Sulfate and Molybdate Incorporation at the Calcite–Water Interface: Insights from Ab Initio Molecular Dynamics

Sulfur and molybdenum trace impurities in speleothems (stalagmites stalactites) can provide long continuous records of volcanic activity, which are important for past climatic environmental reconstructions. However, the chemistry governing incorporation element-bearing species into calcium carbonate phases forming is not well understood. Our previous work has shown that substitution tetrahedral oxyanions [XO4]2– (X = S Mo) replacing [CO3]2– CaCO3 bulk (except perhaps vaterite) thermodynamically unfavorable with respect to formation competing phases, due larger size different shape anions comparison flat anions, implied most would happen at surface rather than mineral. Here, we present an ab initio molecular dynamics study, exploring these mineral–water interface. We show oxyanion aqueous calcite (10.4) clearly favored over incorporation, lower structural strain on solid. Incorporation step sites even more favorable both oxyanions, thanks additional interface space afforded by line defect accommodate anion. Differences between sulfate molybdate substitutions be mostly explained anions. The difficult incorporate smaller oxyanion. when substituted surface, elastic cost avoided because protrudes out gains stability via interaction water interface, balance results sulfate. detailed molecular-level insights provided our calculations will useful understand chemical basis S- Mo-based speleothem records.