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Room-temperature volume measurements of the complete set of calcite-structure carbonates in the pressure range 0-8.1 GPa revealed systematic differences in compressibilities that depend on cation type, resulting in significant deviations from empirical relations of an inverse linear correlation between K0 and V0. The bulk modulus for MgCO3 lies approximately 18 GPa below the trend for the 3d transition metal carbonates, which show an expected inverse linear correlation of bulk modulus with ambient cell volume (and M-O bond length). The bulk modulus of CdCO3, whose M-O bond length and cell volume are only slightly smaller than those of CaCO3, lies up to 10 GPa above the trend of the 3d transition metal carbonates and about 30 GPa above that of calcite. These deviations in compressibility trends as a function of cell volume (or M-O bond length) are expressed as differences in a axis compressibility but not in c axis compressibility, which shows a nearly linear increase with M-O distance. Hence, systematic behavior is apparently limited to subsets of carbonates in which metal ions share the same valence electron character (i.e., s vs. 3d vs. 4d) and is primarily attributed to unexpected compressibility differences along the a axis.
Penn et al. (Tue,) studied this question.