The rare mineral, monohydrocalcite, CaCO3·H2O, has been identified in a guinea pig bladder stone. Three other natural occurrences have been described, the most recent from lacustrine deposits in the southeast of South Australia. Our unique pathological specimen, a concretion of alternating white and tan bands, one centimeter in diameter, contains 7 percent MgO, 8 percent PO4, less than 1 percent FeO and no Na2O or K2O. Optical characteristics (ϵ = 1.548, ω = 1.594), X-ray diffraction data (P3, a = 10.602, c = 7.548A) and IR spectral data compare well with those of previous descriptions. The three most intense diffraction maxima are d100 = 4.34, d411 = 1.935, and d112 = 3.08A.

The occurrences of monohydrocalcite in nature are probably the result of biological activity. Preservation of this ephemeral phase may be enhanced in confined biologically-maintained environments. The monohydrocalcite of the guinea pig stone becomes magnesian calcite on exposure to the atmosphere. The magnesian calcite of some invertebrate calcified tissues may have a monohydrocalcite precursor. Similar mechanisms of biologic control of the depositional environment may also apply to the South Australian lacustrian occurrence of monohydrocalcite/magnesian calcite and protodolomite/dolomite described from adjacent lagoons. We suggest that formation of the mineral may be determined by distinct biological species with unique specificities and reactions in the aqueous environment.

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