The crystal structure of synthetic kosmochlor, NaCrSi2O6, was studied using single crystal X-ray diffraction at high pressure. A four-pin diamond anvil cell, with 4:1 methanol:ethanol pressure medium, was used to achieve pressures to 9.28 GPa. Unit-cell data were collected at 20 pressures, and intensity data were collected at 13 of these pressures. Fitting the P-V data to a third-order Birch-Murnaghan equation yields Vo = 418.84(3) Å3, Kc = 134(1) GPa 1, and K1o = 2.0(3). Anisotropic compression was observed with unit strain axial ratios of 1:1.82:2.08. The CrO6 octahedron has a bulk modulus Ko = 90(16) GPa1, while the SiO4 tetrahedron has Ko = 313(55) GPa 1, both with K1o= 4. An o-type rotation of the O3-O3-03 linkage was observed with pressure, with <03-03-03 decreasing from 172.8(2)° to 166.1(7)°. Compression in kosmochlor is related to the stacking directions of distorted cubic closest packed O atom monolayers. Unit strain ellipsoids for diopside. hedenbergite, spodumene (C2/c and P21/c), LiScSi2O6 (C2/c and P2l/c), clinoenstatite, orthoenstatite. and anisotropy of olivine and pyroxene.
ASJC Scopus subject areas
- Geochemistry and Petrology