Molybdenum and sulfur incorporation as oxyanion substitutional impurities in calcium carbonate minerals: a computational investigation

Marked increases in sulfur and molybdenum concentration in stalagmites have been proposed as possible evidence of volcanic activity in the past. Thus, speleothems have great potential to deliver long and continuous records of volcanic activity. However, little is known about the chemical nature of these impurities in the calcium carbonate (CaCO3) phases forming stalagmites, which hinders the rationalization of the incorporation mechanisms. While sulfur is known to incorporate as a sulfate anion in CaCO3 polymorphs, the nature and stability of molybdenum incorporation in these minerals has not been investigated yet. Here, we present a computer simulation study, based on density functional theory, comparing the thermodynamics of incorporation of sulfur and molybdenum as tetrahedral oxyanions [XO4]2- (X=S, Mo) in anion sites of CaCO3 polymorphs (calcite, aragonite, vaterite, monohydrocalcite and ikaite). Among the different polymorphs, vaterite incorporates [XO4]2- ions most favourably, which reflects the relatively low density of this carbonate phase. We show that molybdate anions are very unstable (more so than sulfate anions) in the bulk of all three anhydrous carbonate phases, with respect to the formation of naturally occurring competing phases. Most of the Mo impurities found in typical calcite/aragonite stalagmites is therefore likely to concentrate at surface/interface regions such as grain boundaries. Using the calcite (10.4) surface as a model, we show that the energies of substitution are indeed much lower at the surface than at the bulk. Our results suggest that factors affecting the crystallinity of CaCO3 in stalagmites, and therefore the specific surface area, will have a significant effect on the concentration of incorporated molybdenum, which should be a key consideration when interpreting data from Mo-based speleothem archives.