Epitactic Overgrowths of Calcite (CaCO3) on Anhydrite (CaSO4) Cleavage Surfaces

We investigate the crystallographic relationships between calcite crystals overgrown on the three main anhydrite cleavage surfaces, (100), (010), and (001), as a result of the interaction between anhydrite surfaces with carbonate-rich aqueous solutions (0.5 mol/L Na2CO3). Scanning electron microscope (SEM) images and energy back scattered diffraction (EBSD) analyses reveal that this interaction always leads to oriented nucleation and growth of calcite crystals, characterized by a parallelism between the calcite {10 (1) over bar4} faces and the corresponding anhydrite cleavage surface. A high density of oriented calcite crystals was observed on the (001)(Anh) face, while the calcite crystal density was significantly lower on the (100)(Anh) and (010)(Anh) faces. On all investigated anhydrite cleavage surfaces, calcite {10 (1) over bar4} showed at least two excellent matching orientations, both defined by the parallelism between <(4) over bar 41 > 0 and one of the three crystallographic axes of anhydrite. As a result, two different populations of calcite crystals coexisted on the same substrate. In each population four groups of differently orientated crystals could be distinguished, related by symmetric operators inherent to the anhydrite substrates. The coalescence of these differently oriented crystals leads to the formation of twins. The calcite epitactic overgrowth contains an intrinsic intergranular porosity that makes the passivation of anhydrite crystals very unlikely. This characteristic together with the negative molar volume change involved in the anhydrite carbonation reaction makes anhydrite a material suitable to be used in the design of CO, capture strategies.