[S-MP47_1AM2] Physics and Chemistry of Minerals

[SMP47-08] Crystal structure analysis of a new high-pressure strontium silicate

Keisuke SEKINE¹, Takayuki ISHII¹, *Hiroshi KOJITANI¹, Masaki AKAOGI¹ (1.Dept. of Chemistry, Gakushuin University)

Keywords:strontium silicate, high-pressure, single-crystal structure analysis, SiO5 polyhedron

SrSiO₃ is an analog material to CaSiO₃ which is an important component of the Earth’s crust- and mantle-constituting minerals. High-pressure phase relation experiments in SrSiO₃ showed that δ’-SrSiO₃ is stable up to about 10 GPa and decomposes into BaGe₂O₅-III -type SrSi₂O₅ + larnite-type Sr₂SiO₄ between 14 and 20 GPa (Kojitani et al., 2005). Then, hexagonal perovskite-type SrSiO₃ becomes stable above about 20 GPa (Yusa et al., 2005). However, phases except for larnite-type Sr₂SiO₄ appearing in the pressure range between 10 and 14 GPa have been unclear. In this study, crystal structure and composition of one of the unknown phases were determined.A sample for single-crystal structure analysis was synthesized by heating a mixture of pseudowollastonite-type CaSiO₃ and SiO₂ cristobalite (mole ratio of 1:1) with a little amount of water at 12 GPa and 1200℃ for 90 min using a Kawai-type multi-anvil high-pressure apparatus. A single-crystal sample with 120x80x60 μm was used for the single-crystal X-ray diffraction measurement. 953 reflection data were analyzed using the SHELX-97 software. Composition analysis of the high-pressure phase was performed using SEM-EDS.The composition analysis showed that the new high-pressure phase had a composition of Sr₄Si₉O₂₂. The single-crystal structure analysis suggests the monoclinic crystal system with the space group of C2/m. Lattice parameters were determined to be a = 13.3765(4) Å, b = 5.2321(2) Å, c = 11.6193(6) Å, β = 113.976(4)deg. R factor was 1.25%. The framework of the obtained crystal structure consists of two layers by corner-sharing single chains of edge-shared SiO₆ octahedra or SiO₅ rhombic pyramid polyhedra and by corner-shared SiO₄ tetrahedra and SiO₆ octahedra. It should be mentioned that this structure includes the SiO₅ rhombic pyramids which are very rare in silicates. Strontium ions in the structure are arranged between the two layers and are coordinated by seven oxygens. The structure of δ’-SrSiO₃ consists of four-membered rings of SiO₄ tetrahedra and strontium ions coordinated by seven oxygens. On the other hand, BaGe₂O₅-III type SrSi₂O₅ has a framework by corner-shared SiO₆ octahedra and SiO₄ octahedra and coordination number of Si²⁺ is 12. The crystal structure determined in this study is consistent with the fact that its density would be between those of the lower-pressure and higher-pressure phases.