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

Marcus J. Origlieri, Robert T. Downs, Richard M. Thompson, Carolyn J.S. Pommier, M. Bonner Denton, George E. Harlow

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40 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)1025-1032
Number of pages8
JournalAmerican Mineralogist
Issue number7
StatePublished - Jul 2003

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology


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