The unit-cell parameters of natural grunerite were measured in a diamond anvil cell at nine different pressures up to 5.1 GPa. The bulk modulus K0 and its pressure derivative K0′ were calculated by fitting the pressure-volume data to a Birch-Murnaghan equation of state: K0 = 50(1) GPa and K0′ = 13(1). The linear compressibilities βa, βb, and βc are 4.97(6),3.50(4), and 3.62(5) × 10−3 GPa−1, respectively, yielding the ratio βa:βb:βc = 1.418(1): 1.000(2):1.032(5). Grunerite exhibits strong anisotropic compression in the low-pressure region (P < 1 GPa). With increasing pressure, the compression becomes more isotropic. This behavior and the larger volume compressibility compared to that of Ca-rich clinopyroxenes in the pressure range studied are proposed to be due to the strong compression of the empty A cavities in the crystal structure at low pressures.

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