Testing the cation-hydration effect on the crystallization of Ca–Mg–CO 3 systems

Significance Magnesium-bearing carbonate minerals play critical roles in the health and function of the Earth system because they constitute a significant fraction of lithosphere carbon reservoir and build skeletal structures for the majority of marine invertebrate organisms. Despite wide occurrence, high-Mg and sole-Mg phases such as dolomite ([Ca,Mg]CO 3 ) and magnesite (MgCO 3 ) prove virtually impossible to be crystallized under ambient conditions. It has long been believed that Mg 2+ hydration is the cause for such a geological mystery. Here, we probe this hypothesis by investigating Ca–Mg–CO 3 precipitation in the absence of water and find direct proof suggesting the existence of a more intrinsic crystallization barrier. These findings provide a perspective augmenting our understanding in carbonate mineralogy, biomineralization, and mineral-carbonation processes.