Myriad Mapping of nanoscale minerals reveals calcium carbonate hemihydrate in forming nacre and coral biominerals.

Calcium carbonate (CaCO) is abundant on Earth, is a major component of marine biominerals and thus of sedimentary and metamorphic rocks and it plays a major role in the global carbon cycle by storing atmospheric CO into solid biominerals. Six crystalline polymorphs of CaCO are known-3 anhydrous: calcite, aragonite, vaterite, and 3 hydrated: ikaite (CaCO·6HO), monohydrocalcite (CaCO·1HO, MHC), and calcium carbonate hemihydrate (CaCO·½HO, CCHH). CCHH was recently discovered and characterized, but exclusively as a synthetic material, not as a naturally occurring mineral.

Faster Crystallization during Coral Skeleton Formation Correlates with Resilience to Ocean Acidification.

The mature skeletons of hard corals, termed stony or scleractinian corals, are made of aragonite (CaCO). During their formation, particles attaching to the skeleton's growing surface are calcium carbonate, transiently amorphous. Here we show that amorphous particles are observed frequently and reproducibly just outside the skeleton, where a calicoblastic cell layer envelops and deposits the forming skeleton.