Monohydrocalcite is the main constituent of beach rocks surrounding Lakes Fellmongery and Butler near Robe in the south-east of South Australia. It occurs as hard pelletal aggrigates with a sponge-like texture which resembles calcareous tufa.

At Lake Fellmongery, it appears that the monohydrocalcitie is formed in saline waters with high Mg/Ca ratio, high pH, and in the presence of blue-green algae. Isotopic data suggest that the monohydrocalcite is not secreted by the algae but rather that it is precipitated from saline waters.

The composition of monohydrocalcite derived from the most comprehensive chemical analysis is (Ca0.9053 Mg0.0788 Fe0.0022 Sr0.0047 Na20.0133 K20.0008)CO3 · l.15H2O. All the water is firmly bound in the crystal structure, which appears to be stable when dry, but transforms in water to calcite.

X-ray powder diffraction data have been indexed on a hexagonal unit cell containing 9 (CaCO3 · H2O) and yield lattice parameters of a = 10.566Å, c = 7. 573 Å. The strongest reflections are: 4.333 100; 3.078 70; 1.9308 65; 2.829 50; 2.1665 45; 5.283 35.

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