Monoclinc aragonite, the precursor of metastable aragonite formation

Aragonite is a wide-spread crystalline form of CaCO3 on the Earth surface. Although calcite is the thermodynamically stable CaCO3 form at ambient conditions, aragonite precipitates in the ocean and in some continental settings. It is abundant in the shells of various organisms such as molluscs and corals as well as in stromatolites, and its abiotic formation is favored in waters of a Mg2+/Ca2+ ratio > 1.5. Despite decades of research it is still unclear why metastable aragonite is so abundant. Here, we present evidence of a new CaCO3 polymorph, which sheds light on this long-standing question.