Cation Site Occupancy in Natural Ferroan Double Carbonates via Mössbauer and Fe K-Edge X-ray Absorption Spectroscopy

Double carbonates are minerals with a calcite-type structure with alternating cation layers composed of Ca and Mg/Fe coordinated by carbonate groups. While the perfectly ordered AB-stacked crystal is the thermodynamically most stable configuration, natural mineral formation pathways can leave signatures through kinetically trapped disorder, such as AB antisite cation substitutions. This study probes the degree of cation ordering in naturally occurring double-carbonate samples. In particular, the dependence of structural order on the distribution of Fe A and B crystallographic sites (the A site is the Ca layer, and the B site is the Mg/Fe layer) is examined. Mössbauer, X-ray diffraction, X-ray absorption spectroscopies (both the X-ray absorption near edge structure, XANES, and extended X-ray absorption fine structure, EXAFS), and energy-dispersive X-ray spectroscopy were used to collect a comprehensive experimental data set, which we interpret using density functional theory to elucidate the structural effects of cation disorder. Our results show that the A (nominally Ca) site can host a relatively high Fe fraction. We discuss the implications in terms of mineral formation.