Compressional behavior of the aragonite-structure carbonates to 6 GPa

The behaviors of aragonite (CaCO _3 ), strontianite (SrCO _3 ), cerussite (PbCO _3 ), and witherite (BaCO _3 ) at increasing pressure have been studied up to 6 GPa using density functional theory with plane waves. A parallelism of the orthorhombic carbonates with the closed-packed AsNi structure is considered in our analysis, being the CO _3^2- groups not centered in the interstice of the octahedron. The decomposition of the unit-cell volume into atomic contributions using the Quantum Theory of Atoms in Molecules has allowed the analysis of the bulk modulus in atomic contributions. The bulk, axes, interatomic distances, and atomic compressibilities are calculated. The largest compression is on the c crystallographic axis, and the c linear modulus has a linear function with the mineral bulk modulus ( K_0 ). Many of the interatomic distances moduli of the alkaline earth (AE) carbonates show linear functions with the bulk modulus; however, the whole series (including cerussite) only gives linear functions when K_0 is related either with the CC distances modulus or the modulus of the distances of the C to the faces of the octahedron perpendicular to c . These last distances are the projections of the Metal–Oxygen (MO) distances to the center of the octahedron. K_0AE carbonates also show linear functions with the atomic moduli of their cations. However, the whole series show a linear relation with the atomic modulus of C atoms. Therefore, the whole series highlight the importance of the C atoms and their interactions in the mechanism of compression of the orthorhombic carbonate series.