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The high-temperature clinoenstatite (HT-CEn) is one of the important MgSiO3 pyroxene polymorph. The single-crystal of C2/c HT-CEn endmember is firstly synthesized by rapid pressure-temperature quenching from 15-16 GPa and 900-1900 0C [1]. No report that it is produced as single crystal or large domain has been made on the MgSiO3 endmember. The HT-CEn-type modifications are observed in Ca-poor Mg-Fe clinoenstatite and pigeonite and are always found to be unquenchable in rapid cooling. The high pressure and high temperature experiments of MgSiO3 composition were carried out with a Kawai-type multi-anvil apparatus. The samples were quenched by rapidly releasing the oil pressure load and/or by blow out of anvil cell gasket. The space group of C2/c is strictly determined by Rigaku RAPID Weissenberg photographs and synchrotron radiation. Single-crystal X-ray diffraction experiments were performed at ambient conditions using a Rigaku AFC-5 four circle diffractometer. A total of 9383 reflections was measured and averaged in Laue symmetry 2/m to give 766 independent reflections used for the structure refinements. Final reliability factors converged smoothly to R = 0.029. The single-crystal diffraction analysis shows that the unusual bonding distances frozen in this metastable structure. The degree of kinking of the silicate tetrahedral chains is 1750 for HT-CEn. The chain angle for HP-CEn is substantially smaller (1350) and the angle for L-CEn turned to the opposite direction at -1600 (=2000). The degree of kinking increases by being curved in more than 1800 in the transition from HT-CEn to L-CEn. As for the reverse change from the expansion to the stretch, a potential barrier exists in the point of the continuity. It is suggested that the reason which can quench structure under ambient conditions is the present HT-CEn single crystal was formed by the isosymmetric phase transition from HP-CEn. HT-CEn type single-crystals cannot be frozen without pressure.
abstracts
Hydrogarnets, represented by hydrogrossular, are produced by the replacement of (ZO4)4- in garnets by (H4O4)4- (Z: tetrahedral cation) and the vacancies of tetrahedral cations are created by this replacement. Most of reported hydrogarnets crystallizes with cubic symmetry (space group Ia-3d). To our knowledge, Ca3Mn2[SiO4]2.07[H4O4]0.93 with space group I41/acd (tetragonal) [1] has been only reported as a low-symmetry hydrogarnet. In the cubic hydrogarnets, all O atoms are crystallographically equivalent, whereas in low-symmetric one, they can be located at non-equivalent sites. Therefore, the investigation of low-symmetry hydrogarnes is important to gain knowledge of the site preference of H atoms. Recently, we have successfully synthesized the single crystal of a new low-symmetry hydrogarnet CaGe0.924O3H0.304 (= Ca3(CaGe)[GeO4]2.696[H4O4]0.304) with tetragonal space group I41/a, at 3 GPa and 1273 K under the presence of H2O component. This tetragonal hydrogarnet is produced by the partial replacement of (GeO4)4- in high-pressure CaGeO3 garnet, Ca3(CaGe)[GeO4]3, by (H4O4)4-. In the present study, we report the single crystal X-ray diffraction study of this hydrogarnet at 98 and 298 K. In the structure refinement at 298 K, the occupancy parameters resulted in 0.393(2) for tetrahedral Z2(Ge) site, coordinated only by O6 atoms, and showed no significant deviation from 1.0 for the remaining cation sites. The bond valence sums of each atom except O6 atom agree with the valences of occupied atoms, whereas that of O6 atom are 1.50, deviating largely from oxygen valence. Thus, the substitution of OH groups for O atoms in the present sample occurs only at O6 site, which indicates that O6 is the most preferential site for the OH substitution. The position of H atom will be examined from the residual electron density distributions at a low temperature of 98 K, and the hydrogen bonding in the crystal structure will be discussed.
