Crystal chemistry of a high-pressure C2/c clinopyroxene with six-coordinated silicon

Abstract

A (Ca 0.36 Na 0.56 Mg 0.08 )(Mg 0.73 Si 0.27 )Si 2 O 6 clinopyroxene containing both four- and six-coordinated silicon was synthesized at 15 GPa and 1600 °C and its structure determined with single-crystal X-ray diffraction. Unlike the Na(Mg 0.5 Si 0.5 )Si 2 O 6 clinopyroxene that exhibits an ordered P2/n structure with octahedrally coordinated Mg 2+ and Si 4+ occupying two crystallographically distinct M1 sites (Angel et al. 1988), our sample possesses a C2/c symmetry and shows no detectable ordering between Mg 2+ and Si 4+ in the M1 site. The measured unit-cell parameters are a = 9.5792(13), b = 8.7588(12), c = 5.2610(6) Å, β = 107.199(3)°, and V = 421.7(2) Å 3 . The crystal structure of the clinopyroxene reported in this study is comparable to that for omphacites formed under high temperatures and pressures.