al site in each phase. The crystal structures were solved using an ab initio structure determination technique from synchrotron powder X-ray diffraction data utilizing the local structural information from NMR, and were further refined by the Rietveld method. Both phases contain doubly bent chains made of six edge-shared Al polyhedra (including five-coordinated Al), which are joined by PO4 tetrahedra. The P (1) over bar phase is isostructural with FeVO4 and AlVO4. The two phases differ in the packing manner of the chains. This study has demonstrated that the combined application of ab initio structure determination via X-ray powder diffraction and solid-state NMR spectroscopy is a powerful approach to the rapid solution of complex inorganic crystal structures.

DOI： 10.1107/S0108768110051050

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Liquid B2O3 represents an archetypical oxide melt with a superhigh viscosity at the melting temperature. We present the results of the in situ x-ray diffraction study and the in situ viscosity measurements of liquid B2O3 under high pressure up to 8 GPa. Additionally, the B-11 solid state NMR spectroscopy study of B2O3 glasses quenched from the melt at five different pressures has been carried out. Taken together, the results obtained provide understanding of the nature of structural transformations in liquid B2O3. The fraction of the boroxol rings in the melt structure rapidly decreases with pressure. From pressures of about 4.5 GPa, four-coordinated boron states begin to emerge sharply, reaching the fraction 40%-45% at 8 GPa. The viscosity of the B2O3 melt along the melting curve drops by 4 orders of magnitude as the pressure increases up to 5.5 GPa and remains unchanged on further pressure increase.

DOI： 10.1103/PhysRevLett.105.115701

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOC AMER  

A new polymorph of topaz-OH (denoted as topaz-OH H) was recently discovered at higher P-T conditions than has been known thus far (denoted as topaz-OH I). High-resolution (1)H, (29)Si, and (27)Al nuclear magnetic resonance (NMR) and micro-Raman spectroscopy are applied to shed light on the crystal chemistry of both polymorphs. Topaz-OH I, synthesized at 7 GPa and 640 degrees C, is stoichiometric (Si/Al = 0.5) with a largely ordered local structure. Higher P-T topaz-OH I synthesized at conditions close to the polymorphic phase transition boundary, on the other hand, shows lower Si/Al ratios (0.44-0.45) and greater local structural disorder [including a small fraction (similar to 3%) of octahedral Si with a unique (29)Si chemical shift near 133 ppm]. The latter may be accounted for by the development of defects (Si/Al in normally vacant octahedral sites and vacancies in the tetrahedral sites) at higher P-T conditions. Topaz-OH II synthesized at 13.5 similar to 14 GPa and 1300 similar to 1400 degrees C similarly exhibits low Si/Al ratios (0.41-0.46). The NMR and Raman spectra for these topaz-OH II are, in general, broader and revealed a substantial fraction (33-37%) of octahedral Si with a range of (29)Si chemical shifts (-130 to -190 ppm), a small fraction (2-3%) of tetrahedral Al, and a range of (and overall shorter) hydrogen-bonding distances than topaz-OH I. Therefore, the phase transition from topaz-OH I to II is characterized by both a significant increase in the occupied octahedral/tetrahedral site ratio as well as disordering of cation distribution, which is unique from the viewpoint of crystal chemistry.

DOI： 10.2138/am.2010.3471

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOC AMER  

A new high-pressure form of topaz-OH (denoted here topaz-OH II) was obtained at 14 GPa and 1400 degrees C. The X-ray diffraction pattern of this phase can be indexed by an orthorhombic cell with a = 4.72318(5), b = 8.91480(9), and c = 2.77276(3) angstrom. The lengths of a and b are similar, but c is approximately a third of that for a previously reported topaz-OH (denoted topaz-OH l). The structural formula of topaz-OH II can be written as (Al(0.68)Si(0.32))(O(0.66)(OH)(0.34))(2), suggesting significant cation disorder in the structure. The crystal structure of topaz-OH II is solved using powder synchrotron X-ray diffraction data, and refined with constraints provided by a separate multi-nuclear NMR study. The structure has similarities with topaz-OH I and diaspore (alpha-A100H) structures, having partially occupied double edge-shared octahedral chains, and 2 x 1 tunnels with partially occupied tetrahedral and octahedral sites.

DOI： 10.2138/am.2010.3555

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

The solid solutions of strontium-calcium phosphate, gamma-Ca3-xSrx(PO4)(2) (0 &lt;= x &lt;= 2), have been synthesized under high-pressure and high-temperature conditions. The products were well characterized by powder X-ray diffraction measurements and Micro-Raman spectroscopy. All the products are isostructural and belong to R-3m space group. With the increasingcontent of Sr in gamma-Ca3-xSrx(PO4)(2), the unit cell parameters (a, c, c/a, V) linearly increase, and the wavenumbers of Raman vibrational modes (v(3), v(1), v(4), v(2)) also change in linear trends with different slopes. (c) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.matchemphys.2009.11.010

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Language：English   Publisher：WILEY-BLACKWELL  

Solid-state 1H nuclear magnetic resonance (NMR) spectroscopy has developed into a versatile, high-resolution method for elucidating the detailed structures of both crystalline and amorphous materials. Here, we summarize our recent endeavors in applying this method to crystalline and glassy silicates and related inorganic materials. We first present updated correlations between 1H chemical shift and O circle dot O and H circle dot O distances of O-H circle dot O hydrogen bonds, derived from a comprehensive 1H chemical shift database containing both original high-quality fast magic angle spinning (MAS) and two-dimensional combined rotation and multiple pulse spectroscopy (CRAMPS)-MAS NMR data and literature MAS NMR data for a large number of inorganic compounds, including phosphates, silicates, (oxy)hydroxides, borates, sulfates, and carbonates. These correlations may be used to estimate hydrogen-bonding distances for inorganic oxide materials of unknown structures. We then present case studies for the application of high-resolution two-dimensional 1H CRAMPS-MAS NMR to unravel the order/disorder of proton distributions in crystalline high-pressure hydrous silicates. Finally, we summarize some of our results on the water speciation in hydrous (alumino)silicate glasses of a range of compositions obtained through comprehensive 1H MAS NMR, and 29Si-1H and 27Al-1H double-resonance NMR experiments, and analyze them in the framework of a quasi-chemical model.

DOI： 10.1111/j.1551-2916.2009.03468.x

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOC AMER  

High-resolution Si-29 and Al-27 NMR techniques have been applied to resolve the Si-Al distribution and coordination in the high-pressure CaAl4Si2O11 (CAS) phase, a potentially important mineral in subducted crustal materials in the deep mantle that has a unique hexagonal ferrite structure containing two octahedral (M1; M2) and one trigonal bipyramidal sites. The Si-29 MAS NMR spectra of the CAS phase synthesized at 20 GPa and 1400 similar to 1600 degrees C show two broad, asymmetric peaks near -92.7 and -182.7 ppm with an intensity ratio of 1:3, suggesting that 1/4 of the Si are in tetrahedral coordination and 3/4 in octahedral coordination. Therefore, the trigonal bipyramidal and M1 octahedral sites are each occupied by equal proportions of Si and AI, and the former are effectively half-occupied face-sharing tetrahedra (at least for Si). The Al-27 MAS and 3Q MAS NMR spectra contain only one unresolved peak typical of octahedral Al with a range of quadrupolar coupling constants.

DOI： 10.2138/am.2009.3348

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

An investigation into the high-pressure behavior of AX(2)-type iron pnictides was conducted using first-principles calculations based on density functional theory within the generalized gradient approximation. Our results demonstrate that a phase transition from the marcasite to the CuAl2 occurs at 108 GPa for FeP2, at 92 GPa for FeAs2, and at 38 GPa for FeSb2, accompanying a semiconductor-to-metal crossover. A linear relationship between bulk moduli and the inverse specific volume is proposed to be B-0 = 17498/V-0 - 45.9 GPa for the marcasite-type phase and B-0 = 31798/V-0 - 67.5 GPa for the CuAl2-type phase. According to the observed structural evolutions, we claim that the regular marcasite transforms to the CuAl2-type phase and the anomalous marcasite transforms to the pyrite-type phase at high pressures.

DOI： 10.1088/0953-8984/21/18/185403

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Under compression, the As-S liquids are subject to transformations, including polymerization and metallization. We have measured the viscosity of the As-S liquids under high pressures. As a result, large viscosity variations by 4-5 orders of magnitude have been revealed. The viscosity values of the As-S liquids are moderate in the molecular state, very high in the covalent state, and low in the metallic state. Therefore, predicting the viscosity behavior in other melts under pressure is possible, which is of considerable importance for the physics of glass transition, geophysics, and material science.

DOI： 10.1103/PhysRevLett.102.115901

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We have performed X-ray Raman scattering (XRS) measurements on the oxygen K and silicon L absorption edges of four silica minerals: alpha-quartz, alpha-cristobalite, coesite, and stishovite. We have also calculated the partial electron densities of states (DOSs) and compared these with the XRS spectra. This study demonstrates that the short-range structure around the atom of interest strongly influences the XRS spectral features. Importantly, the oxygen K-edge XRS spectra are found to reflect the p-orbital DOS while the silicon L-edge spectra reflect the s- and d-orbital DOSs, even when a product of a momentum transfer and a mean radius of a electron orbit (1s for oxygen and 2p for silicon), Qr, is close to or larger than unity. Building on this, calculations of the partial DOSs for other silica phases are presented, including ultra-high-pressure phases, which provide a good reference for further XRS study of silica and silicate minerals. XRS measurements should be performed on not only either of oxygen or silicon but also on many kinds of constituent elements to reveal the structural change of glasses/melts of silicates under extreme conditions.

DOI： 10.1007/s00269-008-0267-x

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Language：Japanese   Publisher：Japan Association of Mineralogical Sciences  

High-pressure quench experiments of AlPO₄ revealed two new polymorphs at 6-7 GPa. "I" stabilized at 1000°C, and "II" stabilized at 1500°C. ³¹P MAS NMR revealed three tetrahedral P sites of equal abundances in each phase. ²⁷Al MQ MAS NMR unveiled two octahedral and one penta-coordinated Al sites for each phase. We solved the crystal structures using the "FOX" program (direct-space searching method), using the information from NMR to constrain the number and types of polyhedra. The space group of I was P-1, whereas, that of II was P2₁/c. The length of a- and c-axes are similar, however, that of b-axis of II was about twice of that of I. The densities of two phases are almost the same, and are 28% larger than that of quartz phase. Both phases contain short chains made of edge-shared six Al-polyhedra which are interconnected by PO₄ tetrahedra to form 3D network.

DOI： 10.14824/jakoka.2009.0.127.0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

The structural stability of marcasite-type FeP2 at high pressure has been studied by X-ray diffraction, Raman spectroscopy, and theoretical calculations. Experimental results show that no phase transitions happen up to 28GPa at room temperature. The shortest axis of the marcasite-type FeP2 cell, the c-axis, is the most compressible, due to the softening of edge-shared octahedra along the c-axis. The linear pressure coefficients and Gruneisen parameters of four Raman modes are determined. Theoretical calculations further support the experimental results and indicate that FeP2 is still a semiconductor up to 35GPa.

DOI： 10.1080/08957950802597221

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Language：English   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

DOI： 10.2465/jmps.080514

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOC AMER  

To gain new structural insights into phase D and superhydrous B, two phases of potential mantle water reservoir, we have applied a range of one- (1D) and two-dimensional (2D) (1)H and (29)Si NMR techniques, as well as Raman spectroscopy, to samples synthesized at 24 GPa and 900 similar to 1100 degrees C. These data have revealed that phase D is characterized by disordered and varying local structures around both Si and H. The (29)Si NMR spectra of phase D contain a nearly symmetric, broad peak near -177.7 ppm, attributable to octahedral Si with local structural disorder. The high-resolution (1)H CRAMPS spectra of phase D contain a main broad peak near 12.6 ppm with shoulders near 10 and 7 ppm, suggesting a distribution of hydrogen bonding distances. For superhydrous B, our comprehensive 2D (1)H and (29)Si NMR results have clearly revealed that it contains dissimilar hydrogen (H1-H2) pairs and one tetrahedral Si site, consistent with space group Pnn2.

DOI： 10.2139/am.2008.2751

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Near-edge x-ray Raman scattering (XRS) spectra of the silicon L edge have been obtained on SiO(2) glass under compression in a diamond anvil cell up to 74 GPa. Partial densities of states (DOS&apos;s) of electrons, which the spectra reflect, have been calculated. The spectra for the glass do not show significant variations with pressure, whereas distinct differences are observed between quartz and stishovite, providing clear evidence for the nonexistence of sixfold-coordinated silicon by oxygen, that is stishovitelike silicon, in silica glass up to 74 GPa. A post main-edge hump, which is not seen in the XRS spectra and DOS&apos;s of quartz and stishovite, and an edge energy shift suggest that the silicon is in a different coordination environment under pressure. A compression mechanism of SiO(2) glass which involves the formation of fivefold-coordinated silicon following the decrease in Si-O-Si angle is proposed to explain the observed changes up to 74 GPa.

DOI： 10.1103/PhysRevB.78.012203

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We have taken a systematic approach utilizing advanced solid-state NMR techniques to gain new insights into the controversial issue concerning the dissolution mechanisms of water in aluminosilicate melts (glasses). A series of quenched anhydrous and hydrous (similar to 2 wt% H2O) glass samples along the diopside (Di, CaMgSi2O6) - anorthite (An, CaAl2Si2O8) join with varying An components (0, 20, 38, 60, 80, and 100 mol%) have been studied. A variety of NMR techniques, including one-dimensional (ID) H-1 and Al-27 MAS NMR, and Al-27 -&gt; H-1 cross-polarization (CP) MAS NMR, as well as two-dimensional (M) H-1 double-quantum (DQ) MAS NMR Al-27 triple-quantum (3Q) MAS NMR, and Al-27 -&gt; H-1 heteronuclear correlation NMR (HETCOR) and 3QMAS/HETCOR NMR, have been applied. These data revealed the presence of SiOH, free OH ((Ca,Mg)OH) and AlOH species in the hydrous glasses, with the last mostly interconnected with Si and residing in the more polymerized parts of the structure. Thus, there are no fundamental differences in water dissolution mechanisms for Al-free and Al-bearing silicate melts (glasses), both involving two competing processes: the formation of SiOH/AlOH that is accompanied by the depolymerization of the network structure, and the formation of free OH that has an opposite effect. The latter is more important for depolymerized compositions corresponding to mafic and ultramafic magmas.
Aluminum is dominantly present in four coordination (Al-IV), but a small amount of five-coordinate Al (Al-V) is also observed in all the anhydrous and hydrous glasses. Furthermore, six-coordinate Al (Al-VI) is also present in most of the hydrous glasses. As Al of higher coordinations are favored by high pressure, (AlOH)-O-VI and (AlOH)-O-V may become major water species at higher pressures corresponding to those of the Earth's mantle. (c) 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.gca.2008.01.022

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Language：Japanese   Publisher：Japan Association of Mineralogical Sciences  

X-ray radiography in SPring-8 indicates that fluids derived from the downgoing slab can be in supercritical conditions underneath volcanic arcs. Such supercritical fluids separate into aqueous fluid and silicate melt during their ascents. Elemental partitioning between them is key information.

DOI： 10.14824/jakoka.2008.0.44.0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Viscosity is one of the fundamental physical properties of liquids; for different melts it varies in an extremely wide range. Selenium is among the first elementary substances to have manifested, at compression, a phase transformation in the liquid state accompanied by melt metallization. Direct measurements by means of a real-time radiography show that the viscosity of liquid Se under pressure drops by 500 times to a very low level of 8 mPa s. This is the first case of viscosity measurements being performed both for a relatively viscous semiconducting state and a low-viscous metallic state of the same liquid substance. The viscosity of the Se melt strongly decreases with pressure along the melting curve in a semiconducting state and experiences a further significant drop at melt metallization. A similar phenomenon is expected to be observed in many chalcohenide, halogenide, and oxide melts.

DOI： 10.1103/PhysRevLett.99.245901

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

In order to shed light on the proton distributions and order/disorder in high-pressure delta-Al(OH)(3) and delta-AlOOH phases, two-dimensional, high-resolution H-1 CRAMPS (FSLG)-MAS NMR and Al-27 3QMAS NMR spectra have been obtained. For delta-Al(OH)(3), the H-1 CRAMPS-MAS NMR revealed two peaks with an intensity ratio close to 2:1. The Al-27 MAS and 3QMAS NMR suggest a single Al site with a well-defined local structure. For delta-AlOOH, the H-1 and Al-27 NMR indicate the presence of a single H and Al site each. These results are consistent with crystal structures refined from X-ray diffraction. For comparison, 1H MAS and CRAMPS-MAS NMR spectra were also obtained for several other hydroxides/oxyhydroxides, including In(OH)(3) and InOOH that have similar structures to delta-Al(OH)(3) and delta-AlOOH, respectively. These data not only provide additional insights into the proton distributions in these important crystal structure classes but also together provide a better defined quantitative correlation between 1H chemical shift and hydrogen-bonding O center dot center dot center dot O distance.

DOI： 10.1021/jp073968r

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

The second critical endpoint in the peridotite-H2O system has been determied using an X-ray radiography technique together with a Kawai-type, double-stage, multianvil system driven by DIA-type cubic press (SPEED-1500) installed at SPring-8, Japan. The pressure of the second critical endpoint was determined by the appearance and disappearance of round shape in the radiographic images with changing the experimental pressure. In the experiments up to 3.6 GPa, two fluid phases (i.e., aqueous fluid and hydrous silicate melt) were observed. At 4.0 GPa, however, we could not distinguish these two phases in the radiographic images. These observations indicate the second critical endpoint occurs at around 3.8 GPa and 1000°C (corresponding to a depth of ∼110 km) in the peridotite-H2O system. Our experimental results suggest that hydrous silicate melt and aqueous fluid in the Earth's mantle become indistinguishable from each other and that melting temperature of hydrous mantle peridotite can no longer be defined beyond this critical condition. This position of the second critical endpoint could explain the previously observed drastic changes in composition and connectivity of aqueous fluid in mantle peridotite at around 3-4 GPa and could play an important role in magmatism and chemical evolution of the Earth's interior. Copyright 2007 by the American Geophysical Union.

DOI： 10.1029/2005JB004125

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[ 1] The second critical endpoint in the peridotite-H2O system has been determined using an X-ray radiography technique together with a Kawai-type, double-stage, multianvil system driven by DIA-type cubic press (SPEED-1500) installed at SPring-8, Japan. The pressure of the second critical endpoint was determined by the appearance and disappearance of round shape in the radiographic images with changing the experimental pressure. In the experiments up to 3.6 GPa, two fluid phases (i.e., aqueous fluid and hydrous silicate melt) were observed. At 4.0 GPa, however, we could not distinguish these two phases in the radiographic images. These observations indicate the second critical endpoint occurs at around 3.8 GPa and 1000 degrees C ( corresponding to a depth of similar to 110 km) in the peridotite-H2O system. Our experimental results suggest that hydrous silicate melt and aqueous fluid in the Earth's mantle become indistinguishable from each other and that melting temperature of hydrous mantle peridotite can no longer be defined beyond this critical condition. This position of the second critical endpoint could explain the previously observed drastic changes in composition and connectivity of aqueous fluid in mantle peridotite at around 3 - 4 GPa and could play an important role in magmatism and chemical evolution of the Earth's interior.

DOI： 10.1029/2005JB004125

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

dIn order to shed light on the dissolution mechanisms of water in depolymerized aluminosilicate melts/glasses, a comprehensive one(113) and two-dimensional (2D) NMR study has been carried out on hydrous Ca- and Mg-aluminosilicate glasses of a haplobasaltic composition. The applied techniques include 1D H-1 MAS NMR and Al-27 -&gt; H-1 cross-polarization (CP) MAS NMR, and 2D H-1 NOESY and double-quantum (DQ) MAS NMR, Al-27 triple-quantum (3Q) MAS NMR and 27 -&gt; H-1 heteronuclear correlation (HETCOR) and 3QMAS/HETCOR NMR. Ab initio calculations were also performed to place additional constraints on the 1H NMR characteristics of AlOH and Si(OH)AI groups. This study has revealed, for the first time, the presence of free OH (i.e. (Ca, Mg)OH), SiOH and AlOH species, in addition to molecular H2O, in hydrous glasses of a depolymerized alummosilicate composition. The AlOH groups are mostly associated with four-coordinate Al, but some are associated with five- and six-coordinate Al. (c) 2006 Published by Elsevier Inc.

DOI： 10.1016/j.ssnmr.2006.11.001

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Language：Japanese   Publisher：The Ceramic Society of Japan  

In order to shed light on the proton distributions and order/disorder in high-pressure delta-Al(OH)3 and delta-AlOOH and isostructural In(OH)3 and InOOH phases, high-resolution 1H CRAMPS (FSLG)-MAS NMR and Raman spectroscopy have been applied. The 1H CRAMPS-MAS NMR revealed two peaks each for delta-Al(OH)3 and In(OH)3, and one each for delta-AlOOH and InOOH. These results are consistent with crystal structures refined from X-ray and/or neutron diffraction. The Raman spectra are also in general agreement with the NMR results.

DOI： 10.14853/pcersj.2007f.0.118.0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOC AMER  

We have applied one-dimensional (1D) H-1 MAS NMR Al-27 -&gt; H-1 CP MAS NMR, as well as 2D(27)Al triple-quantum (3Q) MAS NMR, Al-21 -&gt; H-1 heteronuclear correlation (HETCOR) and high-resolution 3QMAS/HETCOR NMR techniques to KAlSi3O8 (Or), NaAlSi3O8 (Ab) and NaAlSiO4 (Ne) glasses containing 0 similar to 2 wt% H2O to shed light on the dissolution mechanisms of water in aluminosilicate melts (glasses). An Al Q(3)-OH group, characterized by H-1 chemical shifts of 1.3-1.9 ppm, was identified for all hydrous glasses. Its abundance increases with bulk Al/Si ratio. The Al-27 chemical shifts (partial derivative(Al)(i)) of this species are 64-68 ppm, larger than those of Al Q(4) by 3-6 ppm. Despite this difference, it is only through Al-27 -&gt; H-1 HETCOR and 3QMAS/HETCOR, but not Al-27 MAS or 3QMAS NMR that the peaks are resolved. This study has demonstrated that depolymerization and formation of AlOH/SiOH is a general water dissolution mechanism for polymerized aluminosilicate melts (glasses), and HETCOR NMR experiments involving H-1 are the key to its revelation.

DOI： 10.2138/am.2006.2365

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOC AMER  

Topaz-OH, phase egg, and delta-AIOOH are hydrous phases in the Al2O3-SiO2-H2O system that have been found to be stable at successively higher pressures up to those corresponding to the lower mantle, and thus they may be important water reservoirs in the deep mantle. We have applied H-1, Si-29, and Al-27 nuclear magnetic resonance (NMR) and Raman spectroscopy to shed new light on the structure of these phases. Si-29 and (27)AI NMR results clearly revealed that the Si-Al distribution in phase egg is partially disordered. The presence of structural disorder in topaz-OH was also confirmed. H-1 NMR and Raman data are both consistent with strong, but asymmetric hydrogen bonding in delta-AlOOH and phase egg, and a range of hydrogen bonding distances in topaz-OH. The observed structural disorder and hydrogen bonding could be responsible for the high upper temperature stability limits (1500 similar to 1700 degrees C) of phase egg and topaz-OH, and are also relevant to the incorporation mechanisms of water in nominally anhydrous stishovite.

DOI： 10.2138/am.2006.2064

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Language：Japanese   Publisher：特定非営利活動法人 日本火山学会  

DOI： 10.18940/vsj.2006.0_40

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Publisher：Japan Association of Mineralogical Sciences  

Topaz-OH (Al2SiO4(OH)2), phase egg (AlSiO3OH) and delta-AlOOH are hydrous phases in the Al2O3-SiO2-H2O system that have been found to be stable at successively higher pressures up to those of the lower mantle, and thus may be important in the siliceous sediments of subducting slab. Although the crystal structures of these phases have been determined by X-ray diffraction, this technique is insensitive to H positions and Al-Si disorder. We have applied 1H, 29Si and 27Al NMR and micro-Raman spectroscopy to gain additional insights. Our study revealed that the octahedral Al-Si distribution in phase egg is partially disordered, which is expected to have a significant effect on its physical and thermodynamic properties and stability field. In addition, these data also suggest the presence of some structural disorder in topaz-OH. Finally, our data support that hydrogen bonding is strong in phase egg and delta-AlOOH, but weak in topaz-OH.

DOI： 10.14824/kobutsu.2005.0.45.0

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Publisher：Japan Association of Mineralogical Sciences  

We have calculated electric field gradients (EFG) of various crystals using FPLAPW method. Calculated Cq and asymmetry parameter of oxygen well reproduced experimental NMR data within 5 %, except for hydrogen containing systems. Therefore such calculations allow us to uniquely assign NMR peaks to certain sites with confidence.

DOI： 10.14824/kobutsu.2005.0.23.0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Dissolution of water in magmas significantly affects phase relations and physical properties. To shed new light on the this issue, we have applied H-1 and Si-29 nuclear magnetic resonance. (NMR) lspectroscopic techniques to hydrous silicate glasses (quenched melts) in the CaO-MgO-SiO2 (CMS). Na2O-SiO2,Na2O-CaO-SiO2 and Li2O-SiO2 systems. We have also carried out ab initio molecular orbital calculations on representative clusters to gain insight into the experimental results.
The most prominent result is the identification of a major peak at similar to1.1 to 1.7 ppm in the H-1 MAS NMR spectra for all the hydrous CMS glasses. On the basis of experimental NAIR data for crystalline phase:, and ab initio calculation results. this peak can be unambiguously attributed to (Ca,Mg)OH groups. Such OH groups, like free oxygens, are only linked to metal cations. but not part of the silicate network. and are thus referred to as free hydroxyls in the paper. This represents die first direct evidence for a substantial proportion (similar to13-29%) of the dissolved water as free hydroxyl groups in quenched hydrous silicate melts. We have found that free hydroxyls are favored by (1) more depolymerized melts and (2) network-modifying cations of higher field strength (Z/R-2: Z: charge, R: cation-oxygen bond length) in the order Me &gt; Ca &gt; Na. Their formation is expected to cause an increase in the melt polymerization, contrary to the effect of SiOHi formation. The Si-29 MAS NMR results are consistent with such an interpretation. This water dissolution mechanism could be particularly important for ultramafic and mafic magmas.
The H-1 MAS NMR spectra for glasses of all the studied compositions contain peaks in the 4 to 17 ppm region, attributable to SiOH of a range of strength of hydrogen bonding and molecular H2O. The relative population of SiOH with strong hydrogen bonding grows with decreasing field strength of the network-modifying cations. Ab initio calculations confirmed that this trend largely reflects hydrogen bonding with nonbridging oxygens. Copyright (C) 2004 Elsevier Ltd.

DOI： 10.1016/j.gca.2004.08.016

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

To determine the second critical end point in silicate-H2O systems. a new method for the direct observations of immiscible fluids has been developed using a synchrotron X-ray radiography technique. High-pressure and high-temperature experiments were carried out with a Kawai-type, double-stage, multianvil high-pressure apparatus (SPEED-1500) installed at BL04B1. SPring-S, Japan. The Sr-plagioclase (SrAl2Si2O8)-H2O system was used as an illustrative example. A new sample container compose of a metal (Pt) tube with a pair of lids, made of single crystal diamonds, was used under pressures between 3.0 and 4.3 GPa. and temperatures up to similar to 1600degreesC. The sample in the container could be directly observed through the diamond lids with X-ray radiography. At around 980 to 1060degreesC and pressures between 3.0 and 4.0 GPa, light gray spherical bubbles moving upward through the dark gray matrix were observed. The light gray spheres that absorb less X-rays represent an aqueous fluid. whereas the dark gray matrix represents a silicate melt. These two immiscible phases (aqueous fluid and silicate melt) were observed up to 4.0 GPa. At 4.3 GPa. no bubbles were observed. These observations suggest that the second critical end point in the Sr-plagioclase-H2O system occurs at around 4.2 +/- 0.2 GPa and 1020 +/- 50degreesC. Our new technique can be applied to the direct observations of various systems with two coexisting fluids under deep mantle conditions. Copyright (C) 2004 Elsevier Ltd.

DOI： 10.1016/j.gca.2004.07.015

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Information about the state of Sulfur in silicate melts and glasses is important in both earth sciences and materials sciences. Because of its variety of valence states from S2- (sulfide) to S6+ (Sulfate). the speciation of sulfur dissolved in silicate melts and glasses is expected to be highly dependent on the oxygen fugacity. To place new constraint on this issue, we have synthesized sulfur-bearing sodium silicate glasses (quenched melts) front starting materials containing sulfur of different valence states (Na2SO4, Na2SO3, Na2S2O3 and native S) using an internally heated gas pressure vessel, and have applied electron-induced SKalpha X-ray fluorescence, micro-Raman and NMR spectroscopic techniques to probe their structure. The wavelength shift of SKa X-rays revealed that the differences in the valence state of sulfur in the starting compounds are largely retained in the synthesized sulfur-bearing glasses, with a small reduction for more oxidized samples. The Si-29 MAS NMR spectra of all the glasses contain no peaks attributable to the SiO4-nSn (with n &gt; 0) linkages. The Raman spectra are consistent with the coexistence of sodium sulfate (Na2SO4) species and one or more types of more reduced sulfur species containing S-S linkages in all the sulfur-bearing silicate glasses, with the former dominant in glasses produced from Na2SO4-doped starting materials, and the latter more abundant in more reduced glasses. The 29Si MAS MAR and Raman spectra also revealed changes in the silicate network Structure of the sulfur-bearing glasses, which can be interpreted in terms of change:, in the chemical composition and sulfur speciation. Copyright (C) 2004 Elsevier Ltd.

DOI： 10.1016/j.gca.2004.08.029

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DOI： 10.1016/j.gca.2004.08.025

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The phase boundary of the post-spinel transition in Mg2SiO4 was re-investigated by means of high P-T in situ X-ray diffractometry with a gold pressure marker in a Kawai-type apparatus. Rapid and continuous temperature changes were conducted to initiate dissociation of spinel, which tends to be inert after long annealing. Isothermal decompression at high temperature was conducted to form spinel from perovskite plus periclase. The phase boundary is located at ca. 22 GPa in the temperature range from 1550 to 2100 K, which is 1-1.5 GPa lower than the 660 km discontinuity. This discrepancy might be explained in terms of the pressure effect of thermocouple emf and inaccurate equation of state (EOS) for the pressure maker. The transition is found to be less sensitive to temperature than reported previously, with a Clapeyron slope ranging from -2 to -0.4 MPa/K. This small Clapeyron slope implies that the post-spinel transition would not be an effective barrier to mantle convection. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0031-9201(03)00019-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Ab initio molecular orbital calculations have been carried out for silicate, aluminosilicate, and aluminate clusters to study the NMR characteristics of various types of hydroxyls (OH) that are possibly present in hydrous silicate glasses and melts. The clusters have been optimized with the density functional theory (B3LYP/6-311+G(2df,p)) and their NMR parameters calculated at HF/G-311+G(2df,p). Our calculations suggest that the O-17 and H-2 quadrupolar coupling constants (C-Q(O) and C-Q(H)) and H-1 chemical shift (delta (H)(i)) of SiOH, AlOH, and bridging OH's (Si(OH)Al, Al(OH)Al) all show good correlation with the O-H and O-H . . .O distances. The calculated C-Q(O), C-Q(H), and delta (H)(i) values agree well with those of the experimental data for OH groups with similar O-H . . .O distances in crystalline phases. Hydroxyls with stronger hydrogen bonding tend to yield smaller C-Q(O) and C-Q(H) and larger delta (H)(i). SiOH and bridging OH's of comparable hydrogen-bonding strengths give similar O-17 and H-1 (H-2) NMR parameters. AlOH have a tendency not to form strong Al-O-H . . .O type hydrogen bonding and, thus, give relatively large C-Q(O) and C-Q(H) and small delta (H)(i). On the basis of these calculation results, together with information for hydrogen-bonding strengths estimated from experimental vibrational spectra and H-1 NMR data, we were able to predict O-17 NMR parameters for hydroxyls in hydrous silicate glasses. The observed O-17 NMR peaks for silica gel and hydrous albite glass, that have been attributed to SIGH, are significantly narrower than expected from C-Q(O), suggesting that at least some of the SiOH, if present, must be nonrigid. The observed broad O-17 NMR peaks for hydrous albite and alkali silicate glasses, originally attributed to molecular H2O, could equally well be ascribed to rigid hydroxyls with weak hydrogen bonding.

DOI： 10.1021/jp0040751

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Si-29 magic-angle-spinning nuclear-magnetic resonance has been measured on spinel-type cubic silicon nitride (c-Si3N4). c-Si3N4 shows two Si-29 resonances at -50.0 +/-0.2 and -225.0 +/-0.2 ppm, corresponding to the tetrahedron SN4 and the octahedron SiN6, respectively. Integration of the spectrum gives SN4/SiN6 about one half that of the spinel structure. Ab initio self-consistent field Hartree-Fock molecular orbital calculations also indicate that the chemical shift for octahedral Si is more negative in nitride than in oxides. (C) 2001 American Institute of Physics.

DOI： 10.1063/1.1372199

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In situ observations of the ilmenite-perovskite transition in MgSiO3 were carried out in a multianvil high-pressure apparatus interfaced with synchrotron radiation. The phase boundary between ilmenite and perovskite in the temperature range of 1300-1600 K was determined to be P (GPa) = 28.4(+/-0.4) - 0.0029(+/-0.0020)T (K) based on Jamieson's equation of state of gold [Jamieson et at., 1982] and P (GPa) = 27.3(+/-0.4) - 0.0035(+/-0.0024)T (K) based on Anderson's equation of state of gold [Anderson et al., 1989]. The consistency of our results, using Jamieson's equation of state, with previous studies obtained by quench methods leads us to conclude that the 660 km seismic discontinuity in the mantle can be attributed a phase transition to perovskite phase. However, the phase boundary based on the Anderson's equation of state implies that the depth of the 660-km seismic discontinuity does not match the pressure of this transition.

DOI： 10.1029/1999GL008446

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Ab initio molecular orbital calculations (Hartree-Fock, HF and density functional theories, DFTs) have been carried out for SiO2 polymorphs coesite, low cristobalite, and α-quartz, in order to investigate the reliability of this method for predicting 29Si and 17O nuclear magnetic resonance (NMR) properties of silicates. Oxygen- and silicon-centered clusters consisting of one (1T) to three tetrahedral (3T) shells (one to four atomic shells), taken from real crystal structure, have been investigated. It is found that for reasonable predication of both the 29Si and 17O chemical shifts (δi Si and δi O), the minimum cluster is one that gives the correct second neighbors to the nucleus of interest. Both the δi Si and δi O have reached convergence with respect to cluster size at the OH-terminated two tetrahedral (2T) shell (three atomic shells around Si and four atomic shells around O) model. At convergence, the calculated δi Si values agree well (within ±1 ppm) with experimental data. The calculated 17O electric field gradient (EFG)-related parameters also agree with experimental data within experimental uncertainties. The calculation also reproduces small differences in δi O for O sites with similar tetrahedral connectivities, but shows deviations up to about 10 ppm in relative difference for O sites with different tetrahedral connectivities. The poor performance for the latter is mainly due to the approximations of the HF method. Our study thus suggests that the ab initio calculation method is a reliable mean for predicting 29Si and 17O NMR parameters for silicates. Such an approach should find application not only to well-ordered crystalline phases, but also to disordered materials, by combining with other techniques, such as the molecular dynamics simulation method. © 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0926-2040(00)00075-8

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DOI： 10.1021/jp992108a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER VERLAG  

In order to gain insight into the correlations between Si-29, O-17 and H-1 NMR properties (chemical shift and quadrupolar coupling parameters) and local structures in silicates, ab initio self-consistent field Hartree-Fock molecular orbital calculations have been carried out on silicate clusters of various polymerizations and intertetrahedral (Si-O-Si) angles. These include Si(OH)(4) monomers (isolated as well as interacting), Si2O(OH)(6) dimers (Cz symmetry) with the Si-O-Si angle fixed at 5 degrees intervals from 120 degrees to 180 degrees, Si3O2(OH)(8) linear trimers (C-2 symmetry) with varying Si-O-Si angles, Si3O3(OH)(6) three-membered rings (D-3 and C-1 symmetries), Si4O4(OH)(8) four-membered ring (C-4 symmetry) and Si8O12(OH)(8) octamer (D-4 symmetry). The calculated Si-29, O-17 and H-1 isotropic chemical shifts (delta(i)(Si), delta(i)(O) and delta(i)(H)) for these clusters are all close to experimental NMR data for similar local structures in crystalline silicates. The calculated O-17 quadrupolar coupling constants (QCC) of the bridging oxygens (Si-O-Si) are also in good agreement with experimental data. The calculated O-17 QCC of silanols (Si-O-H) are much larger than those of the bridging oxygens, but unfortunately there are no experimental data for similar groups in well-characterized crystalline phases for comparison. There is a good correlation between delta(i)(Si) and the mean Si-O-Si angle for both Q(1) and Q(2), where Q(n) denotes Si with n other tetrahedral Si next-nearest neighbors. Both the delta(i)(O) and the O-17 electric field gradient asymmetry parameter, eta of the bridging oxygens have been found to depend strongly on the O site symmetry, in addition to the Si-O-Si angle. On the other hand, the O-17 QCC seems to be influenced little by structural parameters other than the Si-O-Si angle, and is thus expected to be the most reliable O-17 NMR parameter that can be used to decipher Si-O-Si angle distribution information. Both the O-17 QCC and the H-2 QCC of silanols decrease with decreasing length of hydrogen bond to a second O atom (Si-O-H ... O), and the delta(i)(H) increase with the same parameter.

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The crystal structure of αCaSi2O5 synthesized at conditions of 1500°C and 10 GPa, has been solved and refined in centrosymmetric space group P1, using single crystal X-ray diffraction data. The composition (Z = 4) and unit cell are Ca1.02Si1.99O5 by EPMA analysis and a = 7.243(2) A, b = 7.546(4) A, c = 6.501(4) a, α = 81.43(5)°, β = 84.82(4)°, γ = 69.60(3)°, V = 329.5(3) A3, yielding the density value, 3.55 g/cm3. The structures the square-pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0.33 A. The substantial deviation of valence sume for Ca indicates the existence of local strain and the instability of α-CaSi2O5 at room temperature.

DOI： 10.1007/s002690050132

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The crystal structure of alpha-CaSi2O5 synthesized at conditions of 1500 degrees C and 10 GPa, has been solved and refined in centrosymmetric space group P (1) over bar, using single crystal X-ray diffraction data. The composition (Z=4) and unit cell are Ca1.02Si1.99O5 by EPMA analysis and a=7.243(2) Angstrom, b=7.546(4) Angstrom, c=6.501(4) Angstrom, alpha=81.43(5)degrees, beta=84.82(4)degrees, gamma=69.60(3)degrees, V=329.5(3) Angstrom(3), yielding the density value, 3.55 g/cm(3). The structure is closely related to that of titanite, CaTiSiO5 and features the square-pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0.33 Angstrom. The substantial deviation of valence sum for Ca indicates the existence of local strain and the instability of alpha-CaSi2O5 at room pressure.

DOI： 10.1007/s002690050132

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Preliminary studies have been made of phase relations in the K2Si49, Na2Si2O5, Na2Si3O7 and Na2Si4O9 systems up to 14 GPa. Several high-pressure sodium silicate phases have been observed for the first time and were characterized by means of powder X-ray diffraction and 29Si MAS NMR techniques. In the K2Si4O9 system, the wadeite (K2ZrSi3O9)-type phase was found to be stable and melt congruently at least up to 12 GPa. In the Na2Si2O5 system, phase C. a Na2Si2O5 polymorph previously reported at 10-40 MPa, was observed at 2.5 GPa. However, at 5-6 GPa, a previously unknown phase (ε-Na2Si2O5) appeared. This phase was replaced by yet another new phase (ζ-Na2Si2O5) at 8-10 GPa. In the Na2Si3O7 system, a new high-pressure Na2Si3O7 phase was detected at about 10 GPa. In the Na2Si4O9 system, a new Na2Si4O9 phase appeared at 6-8 GPa and it decomposed to stishovite (SiO2) plus the high-pressure Na2Si3O7 phase at 9j 10 GPa. The 29Si MAS NMR spectra revealed that the εNa2Si2O5 phase contains only tetrahedral Si sites, whereas the ζ-Na2Si2O5, Na2Si3O7 and Na2Si4O9 phases contain both tetrahedral and octahedral Si sites. Recently, Fleet and Henderson [Fleet, M., Henderson, G.S., 1995a. Epsilon sodium disilicate: A high-pressure layer structure [Na2Si2O5]. J. Solid State Chem., 119: 400-414
Fleet, M., Henderson, G.S., 1995b. Sodium trisilicate: A new high-pressure silicate structure (Na2Si[Si2O7]). Phys. Chem. Min., 22: 383-386.] and Fleet [Fleet, M., 1996. Sodium tetrasilicate: A complex high-pressure framework silicate (Na6Si3[Si9O27]). Am. Mineral., 81: 1105-1110.] have determined the structures of the ε-Na2Si2O5, Na2Si3O7 and Na2Si4O9 phases. Subsolidus phase transformations with pressure for these alkali silicate systems can be described in terms of reduction of Si-O-Si angles at lower pressures and formation of octahedral Si through conversion of nonbridging oxygens to bridging oxygens at higher pressure. Similar structural changes might be expected for alkali silicate glasses (and melts) within this pressure range.

DOI： 10.1016/S0031-9201(97)00120-9

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Preliminary studies have been made of phase relations in the K2SI4O9, Na2Si2O5, Na2Si3O7 and Na2Si4O9 systems up to 14 GPa. Several high-pressure sodium silicate phases have been observed for the first time and were characterized by means of powder X-ray diffraction and Si-29 MAS NMR techniques; In the K2Si4O9 system, the wadeite (K2ZrSi3O9)-type phase was found to be stable and melt congruently at least up to 12 GPa. In the Na2Si2O5 system, phase C, a Na2Si2O5 polymorph previously reported at 10-40 MPa, was observed at 2.5 GPa. However, at 5-6 GPa, a previously unknown phase (epsilon-Na2Si2O5) appeared. This phase was replaced by yet another new phase(zeta-Na2Si2O5) at 8-10 GPa. In the Na2Si3O7 system, a new high-pressure Na2Si3O7 phase was detected at about 10 GPa. In the Na2Si4O9 system, a new Na2Si3O7 phase appeared at 6-8 GPa and it decomposed to stishovite (SiO2) plus the high-pressure Na2Si3O7 phase at greater than or equal to 10 GPa. The Si-29 MAS NMR spectra revealed that the epsilon-Na2Si2O5 phase contains only tetrahedral Si sites, whereas the zeta-Na2Si2O5, Na2Si3O7 and Na2Si4O9 phases contain both tetrahedral and octahedral Si sites. Recently, Fleet and Henderson [Fleet, M., Henderson, G.S., 1995a. Epsilon sodium disilicate: A high-pressure layer structure [Na2Si2O5]. J. Solid State Chem., 119: 400-404; Fleet, M., Henderson, G.S., 1995b. Sodium trisilicate: A new high-pressure silicate structure (Na2Si[Si2O7]). Phys. Chem. Min., 22. 383-386.] and Fleet [Fleet, M., 1996. Sodium tetrasilicate: A complex high-pressure framework silicate (Na2Si3[Si9O27]). Am. Mineral., 81. 1105-1110.] have determined the structures of the epsilon-Na2Si2O5, Na2Si3O7 and Na2Si4O9 phases. Subsolidus phase transformations with pressure for these alkali silicate systems can be described in terms of reduction of Si-O-Si angles at lower pressures and formation of octahedral Si through conversion of nonbridging oxygens to bridging oxygens at higher pressures. Similar structural changes might be expected for alkali silicate glasses (and melts) within this pressure range. (C) 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0031-9201(97)00120-9

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Brownmillerite(Cr2Al2O5-Ca2Fe2O5 solid solution) structure can be regarded as an oxygen-ion deficient perovskite structure. Because of high proportion of the oxygen vacancies in the structure, this material could be a candidate of fast oxide-ion conductor. Goodenough et al. indeed observed a first-order transition to a fast oxide-ion conductor at 930 degrees C for Ba2In2O5 which adapts brownmillerite structure at ambient temperature. Molecular dynamics simulation was employed to study oxygen ion diffusion and phase transition of Ba2In2O5. The structure was well simulated at 300 K. When the system was heated, the original orthogonal cell transformed to a tetragonal cell at 2300 K. Inspection of the structure revealed that oxygen ions started to migrate from their original sites to nearest vacant oxygen sites at this temperature. The diffusion was restricted for the oxygen sites around In-tetrahedron, resulting highly anisotropic diffusion on the ac plane. At 4600 K it further transformed to an oxygen vacancies-disordered cubic perovskite structure. Although predicted transition temperature were apparently overestimated, the transition way to the phases with high oxygen ion diffusivity is consistent with the experimental results from electrical conductivity measurements. The high temperature cubic phase shows large ion conductivity. It is of interest to examine whether or not the cubic phase stabilizes in the low temperature region by making solid solution of another elements. We found that the cubic phase is stabilized below 500 degrees C without any decrease of conductivity in BaIn1.9Ce0.1Oy and Ba2In1.8Nb0.2O5.

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DOI： 10.1007/s002690050132

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Verlag  

In order to gain insight into the correlations between 29Si, 17O and 1H NMR properties (chemical shift and quadrupolar coupling parameters) and local structures in silicates, ab initio self-consistent field Hartree-Fock molecular orbital calculations have been carried out on silicate clusters of various polymerizations and intertetrahedral (Si-O-Si) angles. These include Si(OH)4 monomers (isolated as well as interacting), Si2O(OH)6 dimers (C2 symmetry) with the Si-O-Si angle fixed at 5° intervals from 120° to 180°, Si3O2(OH)8 linear trimers (C2 symmetry) with varying Si-O-Si angles, Si3O3(OH)6 three-membered rings (D3 and C1 symmetries), Si4O4(OH)8 four-membered ring (C4 symmetry) and Si8O12(OH)8 octamer (D4 symmetry). The calculated 29Si, 17O and 1H isotropic chemical shifts (δi Si, δi O and δi H) for these clusters are all close to experimental NMR data for similar local structures in crystalline silicates. The calculated 17O quadrupolar coupling constants (QCC) of the bridging oxygens (Si-O-Si) are also in good agreement with experimental data. The calculated 17O QCC of silanols (Si-O-H) are much larger than those of the bridging oxygens, but unfortunately there are no experimental data for similar groups in well-characterized crystalline phases for comparison. There is a good correlation between δi Si and the mean Si-O-Si angle for both Q1 and Q2, where Qn denotes Si with n other tetrahedral Si next-nearest neighbors. Both the δi O and the 17O electric field gradient asymmetry parameter, η of the bridging oxygens have been found to depend strongly on the O site symmetry, in addition to the Si-O-Si angle. On the other hand, the 17O QCC seems to be influenced little by structural parameters other than the Si-O-Si angle, and is thus expected to be the most reliable 17O NMR parameter that can be used to decipher Si-O-Si angle distribution information. Both the 17O QCC and the 2H QCC of silanols decrease with decreasing length of hydrogen bond to a second O atom (Si-O-H⋯O), and the δi H increase with the same parameter. © Springer-Verlag 1998.

DOI： 10.1007/s002690050157

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CERAMIC SOC JAPAN-NIPPON SERAMIKKUSU KYOKAI  

The effect of coordination number on Al-27 NMR chemical shift was studied using ab initio molecular orbital calculation. In order to mimic Al of four-, five-and six-coordinations Al, the clusters of Al(OH)(4)(-), Al(OH)(5)(2-) and Al(OH)(6)(3-) were employed for the calculation. The obtained shifts for Al(OH)(4)(-), Al(OH)(5)(2-) and Al(OH)(6)(3-) were +70.3, +31.5 and -3.9 ppm, respectively, and were in reasonable agreement with observed shifts of Al-containing oxides. Present calculation supports the assignment of +30 ppm peak observed in aluminosilicate glasses as five-coordination state. Ab initio molecular orbital calculation can be used to establish the correlation between local structure and NMR chemical shift of Al-containing materials.

DOI： 10.2109/jcersj.105.91

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Molecular dynamics simulation was employed to study oxygen ion diffusion in Ba2In2O5 with brownmillerite structure. When the system was heated, the original orthorhombic cell changed to a tetragonal cell at 2300 K. Inspection of the structure revealed that oxygen ions jump From their original sites to the nearest vacant sites. The diffusion was restricted for the sites around the tetrahedrally coordinated In ion, resulting in highly anisotropic diffusion on the ac plane. At 4600 K it further transformed to an oxygen disordered cubic perovskite structure, in which all oxygen ions contributed to diffusion. The predicted transition was consistent with the observed transition to a fast oxide-ion conductor for this compound, although predicted temperature was overestimated. The effect of composition on the transition was studied by simulating the A(2)B(2)O(5) (A(2+) = Ba, Sr, Ca; B3+ = Al, Fe, In) systems. From these results, oxide-ion conductors with lower transition temperatures could be predicted.

DOI： 10.1016/S0921-5107(96)01621-2

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In order to identify five-coordinated Si(Si^v) species in crystalline and amorphous silicates by NMR spectroscopy, ²⁹Si NMR chemical shifts for the clusters of Si(CH₃)₄ (TMS), Si(OH)₄, Si(OH)₅⁻ and Si(OH)₆⁻² were calculated using ab initio molecular orbital theory. The calculated shifts relative to TMS cluster were −71.7 and −179.0 ppm for Si(OH)₄ and Si(OH)₆⁻², respectively, and are in good agreement with observed shifts. The calculated shift for Si(OH)₅⁻ was −127.4 ppm, falls just between the shifts of Si(OH)₄ and Si(OH)₆⁻². The chemical shift for Si^v species in polymerized silicates is expected to appear at about −150 ppm, if the effect of polymerization is considered. Present calculation thus supports the assignment of −150 ppm peak as Si^v species, which has been found in the alkali-silicate glasses quenched from high pressures.

DOI： 10.2465/minerj.18.1

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The structure of phase F, a non-stoichiometric hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions of 17 GPa and 1000 degrees C, has been solved and refined in space group P6(3)cm, using synchrotron X-ray data for a single crystal of a size 18 x 24 x 30 mu m. The composition and unit cell for phase F are Mg3.35Si5.51H7.26O18, a = 5.073(3) Angstrom c = 14.013(9) Angstrom, V = 312.3(5) Angstrom(3). The structure contains layers with many similarities to superhydrous phase B. The layers of oxygen atoms are stacked in the ABCBAC-type double cubic closest packing arrangement. The bulk modulus of phase F was estimated from the structural and compositional relationship to superhydrous phase B and periclase.

DOI： 10.1007/BF00202769

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Language：Japanese   Publisher：The Ceramic Society of Japan  

The stability of an infinite-layer compound ACuO_2 (A=Sr, Ca) at high temperature was studied by molecular dynamics simulation. For SrCuO_2 the infinite-layer structure was well reproduced at 300 K. However inspection of the simulated structure revealed that oxygen ions displaced from initially planar CuO_2 layer to the c-direction by ±19 pm alternatively. This structure was stable up to 1200 K and transformed to an amorphous phase above 1300 K. For CaCuO_2 the infinite-layer structure was unstable even at 300 K. Simulations in which cation in A site was substituted revealed that large cation in A site is responsible for the stabilization of the structure, consistent with the experimental results.

DOI： 10.2109/jcersj.103.529

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Language：Japanese   Publisher：The Crystallographic Society of Japan  

DOI： 10.5940/jcrsj.37.Supplement_115

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MINERALOGICAL SOC AMER  

In order to understand the correlations between O-17 NMR parameters and the local structure around 0 in high-pressure silicates, we have obtained O-17 MAS and static NMR spectra on several crystalline alkali silicate and silica phases, as well as on ambient pressure glasses. Our results show that 110 NMR is capable of distinguishing nonbridging O atoms (NBO), bridging atoms (BO: [4]Si-O-[4]Si), O atoms in the [4]Si-O-[6]Si linkage, and O atoms coordinated to three [6]Si ([3]O). NBO are characterized by small quadrupolar coupling constants (e2qQ/h, approximately 2.3 MHz), but their isotropic chemical shifts (delta(i)) are very sensitive to the type of network-modifying cations. The [3]O (in stishovite), on the other hand, has by far the largest e2qQ/h value (6.5 MHz) and a larger delta(i) (109 ppm) than any BO in silicates. BO and [4]Si-O-[6]Si both have intermediate e2qQ/h values (approximately 4.5-5.7 MHz), and the delta(i) value of the latter (97 ppm in wadeite-K2Si4O9) is larger than those of the former (approximately 50-65 ppm).
We have also applied O-17 NMR to study the structure of alkali silicate glasses quenched from melts at high pressure. The O-17 NMR spectra of a K2Si4O9 glass quenched from 6 GPa and of Na2Si4O9 glasses quenched from 6 to 10 GPa are consistent with the presence of O sites in the [4]Si-O-[6]Si (and possibly also [4]Si-O-[5]Si) linkage, supporting our previous model developed from Si-29 NMR. In addition, O-17 static NMR also reveals the development of another new type of O site, possibly one that is connected with two or three [6]Si or [5]Si atoms, in the Na2Si4O9 glass quenched from 10 GPa. Thus, [6]Si and [5]Si may be largely isolated from one another below 10 GPa, whereas at higher pressure, clustering of these Si species may become significant in Na2Si4O9 melts.

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Language：Japanese   Publisher：Japan Association of Mineralogical Sciences  

Recent studies of structural changes of silicate minerals under high temperature and high pressure were reviewed. Recently it was found that several high pressure phases transform to amorphous phase or metastable phase during decompression. Examples are MnTiO₃ perovskite/LiNbO₃ phase transition and C2/c /P2₁/c transition in MgSiO₃ enstatite. In order to study these unquenchable phases it is essential to use in-situ observation technique. Existing phase diagrams based on quench experiments should be reinvestigated. For better understanding of the earth's mantle, structure and physical properties of mantle minerals must be studies at its stable P-T conditions rather than at ambient condition.

DOI： 10.2465/gkk1952.23.167

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Stishovite, a high-pressure SiO2 polymorph in which each Si is coordinated by six O atoms, transforms to an amorphous phase when undergoing heat treatment below the glass transition temperature at ambient pressure. We have applied Si-29 magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) to study this amorphization process. We found that the amorphous phase generated after heating stishovite for up to 3 days at around 600-degrees-C consisted exclusively of four-coordinate Si, similar to glasses quenched from melts at ambient pressure. Furthermore, our data suggest that there are subtle structural differences between the amorphous phase transformed from stishovite at 600-degrees-C and glasses quenched from melts at ambient pressure: the amorphous phase from stishovite had a smaller mean Si-O-Si angle initially, and it gradually relaxed toward the latter with increasing heating time. There was no detectable change in the stishovite structure even after about 80% of it had been converted to the amorphous phase. The mechanism of the amorphization of stishovite is discussed in light of these results.

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Stishovite, a high-pressure SiO2 polymorph in which each Si is coordinated by six O atoms, transforms to an amorphous phase when undergoing heat treatment below the glass transition temperature at ambient pressure. We have applied Si-29 magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) to study this amorphization process. We found that the amorphous phase generated after heating stishovite for up to 3 days at around 600-degrees-C consisted exclusively of four-coordinate Si, similar to glasses quenched from melts at ambient pressure. Furthermore, our data suggest that there are subtle structural differences between the amorphous phase transformed from stishovite at 600-degrees-C and glasses quenched from melts at ambient pressure: the amorphous phase from stishovite had a smaller mean Si-O-Si angle initially, and it gradually relaxed toward the latter with increasing heating time. There was no detectable change in the stishovite structure even after about 80% of it had been converted to the amorphous phase. The mechanism of the amorphization of stishovite is discussed in light of these results.

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The unique cation-disordered crystal structures of two samples of phase E, a non-stoichiometric, hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions above 13 GPa and 1000-degrees-C, have been solved and refined in space group R3mBAR. The compositions and unit cells for the two materials, assuming six oxygens per cell, are Mg2.08Si1.16H3.20O6, a = 2.9701(1) angstrom, c = 13.882(1) angstrom, V = 106.05(4) angstrom3 for sample 1, and Mg2.17Si1.01H3.62O6, a = 2.9853(6) angstrom, c = 13.9482(7) angstrom, V = 107.65(4) angstrom3 for sample 2. The structure contains layers with many features of brucite-type units, with the layers stacked in a rhombohedral arrangement. The layers are cross linked by silicon in tetrahedral coordination and magnesium in octahedral coordination, as well as hydrogen bonds. Interlayer octahedra share edges with intralayer octahedra. Interlayer tetrahedra would share faces with intralayer octahedra. To avoid this situation, there are vacancies within the layers. There is, however, no long-range order in the occupation of these sites, as indicated by the lack of a superstructure. Selected-area electron diffraction patterns show walls of diffuse intensity similar in geometry and magnitude to those observed in short-range-ordered alloys and Hagg phases. Phase E thus appears to represent a new class of disordered silicates, which may be thermodynamically metastable.

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DOI： 10.2465/minerj.16.278

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Language：Japanese   Publisher：The Crystallographic Society of Japan  

DOI： 10.5940/jcrsj.35.Supplement_68

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Language：Japanese   Publisher：Japan Association of Mineralogical Sciences  

Recent studies on pressure-induced structural changes in silicate glass (melt) by various spectroscopic techniques are reviewed. The following changes have been observed with increasing pressure; reduction of Si-O-Si angle, change in Qⁿ distribution, and formation of high-coordination Si (Si^v and Si^VI). Systematic change of high-coor-dination Si with pressure and composition reveals that high-coordination Si is produced by consuming non-bridging ox-ygens in alkali-silicates. The importance of Si^v for dynamic process is discussed.

DOI： 10.2465/gkk1952.21.149

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We have studied quenched high pressure phases in the CaSiO3 system by x-ray diffraction (XRD) and Si-29 MAS NMR. XRD study of the previously reported "epsilon-CaSiO3 phase" synthesized at 12 GPa and 1500-degrees-C reveals that it is actually a mixture of beta-Ca2SiO4 (larnite) and a previously unknown CaSi2O5 phase. This result is supported by the Si-29 NMR spectra. Furthermore, both the XRD and the NMR data suggest that the CaSi2O5 phase may have a titanite (CaTiSiO5) structure in which Ti is replaced by an octahedral Si. Samples quenched from 15 GPa and 1500-degrees-C consist mostly of an amorphous phase, but a small amount of CaSiO3-perovskite was identified by both XRD and NMR. The Si-29 NMR spectrum of the amorphous phase suggests that its local structure is similar to that of a glass quenched from melt at 1 bar.

DOI： 10.1029/91GL00463

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER ASSOC ADVANCEMENT SCIENCE  

Most of the earth's mantle is made up of high-pressure silicate minerals that contain octahedrally coordinated silicon (Si(VI), but many thermodynamically important details of cation site ordering remain unknown. Silicon-29 nuclear magnetic resonance (NMR) spectroscopy is potentially very useful for determining short-range structure. A systematic study of silicon-29 chemical shifts for Si(VI) has revealed empirical correlations between shift and structure that are useful in understanding several new calcium silicates. The observed ordering state of a number of high-pressure magnesium silicates is consistent with the results of previous x-ray diffraction studies.

DOI： 10.1126/science.251.4991.294

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The Si-29 and Na-23 NMR, Raman, and infrared spectra of glasses quenched from liquids at pressures up to 12 GPa for three alkali silicate compositions (Na2Si2O5, Na2Si4O9, K2Si4O9) and silica (SiO2) reveal systematic changes in the melt structure with pressure. The most novel change is the occurrence of [5]Si and [6]Si species at high pressures identified by peaks in the Si-29 MAS NMR spectra of the alkali silicate glasses. The abundances of both [5]Si and [6]Si increase with pressure. At a given pressure there are more such species in the Na2Si4O9 composition than in Na2Si2O5 composition. These species are not observed in a SiO2 glass quenched at 6 GPa. The occurrence of these highly coordinated Si species is consistent with changes in the Raman and infrared spectra of the alkali silicate glasses, although the peak assignments in these spectra are not unique. The change in Si coordination with pressure and composition in these compositions can be accounted for by a model in which [5]Si and [6]Si form at the expense of nonbridging O atoms. This model may be generalized to all partially depolymerized melts. Changes in the tetrahedral structure with increasing pressure include Q speciation disproportionation, reduction of mean Si-O-Si angle, and development of new [4]Si species that share O with [5]Si or [6]Si. The observed structural changes have implications for the physical and thermodynamic properties of silicate melts. [5]Si has been previously envisaged as a transient state for the viscous flow and diffusion processes in silicate melts. The occurrence of [5]Si in the high pressure glasses suggests that [5]Si represents a local energy minimum and its stabilization may partly account for the enhancement of these dynamic processes. Increased configurational entropy as a result of greater distribution of Si coordinations and Q species at high pressures could also contribute to the reduced melt viscosity.

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Pressure-induced phase transformation of orthoenstatite to clinoenstatite has been studied at 7-10 GPa using a multi-anvil high pressure device with low stress (&lt; 10 MPa) conditions. At 1000-degrees-C clinoenstatite was stabilized at pressures above 7.5 GPa. The obtained phase boundary is consistent with natural observations and previous experimental studies performed under quasi-hydrostatic condition, suggesting that clinoenstatite is a stable high pressure phase. Large differences in dP/dT slope between this result and those of earlier studies performed with piston cylinder and belt apparatus may be attributed to large differential stress in the high pressure cells of latter studies. The present study suggests that clinoenstatite can be stabilized by either hydrostatic pressure or differential stress and that the latter tends to shift the transformation boundary defined under hydrostatic condition to lower pressure.

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DOI： 10.1016/0031-9201(91)90085-V

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Language：Japanese   Publisher：The Crystallographic Society of Japan  

DOI： 10.5940/jcrsj.33.Supplement_114

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

DOI： 10.1029/GL017i011p01989

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER ASSOC ADVANCEMENT SCIENCE  

DOI： 10.1126/science.245.4921.962

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Language：English   Publisher：MINERALOGICAL SOC AMER  

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High-pressure phase relations in the system Mg4Si4O12Mg3Al2Si3O12 were investigated at pressures from 10 to 22 GPa and at a temperature of 1000°C. The experimental results indicate that the maximum solubility of enstatite (Mg4Si4O12) in pyrope (Mg3Al2Si3O12) is 80 mol% enstatite at 15 GPa. At pressures between 15 and 19 GPa, the solubility is insensitive to pressure. At 19 GPa, garnet-ilmenite transformation first occurs in enstatite composition. At pressures above 19 GPa, the compositional range of the ilmenite solid solution expands toward pyrope composition with increasing pressure. Based on the present experiments, if pyrolite mantle is assumed, the pyroxene-garnet transformation appears at 350-400 km depth, and garnet + modified spinel and garnet + spinel phase assemblages become stable successively up to ∼ 580 km depth. At greater depths, garnet decomposes to ilmenite + garnet and hence the pyrolite mantle consists of ilmenite + garnet + spinel assemblage. © 1987.

DOI： 10.1016/0031-9201(87)90139-7

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29Si MAS NMR is a well-established tool for quantitative characterization of Si-Al distribution in aluminosilicates. In framework aluminosilicates, all tetrahedrally coordinated Si/Al (T) are linked via bridging oxygens to four other Si/Al (Q 4). Each crystallographically unique T site is commonly assumed to give at most five 29Si NMR peaks with chemical shift spacing around 5 ppm, corresponding to five Si units each linked to nSi and (4 - n)Al (n = 0-4), abbreviated as Si(nSi) hereafter. Here we report a detailed one- and two-dimensional (2D) 29Si NMR study on K-cymrite (KAlSi 3O 8·H 2O), which possesses a double-layered structure with all Si/Al distributed in one crystallographically unique T site (Q 4). Contrary to general belief, more than five 29Si MAS NMR peaks were resolved. These peaks are attributable, via 2D J-coupling mediated experiments, to Si(nSi)(n = 0-4), with two peaks each (∼2 ppm spacing) for n = 1-3. The latter corresponds to different permutations of the nSi(4 - n)Al over neighboring T sites of two distinct Si-O-T angles (139 and 180°). This suggests for the first time that the 29Si chemical shift varies with Si-O-Si and Si-O-Al angles differently. Si-Al exchange over neighboring T sites of unequal Si-O-T angles is another possible cause for multiple 29Si NMR peaks. © 2012 American Chemical Society.

DOI： 10.1021/jp3008859

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DOI： 10.1073/pnas.1207687109

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Language：English   Publisher：MAIK NAUKA/INTERPERIODICA/SPRINGER  

It is known that a number of compressed melts undergo structural phase transitions. Data on the structural changes at high pressures in chalcogenides (AsS, As2S3) and oxide (B2O3) melts with a network structure have been reviewed. Viscosity is one of the fundamental physical properties of a liquid. For various melts, it varies in a very wide range. Structural transformations in melts induce the corresponding changes in all physical properties, in particular viscosity. The measurements of the viscosity of a number of melts at high pressures and temperatures by the radiographic method have been reported. Changes in the viscosity by several orders of magnitude have been detected when the pressure is varied by several gigapascals. The diffusion mechanism in network-structure melts at various pressures has been analyzed. The prediction of the behavior of the viscosity of various melts at superhigh pressures is of high importance for the physics of glass transition, geophysics, and materials science.

DOI： 10.1134/S0021364011140050

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Language：English   Publisher：WILEY-BLACKWELL  

The crystal structures of two new high-pressure AlPO4 phases are reported. One phase synthesized at 6 GPa and 1523 K is triclinic (P (1) over bar) whilst the other phase synthesized at 7 GPa and 1773 K is monoclinic (P2(1)/c). P-31 MAS (magic-angle spinning) NMR suggests three tetrahedral P sites with equal abundance in both phases. Al-27 3Q MAS NMR spectra provided evidence for two octahedral sites and one five-coordinated Al site in each phase. The crystal structures were solved using an ab initio structure determination technique from synchrotron powder X-ray diffraction data utilizing the local structural information from NMR, and were further refined by the Rietveld method. Both phases contain doubly bent chains made of six edge-shared Al polyhedra (including five-coordinated Al), which are joined by PO4 tetrahedra. The P (1) over bar phase is isostructural with FeVO4 and AlVO4. The two phases differ in the packing manner of the chains. This study has demonstrated that the combined application of ab initio structure determination via X-ray powder diffraction and solid-state NMR spectroscopy is a powerful approach to the rapid solution of complex inorganic crystal structures.

DOI： 10.1107/S0108768110051050

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Language：English   Publisher：AMER PHYSICAL SOC  

Liquid B2O3 represents an archetypical oxide melt with a superhigh viscosity at the melting temperature. We present the results of the in situ x-ray diffraction study and the in situ viscosity measurements of liquid B2O3 under high pressure up to 8 GPa. Additionally, the B-11 solid state NMR spectroscopy study of B2O3 glasses quenched from the melt at five different pressures has been carried out. Taken together, the results obtained provide understanding of the nature of structural transformations in liquid B2O3. The fraction of the boroxol rings in the melt structure rapidly decreases with pressure. From pressures of about 4.5 GPa, four-coordinated boron states begin to emerge sharply, reaching the fraction 40%-45% at 8 GPa. The viscosity of the B2O3 melt along the melting curve drops by 4 orders of magnitude as the pressure increases up to 5.5 GPa and remains unchanged on further pressure increase.

DOI： 10.1103/PhysRevLett.105.115701

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A new polymorph of topaz-OH (denoted as topaz-OH II) was recently discovered at higher P-T conditions than has been known thus far (denoted as topaz-OH I). High-resolution 1H, 29Si, and 27Al nuclear magnetic resonance (NMR) and micro-Raman spectroscopy are applied to shed light on the crystal chemistry of both polymorphs. Topaz-OH I, synthesized at 7 GPa and 640 °C, is stoichiometric (Si/Al = 0.5) with a largely ordered local structure. Higher P-T topaz-OH I synthesized at conditions close to the polymorphic phase transition boundary, on the other hand, shows lower Si/Al ratios (0.44-0.45) and greater local structural disorder [including a small fraction (∼3%) of octahedral Si with a unique 29Si chemical shift near -133 ppm]. The latter may be accounted for by the development of defects (Si/Al in normally vacant octahedral sitesand vacancies in the tetrahedral sites) at higher P-T conditions. Topaz-OH II synthesized at 13.5∼14 GPa and 1300∼1400 °C similarly exhibits low Si/Al ratios (0.41-0.46). The NMR and Raman spectra for these topaz-OH II are, in general, broader and revealed a substantial fraction (33-37%) of octahedral Si with a range of 29Si chemical shifts (-130 to -190 ppm), a small fraction (2-3%) of tetrahedral Al, and a range of (and overall shorter) hydrogen-bonding distances than topaz-OH I. Therefore, the phase transition from topaz-OH I to II is characterized by both a significant increase in the occupied octahedral/tetrahedral site ratio as well as disordering of cation distribution, which is unique from the viewpoint of crystal chemistry.

DOI： 10.2138/am.2010.3471

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A new high-pressure form of topaz-OH (denoted here topaz-OH II) was obtained at 14 GPa and 1400 °C. The X-ray diffraction patternof this phase can be indexed by an orthorhombic cell with a = 4.72318(5), b = 8.91480(9), and c = 2.77276(3) Å. The lengths of a andb are similar, but c is approximately a third of that for a previously reported topaz-OH (denoted topaz-OH I). The structural formula of topaz-OH II can be written as (Al 0.68Si0.32(O0.66(OH)0.34) 2, suggesting significant cation disorder in the structure. The crystal structure of topaz-OH II is solved using powder synchrotron X-ray diffraction data, and refined with constraints provided by a separate multi-nuclear NMR study. The structure has similarities with topaz-OH I and diaspore (α-AlOOH) structures, having partially occupied double edge-shared octahedral chains, and 2 × 1 tunnels with partially occupied tetrahedral and octahedral sites.

DOI： 10.2138/am.2010.3555

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

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Language：English   Publishing type：Book review, literature introduction, etc.   Publisher：WILEY-BLACKWELL  

Solid-state 1H nuclear magnetic resonance (NMR) spectroscopy has developed into a versatile, high-resolution method for elucidating the detailed structures of both crystalline and amorphous materials. Here, we summarize our recent endeavors in applying this method to crystalline and glassy silicates and related inorganic materials. We first present updated correlations between 1H chemical shift and O circle dot O and H circle dot O distances of O-H circle dot O hydrogen bonds, derived from a comprehensive 1H chemical shift database containing both original high-quality fast magic angle spinning (MAS) and two-dimensional combined rotation and multiple pulse spectroscopy (CRAMPS)-MAS NMR data and literature MAS NMR data for a large number of inorganic compounds, including phosphates, silicates, (oxy)hydroxides, borates, sulfates, and carbonates. These correlations may be used to estimate hydrogen-bonding distances for inorganic oxide materials of unknown structures. We then present case studies for the application of high-resolution two-dimensional 1H CRAMPS-MAS NMR to unravel the order/disorder of proton distributions in crystalline high-pressure hydrous silicates. Finally, we summarize some of our results on the water speciation in hydrous (alumino)silicate glasses of a range of compositions obtained through comprehensive 1H MAS NMR, and 29Si-1H and 27Al-1H double-resonance NMR experiments, and analyze them in the framework of a quasi-chemical model.

DOI： 10.1111/j.1551-2916.2009.03468.x

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Language：English   Publisher：MINERALOGICAL SOC AMER  

High-resolution Si-29 and Al-27 NMR techniques have been applied to resolve the Si-Al distribution and coordination in the high-pressure CaAl4Si2O11 (CAS) phase, a potentially important mineral in subducted crustal materials in the deep mantle that has a unique hexagonal ferrite structure containing two octahedral (M1; M2) and one trigonal bipyramidal sites. The Si-29 MAS NMR spectra of the CAS phase synthesized at 20 GPa and 1400 similar to 1600 degrees C show two broad, asymmetric peaks near -92.7 and -182.7 ppm with an intensity ratio of 1:3, suggesting that 1/4 of the Si are in tetrahedral coordination and 3/4 in octahedral coordination. Therefore, the trigonal bipyramidal and M1 octahedral sites are each occupied by equal proportions of Si and AI, and the former are effectively half-occupied face-sharing tetrahedra (at least for Si). The Al-27 MAS and 3Q MAS NMR spectra contain only one unresolved peak typical of octahedral Al with a range of quadrupolar coupling constants.

DOI： 10.2138/am.2009.3348

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

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Language：English   Publisher：IOP PUBLISHING LTD  

An investigation into the high-pressure behavior of AX(2)-type iron pnictides was conducted using first-principles calculations based on density functional theory within the generalized gradient approximation. Our results demonstrate that a phase transition from the marcasite to the CuAl2 occurs at 108 GPa for FeP2, at 92 GPa for FeAs2, and at 38 GPa for FeSb2, accompanying a semiconductor-to-metal crossover. A linear relationship between bulk moduli and the inverse specific volume is proposed to be B-0 = 17498/V-0 - 45.9 GPa for the marcasite-type phase and B-0 = 31798/V-0 - 67.5 GPa for the CuAl2-type phase. According to the observed structural evolutions, we claim that the regular marcasite transforms to the CuAl2-type phase and the anomalous marcasite transforms to the pyrite-type phase at high pressures.

DOI： 10.1088/0953-8984/21/18/185403

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Under compression, the As-S liquids are subject to transformations, including polymerization and metallization. We have measured the viscosity of the As-S liquids under high pressures. As a result, large viscosity variations by 4-5 orders of magnitude have been revealed. The viscosity values of the As-S liquids are moderate in the molecular state, very high in the covalent state, and low in the metallic state. Therefore, predicting the viscosity behavior in other melts under pressure is possible, which is of considerable importance for the physics of glass transition, geophysics, and material science.

DOI： 10.1103/PhysRevLett.102.115901

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We have performed X-ray Raman scattering (XRS) measurements on the oxygen K and silicon L absorption edges of four silica minerals: alpha-quartz, alpha-cristobalite, coesite, and stishovite. We have also calculated the partial electron densities of states (DOSs) and compared these with the XRS spectra. This study demonstrates that the short-range structure around the atom of interest strongly influences the XRS spectral features. Importantly, the oxygen K-edge XRS spectra are found to reflect the p-orbital DOS while the silicon L-edge spectra reflect the s- and d-orbital DOSs, even when a product of a momentum transfer and a mean radius of a electron orbit (1s for oxygen and 2p for silicon), Qr, is close to or larger than unity. Building on this, calculations of the partial DOSs for other silica phases are presented, including ultra-high-pressure phases, which provide a good reference for further XRS study of silica and silicate minerals. XRS measurements should be performed on not only either of oxygen or silicon but also on many kinds of constituent elements to reveal the structural change of glasses/melts of silicates under extreme conditions.

DOI： 10.1007/s00269-008-0267-x

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Language：English   Publisher：TAYLOR & FRANCIS LTD  

The structural stability of marcasite-type FeP2 at high pressure has been studied by X-ray diffraction, Raman spectroscopy, and theoretical calculations. Experimental results show that no phase transitions happen up to 28GPa at room temperature. The shortest axis of the marcasite-type FeP2 cell, the c-axis, is the most compressible, due to the softening of edge-shared octahedra along the c-axis. The linear pressure coefficients and Gruneisen parameters of four Raman modes are determined. Theoretical calculations further support the experimental results and indicate that FeP2 is still a semiconductor up to 35GPa.

DOI： 10.1080/08957950802597221

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To gain new structural insights into phase D and superhydrous B, two phases of potential mantle water reservoir, we have applied a range of one- (1D) and two-dimensional (2D) (1)H and (29)Si NMR techniques, as well as Raman spectroscopy, to samples synthesized at 24 GPa and 900 similar to 1100 degrees C. These data have revealed that phase D is characterized by disordered and varying local structures around both Si and H. The (29)Si NMR spectra of phase D contain a nearly symmetric, broad peak near -177.7 ppm, attributable to octahedral Si with local structural disorder. The high-resolution (1)H CRAMPS spectra of phase D contain a main broad peak near 12.6 ppm with shoulders near 10 and 7 ppm, suggesting a distribution of hydrogen bonding distances. For superhydrous B, our comprehensive 2D (1)H and (29)Si NMR results have clearly revealed that it contains dissimilar hydrogen (H1-H2) pairs and one tetrahedral Si site, consistent with space group Pnn2.

DOI： 10.2139/am.2008.2751

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Near-edge x-ray Raman scattering (XRS) spectra of the silicon L edge have been obtained on SiO(2) glass under compression in a diamond anvil cell up to 74 GPa. Partial densities of states (DOS&apos;s) of electrons, which the spectra reflect, have been calculated. The spectra for the glass do not show significant variations with pressure, whereas distinct differences are observed between quartz and stishovite, providing clear evidence for the nonexistence of sixfold-coordinated silicon by oxygen, that is stishovitelike silicon, in silica glass up to 74 GPa. A post main-edge hump, which is not seen in the XRS spectra and DOS&apos;s of quartz and stishovite, and an edge energy shift suggest that the silicon is in a different coordination environment under pressure. A compression mechanism of SiO(2) glass which involves the formation of fivefold-coordinated silicon following the decrease in Si-O-Si angle is proposed to explain the observed changes up to 74 GPa.

DOI： 10.1103/PhysRevB.78.012203

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We have taken a systematic approach utilizing advanced solid-state NMR techniques to gain new insights into the controversial issue concerning the dissolution mechanisms of water in aluminosilicate melts (glasses). A series of quenched anhydrous and hydrous (similar to 2 wt% H2O) glass samples along the diopside (Di, CaMgSi2O6) - anorthite (An, CaAl2Si2O8) join with varying An components (0, 20, 38, 60, 80, and 100 mol%) have been studied. A variety of NMR techniques, including one-dimensional (ID) H-1 and Al-27 MAS NMR, and Al-27 -&gt; H-1 cross-polarization (CP) MAS NMR, as well as two-dimensional (M) H-1 double-quantum (DQ) MAS NMR Al-27 triple-quantum (3Q) MAS NMR, and Al-27 -&gt; H-1 heteronuclear correlation NMR (HETCOR) and 3QMAS/HETCOR NMR, have been applied. These data revealed the presence of SiOH, free OH ((Ca,Mg)OH) and AlOH species in the hydrous glasses, with the last mostly interconnected with Si and residing in the more polymerized parts of the structure. Thus, there are no fundamental differences in water dissolution mechanisms for Al-free and Al-bearing silicate melts (glasses), both involving two competing processes: the formation of SiOH/AlOH that is accompanied by the depolymerization of the network structure, and the formation of free OH that has an opposite effect. The latter is more important for depolymerized compositions corresponding to mafic and ultramafic magmas.
Aluminum is dominantly present in four coordination (Al-IV), but a small amount of five-coordinate Al (Al-V) is also observed in all the anhydrous and hydrous glasses. Furthermore, six-coordinate Al (Al-VI) is also present in most of the hydrous glasses. As Al of higher coordinations are favored by high pressure, (AlOH)-O-VI and (AlOH)-O-V may become major water species at higher pressures corresponding to those of the Earth's mantle. (c) 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.gca.2008.01.022

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DOI： 10.2465/jmps.080514

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Language：Japanese   Publisher：The Geochemical Society of Japan  

Knowledge of the atomic structures of Earth's materials is indispensible for the understanding/modeling of macroscopic geochemical processes. Solid-state NMR spectroscopy offers a rich variety of advanced multi-nuclear, multi-dimensional techniques that can provide not only quantitative information about local structures around different elements (isotopes), but also direct information concerning atomic connectivities. Recently, we have applied some of these techniques to unravel the structures of (1) high-pressure hydrous minerals in the MgO-SiO₂-H₂O and Al₂O₃-SiO₂-H₂O systems (e.g. phase egg, δ-AlOOH, phase D, superhydrous B), which represent potential water reservoirs in the Earth's mantle, and (2) hydrous (alumino) silicate glasses (quenched melts), which serve as analogs for natural magmas. For the hydrous minerals, the states of Si-Al and Si-Mg order/disorder among octahedral sites and hydrogen distribution and hydrogen-bonding distances were clearly revealed. Such information would have been difficult to obtain by any other single technique. For the hydrous aluminosilicate melts, advanced NMR techniques provided the key information needed to end a long-standing controversy concerning the water dissolution mechanisms. Some of these results are summarized here to demonstrate the usefulness and wonder of advanced solid-state NMR spectroscopy.

DOI： 10.14934/chikyukagaku.42.133

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Viscosity is one of the fundamental physical properties of liquids; for different melts it varies in an extremely wide range. Selenium is among the first elementary substances to have manifested, at compression, a phase transformation in the liquid state accompanied by melt metallization. Direct measurements by means of a real-time radiography show that the viscosity of liquid Se under pressure drops by 500 times to a very low level of 8 mPa s. This is the first case of viscosity measurements being performed both for a relatively viscous semiconducting state and a low-viscous metallic state of the same liquid substance. The viscosity of the Se melt strongly decreases with pressure along the melting curve in a semiconducting state and experiences a further significant drop at melt metallization. A similar phenomenon is expected to be observed in many chalcohenide, halogenide, and oxide melts.

DOI： 10.1103/PhysRevLett.99.245901

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In order to shed light on the proton distributions and order/disorder in high-pressure delta-Al(OH)(3) and delta-AlOOH phases, two-dimensional, high-resolution H-1 CRAMPS (FSLG)-MAS NMR and Al-27 3QMAS NMR spectra have been obtained. For delta-Al(OH)(3), the H-1 CRAMPS-MAS NMR revealed two peaks with an intensity ratio close to 2:1. The Al-27 MAS and 3QMAS NMR suggest a single Al site with a well-defined local structure. For delta-AlOOH, the H-1 and Al-27 NMR indicate the presence of a single H and Al site each. These results are consistent with crystal structures refined from X-ray diffraction. For comparison, 1H MAS and CRAMPS-MAS NMR spectra were also obtained for several other hydroxides/oxyhydroxides, including In(OH)(3) and InOOH that have similar structures to delta-Al(OH)(3) and delta-AlOOH, respectively. These data not only provide additional insights into the proton distributions in these important crystal structure classes but also together provide a better defined quantitative correlation between 1H chemical shift and hydrogen-bonding O center dot center dot center dot O distance.

DOI： 10.1021/jp073968r

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[ 1] The second critical endpoint in the peridotite-H2O system has been determined using an X-ray radiography technique together with a Kawai-type, double-stage, multianvil system driven by DIA-type cubic press (SPEED-1500) installed at SPring-8, Japan. The pressure of the second critical endpoint was determined by the appearance and disappearance of round shape in the radiographic images with changing the experimental pressure. In the experiments up to 3.6 GPa, two fluid phases (i.e., aqueous fluid and hydrous silicate melt) were observed. At 4.0 GPa, however, we could not distinguish these two phases in the radiographic images. These observations indicate the second critical endpoint occurs at around 3.8 GPa and 1000 degrees C ( corresponding to a depth of similar to 110 km) in the peridotite-H2O system. Our experimental results suggest that hydrous silicate melt and aqueous fluid in the Earth's mantle become indistinguishable from each other and that melting temperature of hydrous mantle peridotite can no longer be defined beyond this critical condition. This position of the second critical endpoint could explain the previously observed drastic changes in composition and connectivity of aqueous fluid in mantle peridotite at around 3 - 4 GPa and could play an important role in magmatism and chemical evolution of the Earth's interior.

DOI： 10.1029/2005JB004125

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In order to shed light on the dissolution mechanisms of water in depolymerized aluminosilicate melts/glasses, a comprehensive one- (1D) and two-dimensional (2D) NMR study has been carried out on hydrous Ca- and Mg-aluminosilicate glasses of a haplobasaltic composition. The applied techniques include 1D 1H MAS NMR and 27Al→1H cross-polarization (CP) MAS NMR, and 2D 1H NOESY and double-quantum (DQ) MAS NMR, 27Al triple-quantum (3Q) MAS NMR and 27Al→1H heteronuclear correlation (HETCOR) and 3QMAS/HETCOR NMR. Ab initio calculations were also performed to place additional constraints on the 1H NMR characteristics of AlOH and Si(OH)Al groups. This study has revealed, for the first time, the presence of free OH (i.e. (Ca, Mg)OH), SiOH and AlOH species, in addition to molecular H2O, in hydrous glasses of a depolymerized aluminosilicate composition. The AlOH groups are mostly associated with four-coordinate Al, but some are associated with five- and six-coordinate Al. © 2006.

DOI： 10.1016/j.ssnmr.2006.11.001

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We have applied one-dimensional (1D) H-1 MAS NMR Al-27 -&gt; H-1 CP MAS NMR, as well as 2D(27)Al triple-quantum (3Q) MAS NMR, Al-21 -&gt; H-1 heteronuclear correlation (HETCOR) and high-resolution 3QMAS/HETCOR NMR techniques to KAlSi3O8 (Or), NaAlSi3O8 (Ab) and NaAlSiO4 (Ne) glasses containing 0 similar to 2 wt% H2O to shed light on the dissolution mechanisms of water in aluminosilicate melts (glasses). An Al Q(3)-OH group, characterized by H-1 chemical shifts of 1.3-1.9 ppm, was identified for all hydrous glasses. Its abundance increases with bulk Al/Si ratio. The Al-27 chemical shifts (partial derivative(Al)(i)) of this species are 64-68 ppm, larger than those of Al Q(4) by 3-6 ppm. Despite this difference, it is only through Al-27 -&gt; H-1 HETCOR and 3QMAS/HETCOR, but not Al-27 MAS or 3QMAS NMR that the peaks are resolved. This study has demonstrated that depolymerization and formation of AlOH/SiOH is a general water dissolution mechanism for polymerized aluminosilicate melts (glasses), and HETCOR NMR experiments involving H-1 are the key to its revelation.

DOI： 10.2138/am.2006.2365

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Topaz-OH, phase egg, and delta-AIOOH are hydrous phases in the Al2O3-SiO2-H2O system that have been found to be stable at successively higher pressures up to those corresponding to the lower mantle, and thus they may be important water reservoirs in the deep mantle. We have applied H-1, Si-29, and Al-27 nuclear magnetic resonance (NMR) and Raman spectroscopy to shed new light on the structure of these phases. Si-29 and (27)AI NMR results clearly revealed that the Si-Al distribution in phase egg is partially disordered. The presence of structural disorder in topaz-OH was also confirmed. H-1 NMR and Raman data are both consistent with strong, but asymmetric hydrogen bonding in delta-AlOOH and phase egg, and a range of hydrogen bonding distances in topaz-OH. The observed structural disorder and hydrogen bonding could be responsible for the high upper temperature stability limits (1500 similar to 1700 degrees C) of phase egg and topaz-OH, and are also relevant to the incorporation mechanisms of water in nominally anhydrous stishovite.

DOI： 10.2138/am.2006.2064

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DOI： 10.1016/j.gca.2004.08.025

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To determine the second critical end point in silicate-H2O systems. a new method for the direct observations of immiscible fluids has been developed using a synchrotron X-ray radiography technique. High-pressure and high-temperature experiments were carried out with a Kawai-type, double-stage, multianvil high-pressure apparatus (SPEED-1500) installed at BL04B1. SPring-S, Japan. The Sr-plagioclase (SrAl2Si2O8)-H2O system was used as an illustrative example. A new sample container compose of a metal (Pt) tube with a pair of lids, made of single crystal diamonds, was used under pressures between 3.0 and 4.3 GPa. and temperatures up to similar to 1600degreesC. The sample in the container could be directly observed through the diamond lids with X-ray radiography. At around 980 to 1060degreesC and pressures between 3.0 and 4.0 GPa, light gray spherical bubbles moving upward through the dark gray matrix were observed. The light gray spheres that absorb less X-rays represent an aqueous fluid. whereas the dark gray matrix represents a silicate melt. These two immiscible phases (aqueous fluid and silicate melt) were observed up to 4.0 GPa. At 4.3 GPa. no bubbles were observed. These observations suggest that the second critical end point in the Sr-plagioclase-H2O system occurs at around 4.2 +/- 0.2 GPa and 1020 +/- 50degreesC. Our new technique can be applied to the direct observations of various systems with two coexisting fluids under deep mantle conditions. Copyright (C) 2004 Elsevier Ltd.

DOI： 10.1016/j.gca.2004.07.015

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Information about the state of Sulfur in silicate melts and glasses is important in both earth sciences and materials sciences. Because of its variety of valence states from S2- (sulfide) to S6+ (Sulfate). the speciation of sulfur dissolved in silicate melts and glasses is expected to be highly dependent on the oxygen fugacity. To place new constraint on this issue, we have synthesized sulfur-bearing sodium silicate glasses (quenched melts) front starting materials containing sulfur of different valence states (Na2SO4, Na2SO3, Na2S2O3 and native S) using an internally heated gas pressure vessel, and have applied electron-induced SKalpha X-ray fluorescence, micro-Raman and NMR spectroscopic techniques to probe their structure. The wavelength shift of SKa X-rays revealed that the differences in the valence state of sulfur in the starting compounds are largely retained in the synthesized sulfur-bearing glasses, with a small reduction for more oxidized samples. The Si-29 MAS NMR spectra of all the glasses contain no peaks attributable to the SiO4-nSn (with n &gt; 0) linkages. The Raman spectra are consistent with the coexistence of sodium sulfate (Na2SO4) species and one or more types of more reduced sulfur species containing S-S linkages in all the sulfur-bearing silicate glasses, with the former dominant in glasses produced from Na2SO4-doped starting materials, and the latter more abundant in more reduced glasses. The 29Si MAS MAR and Raman spectra also revealed changes in the silicate network Structure of the sulfur-bearing glasses, which can be interpreted in terms of change:, in the chemical composition and sulfur speciation. Copyright (C) 2004 Elsevier Ltd.

DOI： 10.1016/j.gca.2004.08.029

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DOI： 10.1016/j.gca.2004.08.016

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The phase boundary of the post-spinel transition in Mg2SiO4 was re-investigated by means of high P-T in situ X-ray diffractometry with a gold pressure marker in a Kawai-type apparatus. Rapid and continuous temperature changes were conducted to initiate dissociation of spinel, which tends to be inert after long annealing. Isothermal decompression at high temperature was conducted to form spinel from perovskite plus periclase. The phase boundary is located at ca. 22 GPa in the temperature range from 1550 to 2100 K, which is 1-1.5 GPa lower than the 660 km discontinuity. This discrepancy might be explained in terms of the pressure effect of thermocouple emf and inaccurate equation of state (EOS) for the pressure maker. The transition is found to be less sensitive to temperature than reported previously, with a Clapeyron slope ranging from -2 to -0.4 MPa/K. This small Clapeyron slope implies that the post-spinel transition would not be an effective barrier to mantle convection. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0031-9201(03)00019-0

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Language：Japanese   Publisher：Tokyo Geographical Society  

To investigate the critical phenomena between aqueous fluid and hydrous silicate magma in the Earth's interior, a new method for making direct observations of immiscible fluids was developed using X-ray radiography technique together with the Kawai-type double stage multianvil system driven by a DIA-type cubic press (SPEED-1500) installed at BL04B1, SPring-8, Japan. Basalt-H₂O and Sr-plagioclase-H₂O systems were investigated. A new sample container comprising of a metal (Pt or AuPd) tube with a pair of lids made of single crystal diamonds was used under pressures between 2 and 5 GPa and temperatures up to 1500°C. The sample inside the container can be directly observed through the diamond lids.<BR>At around 800-900°C and pressures up to about 3 GPa in both basalt-H₂O and Srplagioclase-H₂O systems, we observed light-gray spherical bubbles moving upward in the darkgray matrix. The light-gray spheres that absorb less X-rays represent an aqueous fluid, whereas the dark-gray matrix represents a silicate melt. At least up to 1200°C, immiscible two phases (i.e., both aqueous fluid and silicate melt) were observed up to about 3 GPa in the basalt-H₂O system. At pressures above about 4 GPa, no bubbles appeared when heating to about 1200°C, suggesting the possibility that the second critical end-point in basalt-H₂O system could be around 3 to 4 GPa. Our new technique can be applied to the direct observations of various coexisting 2-fluids under deep mantle conditions that could not be achieved by previous experimental methods.

DOI： 10.5026/jgeography.112.6_970

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Ab initio molecular orbital calculations have been carried out for silicate, aluminosilicate, and aluminate clusters to study the NMR characteristics of various types of hydroxyls (OH) that are possibly present in hydrous silicate glasses and melts. The clusters have been optimized with the density functional theory (B3LYP/6-311+G(2df,p)) and their NMR parameters calculated at HF/G-311+G(2df,p). Our calculations suggest that the O-17 and H-2 quadrupolar coupling constants (C-Q(O) and C-Q(H)) and H-1 chemical shift (delta (H)(i)) of SiOH, AlOH, and bridging OH's (Si(OH)Al, Al(OH)Al) all show good correlation with the O-H and O-H . . .O distances. The calculated C-Q(O), C-Q(H), and delta (H)(i) values agree well with those of the experimental data for OH groups with similar O-H . . .O distances in crystalline phases. Hydroxyls with stronger hydrogen bonding tend to yield smaller C-Q(O) and C-Q(H) and larger delta (H)(i). SiOH and bridging OH's of comparable hydrogen-bonding strengths give similar O-17 and H-1 (H-2) NMR parameters. AlOH have a tendency not to form strong Al-O-H . . .O type hydrogen bonding and, thus, give relatively large C-Q(O) and C-Q(H) and small delta (H)(i). On the basis of these calculation results, together with information for hydrogen-bonding strengths estimated from experimental vibrational spectra and H-1 NMR data, we were able to predict O-17 NMR parameters for hydroxyls in hydrous silicate glasses. The observed O-17 NMR peaks for silica gel and hydrous albite glass, that have been attributed to SIGH, are significantly narrower than expected from C-Q(O), suggesting that at least some of the SiOH, if present, must be nonrigid. The observed broad O-17 NMR peaks for hydrous albite and alkali silicate glasses, originally attributed to molecular H2O, could equally well be ascribed to rigid hydroxyls with weak hydrogen bonding.

DOI： 10.1021/jp0040751

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Language：English   Publisher：AMER INST PHYSICS  

Si-29 magic-angle-spinning nuclear-magnetic resonance has been measured on spinel-type cubic silicon nitride (c-Si3N4). c-Si3N4 shows two Si-29 resonances at -50.0 +/-0.2 and -225.0 +/-0.2 ppm, corresponding to the tetrahedron SN4 and the octahedron SiN6, respectively. Integration of the spectrum gives SN4/SiN6 about one half that of the spinel structure. Ab initio self-consistent field Hartree-Fock molecular orbital calculations also indicate that the chemical shift for octahedral Si is more negative in nitride than in oxides. (C) 2001 American Institute of Physics.

DOI： 10.1063/1.1372199

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In situ observations of the ilmenite-perovskite transition in MgSiO3 were carried out in a multianvil high-pressure apparatus interfaced with synchrotron radiation. The phase boundary between ilmenite and perovskite in the temperature range of 1300-1600 K was determined to be P (GPa) = 28.4(+/-0.4) - 0.0029(+/-0.0020)T (K) based on Jamieson's equation of state of gold [Jamieson et at., 1982] and P (GPa) = 27.3(+/-0.4) - 0.0035(+/-0.0024)T (K) based on Anderson's equation of state of gold [Anderson et al., 1989]. The consistency of our results, using Jamieson's equation of state, with previous studies obtained by quench methods leads us to conclude that the 660 km seismic discontinuity in the mantle can be attributed a phase transition to perovskite phase. However, the phase boundary based on the Anderson's equation of state implies that the depth of the 660-km seismic discontinuity does not match the pressure of this transition.

DOI： 10.1029/1999GL008446

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Ab initio molecular orbital calculations (Hartree-Fock, KF and density functional theories, DFTs) have been carried out for SiO2 polymorphs coesite, low cristobalite, and alpha-quartz, in order to investigate the reliability of this method for predicting Si-29 and O-17 nuclear magnetic resonance (NMR) properties of silicates. Oxygen- and silicon-centered clusters consisting of one (1T) to three tetrahedral (3T) shells (one to four atomic shells), taken from real crystal structure, have been investigated. It is found that for reasonable predication of both the Si-29 and O-17 chemical shifts (delta(i)(Si) and delta(i)(O)), the minimum cluster is one that gives the correct second neighbors to the nucleus of interest. Both the delta(i)(Si) and delta(i)(O) have reached convergence with respect to cluster size at the OH-terminated two tetrahedral (2T) shell (three atomic shells around Si and four atomic shells around O) model. At convergence, the calculated delta(i)(Si) values agree well (within +/- 1 ppm) with experimental data. The calculated O-17 electric field gradient (EFG)-related parameters also agree with experimental data within experimental uncertainties. The calculation also reproduces small differences in delta(i)(O) for O sites with similar tetrahedral connectivities, but shows deviations up to about 10 ppm in relative difference for O sites with different tetrahedral connectivities. The poor performance for the latter is mainly due to the approximations of the HF method. Our study thus suggests that the ab initio calculation method is a reliable mean for predicting Si-29 and O-17 NMR parameters for silicates. Such an approach should find application not only to well-ordered crystalline phases, but also to disordered materials, by combining with other techniques, such as the molecular dynamics simulation method. (C) 2000 Elsevier Science B.V. All rights reserved.

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Ab initio molecular orbital calculations (Hartree-Fock, HF and density functional theories, DFTs) have been carried out for SiO2 polymorphs coesite, low cristobalite, and α-quartz, in order to investigate the reliability of this method for predicting 29Si and 17O nuclear magnetic resonance (NMR) properties of silicates. Oxygen- and silicon-centered clusters consisting of one (1T) to three tetrahedral (3T) shells (one to four atomic shells), taken from real crystal structure, have been investigated. It is found that for reasonable predication of both the 29Si and 17O chemical shifts (δi Si and δi O), the minimum cluster is one that gives the correct second neighbors to the nucleus of interest. Both the δi Si and δi O have reached convergence with respect to cluster size at the OH-terminated two tetrahedral (2T) shell (three atomic shells around Si and four atomic shells around O) model. At convergence, the calculated δi Si values agree well (within ±1 ppm) with experimental data. The calculated 17O electric field gradient (EFG)-related parameters also agree with experimental data within experimental uncertainties. The calculation also reproduces small differences in δi O for O sites with similar tetrahedral connectivities, but shows deviations up to about 10 ppm in relative difference for O sites with different tetrahedral connectivities. The poor performance for the latter is mainly due to the approximations of the HF method. Our study thus suggests that the ab initio calculation method is a reliable mean for predicting 29Si and 17O NMR parameters for silicates. Such an approach should find application not only to well-ordered crystalline phases, but also to disordered materials, by combining with other techniques, such as the molecular dynamics simulation method. © 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0926-2040(00)00075-8

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Ab initio molecular orbital calculations have been carried our for silicate, aluminosilicate, and aluminate clusters in order to study the NMR characteristics of oxygen sites that are possibly present in the structure of aluminosilicate glasses and melts. Of particular interest are the different types of bridging oxygens (oxygens bonded to two tetrahedrally coordinated Si and/or Al) and tricluster oxygens (oxygens linked to three tetrahedrally coordinated Si and/or Al). The calculated values for the O-17 quadrupolar coupling constants (QCC) of Si-O-Si, Si-O-Al, and Al-O-Al bridging oxygens agree reasonably well with experimental NMR data for similar oxygen sites in crystalline silicates, aluminosilicates, or aluminates, and do not show significant dependence on the basis set and theory (Hartree-Fock vs density functional theory) of calculations. The calculated values for the O-17 isotropic chemical shift (delta(i)(O)), on the other hand, show large dependence on the basis set and method of calculations, although the relative differences in delta(i)(O) for the clusters of interest do not vary as much. Our calculations indicate that bridging oxygens give progressively larger O-17 QCC and larger, but overlapping, delta(i)(O) from Al-O-Al to Si-O-Al to Si-O-Si. Coordination of Si-O-Al or Al-O-Al bridging oxygens to Ca2+, a network-modifying cation, tends to decrease their O-17 QCC and increase their delta(i)(O), consistent with experimental NMR data. Tricluster oxygens give progressively larger O-17 QCC and larger delta(i)(O) values from O(Al-3), to O(SiAl2), to O(Si2Al), to O(Si-3). The O(Al-3) and O(SiAl2) tricluster oxygens have O-17 NMR characteristics overlapping with those of Si-O-Al bridging oxygens, and the O(Si2Al) tricluster oxygen overlapping with those of Si-O-Si bridging oxygens. Our calculations suggest that it may be difficult to distinguish O(Al-3), O(SiAl2), or O(Si2Al) tricluster oxygens from bridging oxygens in aluminosilicate glasses on the basis of O-17 NMR data alone, although it should be possible to distinguish O(Als) tricluster oxygen from Al-O-Al bridging oxygen in aluminates using O-17 NMR. The O(Si3) tricluster have larger O-17 QCC and larger delta(i)(O) values than all types of bridging oxygens and thus may be unambiguously identified on the basis of O-17 NMR data, if present in the structure of aluminosilicates or silicates.

DOI： 10.1021/jp992108a

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Language：English   Publisher：SPRINGER VERLAG  

In order to gain insight into the correlations between Si-29, O-17 and H-1 NMR properties (chemical shift and quadrupolar coupling parameters) and local structures in silicates, ab initio self-consistent field Hartree-Fock molecular orbital calculations have been carried out on silicate clusters of various polymerizations and intertetrahedral (Si-O-Si) angles. These include Si(OH)(4) monomers (isolated as well as interacting), Si2O(OH)(6) dimers (Cz symmetry) with the Si-O-Si angle fixed at 5 degrees intervals from 120 degrees to 180 degrees, Si3O2(OH)(8) linear trimers (C-2 symmetry) with varying Si-O-Si angles, Si3O3(OH)(6) three-membered rings (D-3 and C-1 symmetries), Si4O4(OH)(8) four-membered ring (C-4 symmetry) and Si8O12(OH)(8) octamer (D-4 symmetry). The calculated Si-29, O-17 and H-1 isotropic chemical shifts (delta(i)(Si), delta(i)(O) and delta(i)(H)) for these clusters are all close to experimental NMR data for similar local structures in crystalline silicates. The calculated O-17 quadrupolar coupling constants (QCC) of the bridging oxygens (Si-O-Si) are also in good agreement with experimental data. The calculated O-17 QCC of silanols (Si-O-H) are much larger than those of the bridging oxygens, but unfortunately there are no experimental data for similar groups in well-characterized crystalline phases for comparison. There is a good correlation between delta(i)(Si) and the mean Si-O-Si angle for both Q(1) and Q(2), where Q(n) denotes Si with n other tetrahedral Si next-nearest neighbors. Both the delta(i)(O) and the O-17 electric field gradient asymmetry parameter, eta of the bridging oxygens have been found to depend strongly on the O site symmetry, in addition to the Si-O-Si angle. On the other hand, the O-17 QCC seems to be influenced little by structural parameters other than the Si-O-Si angle, and is thus expected to be the most reliable O-17 NMR parameter that can be used to decipher Si-O-Si angle distribution information. Both the O-17 QCC and the H-2 QCC of silanols decrease with decreasing length of hydrogen bond to a second O atom (Si-O-H ... O), and the delta(i)(H) increase with the same parameter.

DOI： 10.1007/s002690050157

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The crystal structure of alpha-CaSi2O5 synthesized at conditions of 1500 degrees C and 10 GPa, has been solved and refined in centrosymmetric space group P (1) over bar, using single crystal X-ray diffraction data. The composition (Z=4) and unit cell are Ca1.02Si1.99O5 by EPMA analysis and a=7.243(2) Angstrom, b=7.546(4) Angstrom, c=6.501(4) Angstrom, alpha=81.43(5)degrees, beta=84.82(4)degrees, gamma=69.60(3)degrees, V=329.5(3) Angstrom(3), yielding the density value, 3.55 g/cm(3). The structure is closely related to that of titanite, CaTiSiO5 and features the square-pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0.33 Angstrom. The substantial deviation of valence sum for Ca indicates the existence of local strain and the instability of alpha-CaSi2O5 at room pressure.

DOI： 10.1007/s002690050132

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Preliminary studies have been made of phase relations in the K2Si49, Na2Si2O5, Na2Si3O7 and Na2Si4O9 systems up to 14 GPa. Several high-pressure sodium silicate phases have been observed for the first time and were characterized by means of powder X-ray diffraction and 29Si MAS NMR techniques. In the K2Si4O9 system, the wadeite (K2ZrSi3O9)-type phase was found to be stable and melt congruently at least up to 12 GPa. In the Na2Si2O5 system, phase C. a Na2Si2O5 polymorph previously reported at 10-40 MPa, was observed at 2.5 GPa. However, at 5-6 GPa, a previously unknown phase (ε-Na2Si2O5) appeared. This phase was replaced by yet another new phase (ζ-Na2Si2O5) at 8-10 GPa. In the Na2Si3O7 system, a new high-pressure Na2Si3O7 phase was detected at about 10 GPa. In the Na2Si4O9 system, a new Na2Si4O9 phase appeared at 6-8 GPa and it decomposed to stishovite (SiO2) plus the high-pressure Na2Si3O7 phase at 9j 10 GPa. The 29Si MAS NMR spectra revealed that the εNa2Si2O5 phase contains only tetrahedral Si sites, whereas the ζ-Na2Si2O5, Na2Si3O7 and Na2Si4O9 phases contain both tetrahedral and octahedral Si sites. Recently, Fleet and Henderson [Fleet, M., Henderson, G.S., 1995a. Epsilon sodium disilicate: A high-pressure layer structure [Na2Si2O5]. J. Solid State Chem., 119: 400-414
Fleet, M., Henderson, G.S., 1995b. Sodium trisilicate: A new high-pressure silicate structure (Na2Si[Si2O7]). Phys. Chem. Min., 22: 383-386.] and Fleet [Fleet, M., 1996. Sodium tetrasilicate: A complex high-pressure framework silicate (Na6Si3[Si9O27]). Am. Mineral., 81: 1105-1110.] have determined the structures of the ε-Na2Si2O5, Na2Si3O7 and Na2Si4O9 phases. Subsolidus phase transformations with pressure for these alkali silicate systems can be described in terms of reduction of Si-O-Si angles at lower pressures and formation of octahedral Si through conversion of nonbridging oxygens to bridging oxygens at higher pressure. Similar structural changes might be expected for alkali silicate glasses (and melts) within this pressure range.

DOI： 10.1016/S0031-9201(97)00120-9

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Preliminary studies have been made of phase relations in the K2Si49, Na2Si2O5, Na2Si3O7 and Na2Si4O9 systems up to 14 GPa. Several high-pressure sodium silicate phases have been observed for the first time and were characterized by means of powder X-ray diffraction and 29Si MAS NMR techniques. In the K2Si4O9 system, the wadeite (K2ZrSi3O9)-type phase was found to be stable and melt congruently at least up to 12 GPa. In the Na2Si2O5 system, phase C. a Na2Si2O5 polymorph previously reported at 10-40 MPa, was observed at 2.5 GPa. However, at 5-6 GPa, a previously unknown phase (ε-Na2Si2O5) appeared. This phase was replaced by yet another new phase (ζ-Na2Si2O5) at 8-10 GPa. In the Na2Si3O7 system, a new high-pressure Na2Si3O7 phase was detected at about 10 GPa. In the Na2Si4O9 system, a new Na2Si4O9 phase appeared at 6-8 GPa and it decomposed to stishovite (SiO2) plus the high-pressure Na2Si3O7 phase at 9j 10 GPa. The 29Si MAS NMR spectra revealed that the εNa2Si2O5 phase contains only tetrahedral Si sites, whereas the ζ-Na2Si2O5, Na2Si3O7 and Na2Si4O9 phases contain both tetrahedral and octahedral Si sites. Recently, Fleet and Henderson [Fleet, M., Henderson, G.S., 1995a. Epsilon sodium disilicate: A high-pressure layer structure [Na2Si2O5]. J. Solid State Chem., 119: 400-414
Fleet, M., Henderson, G.S., 1995b. Sodium trisilicate: A new high-pressure silicate structure (Na2Si[Si2O7]). Phys. Chem. Min., 22: 383-386.] and Fleet [Fleet, M., 1996. Sodium tetrasilicate: A complex high-pressure framework silicate (Na6Si3[Si9O27]). Am. Mineral., 81: 1105-1110.] have determined the structures of the ε-Na2Si2O5, Na2Si3O7 and Na2Si4O9 phases. Subsolidus phase transformations with pressure for these alkali silicate systems can be described in terms of reduction of Si-O-Si angles at lower pressures and formation of octahedral Si through conversion of nonbridging oxygens to bridging oxygens at higher pressure. Similar structural changes might be expected for alkali silicate glasses (and melts) within this pressure range.

DOI： 10.1016/S0031-9201(97)00120-9

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Language：English   Publisher：The Japan Society of High Pressure Science and Technology  

Crystal structure of a-CaSi₂O₅ synthesized at conditions of 1500°C and 10 GPa, has been solved and refined in centrosymmetric space group P1. The structure is closely related to that of titanite, CaTiSiO₅ and consists the square pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0. 33 Å.

DOI： 10.4131/jshpreview.7.131

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Other Link： https://jlc.jst.go.jp/DN/JALC/00053842206?from=CiNii

Language：English   Publisher：SPRINGER VERLAG  

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Language：English   Publishing type：Research paper, summary (international conference)   Publisher：INT UNION CRYSTALLOGRAPHY  

DOI： 10.1107/S0108767396086850

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Language：English   Publisher：Japan Association of Mineralogical Sciences  

In order to identify five-coordinated Si(Si^v) species in crystalline and amorphous silicates by NMR spectroscopy, ²⁹Si NMR chemical shifts for the clusters of Si(CH₃)₄ (TMS), Si(OH)₄, Si(OH)₅⁻ and Si(OH)₆⁻² were calculated using ab initio molecular orbital theory. The calculated shifts relative to TMS cluster were −71.7 and −179.0 ppm for Si(OH)₄ and Si(OH)₆⁻², respectively, and are in good agreement with observed shifts. The calculated shift for Si(OH)₅⁻ was −127.4 ppm, falls just between the shifts of Si(OH)₄ and Si(OH)₆⁻². The chemical shift for Si^v species in polymerized silicates is expected to appear at about −150 ppm, if the effect of polymerization is considered. Present calculation thus supports the assignment of −150 ppm peak as Si^v species, which has been found in the alkali-silicate glasses quenched from high pressures.

DOI： 10.2465/minerj.18.1

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DOI： 10.1016/S0921-5107(96)01621-2

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DOI： 10.2109/jcersj.103.529

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Language：English   Publisher：MINERALOGICAL SOC AMER  

In order to understand the correlations between O-17 NMR parameters and the local structure around 0 in high-pressure silicates, we have obtained O-17 MAS and static NMR spectra on several crystalline alkali silicate and silica phases, as well as on ambient pressure glasses. Our results show that 110 NMR is capable of distinguishing nonbridging O atoms (NBO), bridging atoms (BO: [4]Si-O-[4]Si), O atoms in the [4]Si-O-[6]Si linkage, and O atoms coordinated to three [6]Si ([3]O). NBO are characterized by small quadrupolar coupling constants (e2qQ/h, approximately 2.3 MHz), but their isotropic chemical shifts (delta(i)) are very sensitive to the type of network-modifying cations. The [3]O (in stishovite), on the other hand, has by far the largest e2qQ/h value (6.5 MHz) and a larger delta(i) (109 ppm) than any BO in silicates. BO and [4]Si-O-[6]Si both have intermediate e2qQ/h values (approximately 4.5-5.7 MHz), and the delta(i) value of the latter (97 ppm in wadeite-K2Si4O9) is larger than those of the former (approximately 50-65 ppm).
We have also applied O-17 NMR to study the structure of alkali silicate glasses quenched from melts at high pressure. The O-17 NMR spectra of a K2Si4O9 glass quenched from 6 GPa and of Na2Si4O9 glasses quenched from 6 to 10 GPa are consistent with the presence of O sites in the [4]Si-O-[6]Si (and possibly also [4]Si-O-[5]Si) linkage, supporting our previous model developed from Si-29 NMR. In addition, O-17 static NMR also reveals the development of another new type of O site, possibly one that is connected with two or three [6]Si or [5]Si atoms, in the Na2Si4O9 glass quenched from 10 GPa. Thus, [6]Si and [5]Si may be largely isolated from one another below 10 GPa, whereas at higher pressure, clustering of these Si species may become significant in Na2Si4O9 melts.

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Language：English   Publisher：Springer-Verlag  

Stishovite, a high-pressure SiO2 polymorph in which each Si is coordinated by six O atoms, transforms to an amorphous phase when undergoing heat treatment below the glass transition temperature at ambient pressure. We have applied 29Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) to study this amorphization process. We found that the amorphous phase generated after heating stishovite for up to 3 days at around 600 ° C consisted exclusively of four-coordinate Si, similar to glasses quenched from melts at ambient pressure. Furthermore, our data suggest that there are subtle structural differences between the amorphous phase transformed from stishovite at 600 ° C and glasses quenched from melts at ambient pressure: the amorphous phase from stishovite had a smaller mean Si-O-Si angle initially, and it gradually relaxed toward the latter with increasing heating time. There was no detectable change in the stishovite structure even after about 80% of it had been converted to the amorphous phase. The mechanism of the amorphization of stishovite is discussed in light of these results. © 1993 Springer-Verlag.

DOI： 10.1007/BF00203188

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DOI： 10.2465/minerj.16.278

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Language：English   Publisher：SPRINGER VERLAG  

The unique cation-disordered crystal structures of two samples of phase E, a non-stoichiometric, hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions above 13 GPa and 1000-degrees-C, have been solved and refined in space group R3mBAR. The compositions and unit cells for the two materials, assuming six oxygens per cell, are Mg2.08Si1.16H3.20O6, a = 2.9701(1) angstrom, c = 13.882(1) angstrom, V = 106.05(4) angstrom3 for sample 1, and Mg2.17Si1.01H3.62O6, a = 2.9853(6) angstrom, c = 13.9482(7) angstrom, V = 107.65(4) angstrom3 for sample 2. The structure contains layers with many features of brucite-type units, with the layers stacked in a rhombohedral arrangement. The layers are cross linked by silicon in tetrahedral coordination and magnesium in octahedral coordination, as well as hydrogen bonds. Interlayer octahedra share edges with intralayer octahedra. Interlayer tetrahedra would share faces with intralayer octahedra. To avoid this situation, there are vacancies within the layers. There is, however, no long-range order in the occupation of these sites, as indicated by the lack of a superstructure. Selected-area electron diffraction patterns show walls of diffuse intensity similar in geometry and magnitude to those observed in short-range-ordered alloys and Hagg phases. Phase E thus appears to represent a new class of disordered silicates, which may be thermodynamically metastable.

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

The differential thermal analysis (DTA) technique has been applied in a multi-anvil high-pressure apparatus to study the dehydration reaction of brucite (Mg(OH)2) to periclase (MgO) plus H2O at 4 to 6 GPa. At 4 GPa and 1030-degrees-C, endothermic and exothermic peaks due to dehydration and rehydration were observed during heating and cooling cycles, respectively. These peaks were shifted to higher temperature with increasing pressure (1080-degrees-C at 5 GPa; 1120-degrees-C at 6 GPa), suggesting that the reaction has a positive dT/dP slope within this pressure range. The stability of brucite under deep mantle condition is discussed.

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Language：English   Publisher：ELSEVIER SCIENCE BV  

Phase relations in the Mg(OH)2-SiO2 system have been studied up to 18 GPa. A new hydrous phase (phase E: Mg2.3Si1.3H2.4O6) was encountered at 13-17 GPa and 1000-degrees-C. Phase F (Mg1.2Si1.8H2.4O6), another new hydrous phase, was observed at 15.5 GPa and 800-degrees-C. The stability field of phase A was also briefly studied. These new hydrous phases might be stabilized in the mantle transition zone, especially in subducting slabs at those depths.

DOI： 10.1016/0031-9201(91)90085-V

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

We have studied quenched high pressure phases in the CaSiO3 system by x-ray diffraction (XRD) and Si-29 MAS NMR. XRD study of the previously reported "epsilon-CaSiO3 phase" synthesized at 12 GPa and 1500-degrees-C reveals that it is actually a mixture of beta-Ca2SiO4 (larnite) and a previously unknown CaSi2O5 phase. This result is supported by the Si-29 NMR spectra. Furthermore, both the XRD and the NMR data suggest that the CaSi2O5 phase may have a titanite (CaTiSiO5) structure in which Ti is replaced by an octahedral Si. Samples quenched from 15 GPa and 1500-degrees-C consist mostly of an amorphous phase, but a small amount of CaSiO3-perovskite was identified by both XRD and NMR. The Si-29 NMR spectrum of the amorphous phase suggests that its local structure is similar to that of a glass quenched from melt at 1 bar.

DOI： 10.1029/91GL00463

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Language：English   Publisher：AMER ASSOC ADVANCEMENT SCIENCE  

Most of the earth's mantle is made up of high-pressure silicate minerals that contain octahedrally coordinated silicon (Si(VI), but many thermodynamically important details of cation site ordering remain unknown. Silicon-29 nuclear magnetic resonance (NMR) spectroscopy is potentially very useful for determining short-range structure. A systematic study of silicon-29 chemical shifts for Si(VI) has revealed empirical correlations between shift and structure that are useful in understanding several new calcium silicates. The observed ordering state of a number of high-pressure magnesium silicates is consistent with the results of previous x-ray diffraction studies.

DOI： 10.1126/science.251.4991.294

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Language：English   Publisher：MINERALOGICAL SOC AMER  

The Si-29 and Na-23 NMR, Raman, and infrared spectra of glasses quenched from liquids at pressures up to 12 GPa for three alkali silicate compositions (Na2Si2O5, Na2Si4O9, K2Si4O9) and silica (SiO2) reveal systematic changes in the melt structure with pressure. The most novel change is the occurrence of [5]Si and [6]Si species at high pressures identified by peaks in the Si-29 MAS NMR spectra of the alkali silicate glasses. The abundances of both [5]Si and [6]Si increase with pressure. At a given pressure there are more such species in the Na2Si4O9 composition than in Na2Si2O5 composition. These species are not observed in a SiO2 glass quenched at 6 GPa. The occurrence of these highly coordinated Si species is consistent with changes in the Raman and infrared spectra of the alkali silicate glasses, although the peak assignments in these spectra are not unique. The change in Si coordination with pressure and composition in these compositions can be accounted for by a model in which [5]Si and [6]Si form at the expense of nonbridging O atoms. This model may be generalized to all partially depolymerized melts. Changes in the tetrahedral structure with increasing pressure include Q speciation disproportionation, reduction of mean Si-O-Si angle, and development of new [4]Si species that share O with [5]Si or [6]Si. The observed structural changes have implications for the physical and thermodynamic properties of silicate melts. [5]Si has been previously envisaged as a transient state for the viscous flow and diffusion processes in silicate melts. The occurrence of [5]Si in the high pressure glasses suggests that [5]Si represents a local energy minimum and its stabilization may partly account for the enhancement of these dynamic processes. Increased configurational entropy as a result of greater distribution of Si coordinations and Q species at high pressures could also contribute to the reduced melt viscosity.

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DOI： 10.1029/GL017i011p01989

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Language：English   Publisher：AMER ASSOC ADVANCEMENT SCIENCE  

DOI： 10.1126/science.245.4921.962

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Language：English   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)   Publisher：MINERALOGICAL SOC AMER  

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DOI： 10.1016/0031-9201(87)90139-7

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Event date： 2020.12.2 - 2020.12.4

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