THE STRUCTURE OF HIGH-PRESSURE C2 C FERROSILITE AND CRYSTAL-CHEMISTRYOF HIGH-PRESSURE C2/C PYROXENES/

Single-crystal X-ray diffraction experiments in a diamond-anvil pressu re cell have demonstrated that clinoferrosilite (FeSiO3) with space gr oup P21/c transforms to a pyroxene with space group C2/c at high press ure. The transformation has been reversed at room temperature and pres sures between 1.48 and 1.75 GPa; it is first-order in character and is accompanied by a 3% decrease in the volume of the unit cell. The stru cture of this C2/c polymorph has been refined from diffraction data co llected at 1.87 GPa and contains a single, symmetrically distinct sili cate tetrahedral chain, with a mean Si-O bond length of 1.63 angstrom and a chain extension angle (O3-O3-O3) of 138.4 +/- 0.9-degrees. The M 1 and M2 cation sites are octahedrally coordinated, with mean bond len gths of 2.14 and 2.18 angstrom, respectively. This clinopyroxene is th e Fe analogue of the C2/c phase found in MgSiO3 at pressures above 7 G Pa. The high-pressure C2/c structure of ferrosilite differs considerab ly from the C2/c phase found at elevated temperatures, with a beta ang le some 7-degrees smaller and a silicate chain significantly more kink ed than in the high-temperature form. It is also substantially differe nt from C2/c structures that contain Ca but is very similar (in terms of the sizes and distortions of the cation sites) to orthorhombic ferr osilite. The same structural relationships hold for the MgSiO3 phases with C2/c symmetry. The chemical behavior of these C2/c forms of both MgSiO3 and FeSiO3 is therefore expected to resemble that of the corres ponding orthopyroxenes.