Monohydrocalcite is a mineral that is a hydrous form of calcium carbonate, CaCO3·H2O. It was formerly also known by the name hydrocalcite, which is now discredited by the IMA. It is a trigonal which is white when pure. Monohydrocalcite is not a common rock-forming mineral, but is frequently associated with other calcium and magnesium carbonate minerals, lansfordite.
Monohydrocalcite has been observed in air conditioning systems deposits in caves, both probably formed from spray of carbonate rich fluids. It is well known in Robe of South Australia as a component of beach sands of Lake Fellmongery and Lake Butler, where it is believed to be formed from algal spume. Other lacustrine deposits include Lake Issyk-Kul, Democratic Republic of the Congo , and Solar Lake.
It has been reported as a significant component of the decomposition of ikaite It is also noted for its bizarre occurrences, which include inside the otoliths of the tiger shark of a guinea pig the calcareous corpuscles of a cestode and the final stages of decomposition of the putrefying flesh of the giant saguaro These occurrences suggest a biochemical origin is possible.