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

The crystal structure of a synthetic potassium-rich clinopyroxene, (Ca_0.88K_0.12)(Mg_0.83Al_0.17) (Si_1.98Al_0.02)O₆, was studied using high-pressure single-crystal X-ray diffraction methods. A four-pin diamond 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) ų, 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 compressibilities β_b > β_c ∼ β_a. 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 significant 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 fit into the pyroxene structure.