Colorless crystals of the rare mineral monohydrocalcite on matrix. From: Iba, Richelsdorfer Gebirge, Hesse, Germany. — Photo: David Hospital · CC BY-SA 4.0
Monohydrocalcite
Calcium carbonate monohydrate — a rare hydrated polymorph precipitated in cold lakes and cave environments.
Intro adapted from Wikipedia (CC BY-SA).
Monohydrocalcite is an IMA-recognized mineral species in the carbonate class, with the chemical formula Ca(CO₃)·H₂O.
It crystallizes in the hexagonal crystal system (space group P3_121).
Key properties: specific gravity 2.38; optically uniaxial.
IMA symbol Mhcal, grandfathered status — known before the IMA’s founding, named in 1964.
Found in lake-bed sediments and as tuffaceous deposits on lake margins, formed by precipitation at pH > 8.0 and high Mg: Ca or by biological activity; in caves, in speleothems, crusts, and “moonmilk, ” probably formed from an aerosol.
It is found associated with Calcite, aragonite, hydromagnesite, nesquehonite.
The name refers to having the composition of calcite and the Greek for one, as with an additional H2O.