High-pressure crystal structure of kosmochlor, NaCrSi₂O₆, and systematics of anisotropric compression in pyroxenes

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 V_o = 418.84(3) ų, K_c = 134(1) GPa ¹, and K¹_o = 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 K_o = 90(16) GPa¹, while the SiO₄ tetrahedron has K_o = 313(55) GPa ¹, both with K¹_o= 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 P2_l/c), clinoenstatite, orthoenstatite. and anisotropy of olivine and pyroxene.