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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
Environments
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).
Localities
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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