The bulk elastic properties of natural andradite and grossular have been investigated by synchrotron radiation powder diffraction at high pressure conditions, up to 37 GPa, on the ID9 beamline (ESRF). The P-V data have been interpolated by the Birch-Murnaghan EOS and the Vinet EOS, and the bulk modulus and its first derivative versus P determined. The results obtained by the different EOS models are compared with one another, and with earlier issues. For andradite and grossular the Birch-Murnaghan EOS yields K0 = 158.5(±1.0) GPa and K0’ = 5.77(±0.10) [K0“=−0.056 GPa−1 implied value], and K0=169.3(±1.2) and K0’ = 5.92(±0.14) [K0”=-0.056 GPa−1 implied value], respectively. The elastic properties here reported are compared with previous issues. The cell edge compressibilities, which have been determined by fitting the experimental lattice parameters with polynomials in P, are βa = 0.00208(2) GPa−1, for andradite, and βa = 0.00194(3) GPa−1, for grossular.

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