Formula: Ca(CO3).H2O
Hydrated normal carbonate
Crystal System: Trigonal
Specific gravity: 2.95 measured, 2.48 calculated
Hardness: 2 to 3
Streak: White
Colour: Colourless, white
Luminescence: Fluorescent vivid green under short wave UV, green under long wave UV

Hydrothermal environments
Cave deposits

Monohydrocalcite occurs in lake-bed sediments and as tuffaceous deposits on lake margins, formed by precipitation at pH>8.0 (alkaline) and high Mg:Ca or by biological activity. It occurs in caves, in speleothems and crusts, possibly formed in the presence of organic matter. It occurs rarely in hydrothermal mineral deposits. Associated minerals include calcite, aragonite, hydromagnesite and nesquehonite (HOM).

Monohydrocalcite also occurs in man-made environments, precipitating in the air scrubbers of air-conditioning plants and in the outlets of cold water taps (Webmin).

The occurrences of monohydrocalcite in nature are probably the result of biological activity. A unique occurrence has been found in a guinea pig bladder stone. The monohydrocalcite of the guinea pig stone becomes Mg-rich calcite on exposure to the atmosphere, indicating that the Mg-rich calcite of some invertebrate calcified tissues may have a monohydrocalcite precursor. It is suggested that formation of the mineral may be determined by distinct biological species in the aqueous environment (AM 62.273-277).

The saguaro is a giant cactus that grows to 15 m tall and weighs up to several tons. Roughly 18% of the dry mass consists of the biomineral weddellite. The carbon in the weddellite derives from atmospheric carbon dioxide via photosynthesis. After the death of the saguaro, a series of minerals crystallise in the rotting flesh. These minerals form from elements released from the decay of the cactus by microorganisms and thus this is a type of biologically induced mineralisation. During the initial stages of decay, minerals crystallising from elements released by the putrefying flesh of the cactus include lansfordite, nesquehonite, glushinskite, monohydrocalcite, calcite and vaterite. As the saguaro decays, the ribs and skin remain intact, producing warm, moist pockets in which abundant, glassy lansfordite crystals grow. Further decay leaves a dried hollow shell covered by the saguaro skin, inside of which nesquehonite and monohydrocalcite crystallise. Lansfordite and nesquehonite are unstable in the desert and rapidly become amorphous after exposure to the atmosphere. During the final stages of decay, the remnants of the saguaro consist of weddellite and its transformation product monohydrocalcite, that further alters to calcite (AM 88.1879-1888).


At Lake Butler and Lake Fellmongery, Robe, South East Limestone Coast, South Australia, Lake Fellmongery was the first lake of the region to be found surrounded by beach rock, composed entirely of monohydrocalcite. Similar beach rock was also found around Lake Butler. The monohydrocalcite occurs as hard pelletal aggregates with a sponge-like texture which resembles calcareous tufa. At Lake Fellmongery, it appears that the monohydrocalcite is formed in saline waters with a high Mg:Ca ratio, high pH (alkaline), and in the presence of blue-green algae. It seems that the monohydrocalcite is not secreted by the algae but rather that it is precipitated from the saline waters. The crystal structure appears to be stable when dry, but transforms in water to calcite (AM 60.690-697).
Samples of beachrock were examined and considerable variation in phase composition was observed, although all samples possessed monohydrocalcite, calcite, aragonite and Mg-rich calcite (AM 93.1014-1018).

The type locality is Issyk Kul Lake, Issyk-Kul Region, Kyrgyzstan.

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