STRUCTURE AND CRYSTAL CHEMISTRY OF CaSi₂O₅ SYNTHESIZED AT 1500 deg.C AND 10 GPa: A NEW HIGH PRESSURE CALCIUM SILICATE WITH FIVE-COORDINATED SILICON. Y. Kudoh^l)*, M. Kanzaki²⁾, ¹⁾ Institute of Mineralogy, Petrology, and Economic Geology Faculty of Science, Tohoku University, Sendai 980, Japan, ²⁾Department of Inorganic Materials, Tokyo Institute of Technology, Megro, Tokyo 152, Japan

Crystal structure of CaSi₂O₅ synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions of 1500 deg.C and 10 GPa, has been solved and refined in non-centro-symmetric space group P1, using the X-ray diffraction data measured on a 35x71x71 micron single crystal. The composition (Z=4) and unit cell are Ca_l.02Si_1.99O₅ by E.P.M.A. analysis and a=7.243(2) Å, b=7.546(4) Å, c=6.501(4) Å, [[alpha]]=98.57(5)deg., [[beta]] =95.18(4)deg., [[gamma]]=69.60(3)deg., V=329.5(3) ų, yielding the density value, 3.55 g/cm³. The crystal structure was determined by Patterson method. Final agreement factors for 1973 Fo's observed up to sin [[theta]] / [[lambda]] =0.75 were R=8.0% and Rw=9.4% with isotropic temperature factors. The crystal structure is closely related to that of titanite,CaTiSiO₅. The formula of unit cell contents is:

^viiCa₃^viiiCa^vSi₂^viS i₂^ivSi₄O₂₀.

The structure consists the square pyramid fivefold coordination of silicon by oxygen. This is the first discovery about five-coordinated Si in an inorganic crystalline state.