Compressibility of synthetic potassium-rich clinopyroxene: In-situ high-pressure single-crystal X-ray study

Abstract

The crystal structure of a synthetic potassium-rich clinopyroxene, (Ca0.88K0.12)(Mg0.83Al0.17)(Si1.98 Al0.02)O6, was studied using high-pressure single-crystal X-ray diffraction methods. A four-pin dia- mond anvil cell with 4:1 methanol:ethanol pressure medium was used to achieve pressures to 9.72 GPa. Unit-cell data were measured at 17 pressures, and intensity data were collected at 6 pressures. Fitting the P-V data to the third-order Birch-Murnaghan equation of state yields V0 = 435.49(3) A 3 , K0 = 129(1) GPa, K' = 2.7(3). Anisotropic compression was observed with unit strain axial ratios of 1:1.94:1.90. Unit-cell parameters decrease gradually as a function of pressure with axial compress- ibilities βb > βc ~ βa. They match those found for kosmochlor but are stiffer than those observed for synthetic diopside and hedenbergite. Compressibilities of the bond distances within the M2, M1, and T polyhedra show signiÞ cant anisotropy. The incorporation of K into the clinopyroxene structure has little effect on its compressibility, although the concomitant substitution of Al in M1 from the K-Jd component reduces its compressibility. The K atom is softer than the M2 polyhedron and thus shrinks enough at high pressure to Þ t into the pyroxene structure.