Crystallographic Control in the Replacement of Calcite by Calcium Sulfates

Abstract

The transformation of calcite into calcium sulfate phases in the presence of an aqueous phase is relevant to many technological processes, including remediation of acid mine drainage, exploitation of subsurface reservoirs, geological CO2 sequestration, and the conservation of building stone. At acidic pH, the interaction of sulfate-bearing aqueous solutions with calcite cleavage fragments results in the replacement of gypsum, bassanite, and/or anhydrite depending on the reaction time and temperature. Here we show that a fraction of the calcium sulfate crystals (gypsum, bassanite, or anhydrite) grows in all cases oriented onto the calcite surfaces, forming a product layer that reproduces the initial morphology of the calcite crystals. Calcium sulfates form on calcite surfaces as thick three-dimensional crystals, suggesting a Volmer–Weber mechanism of epitactic growth. The epitaxial relationships found by two-dimensional X-ray diffraction are (010)Gp//(104)Cal; (010)Bs//(104)Cal; and (200)Anh//(104)Cal and are discussed on the basis of the structure of calcite and the different sulfate phases. Apparently, sulfate phases form in all cases with their crystallographic c-axis parallel to the <4̅41> periodic bond chain (PBC) of calcite. Ca–Ca repeating periods in CaSO4 phases along the <001> direction range from 6.336 to 6.491 Å, thus closely matching that of <4̅41> PBC of calcite (6.425 Å).