The structure of monohydrocalcite and the phase composition of the beachrock deposits of Lake Butler and Lake Fellmongery, South Australia

Samples of beachrock from Lakes Butler and Fellmongery, near Robe, South Australia, were examined by neutron and X-ray powder diffraction. Considerable variation in phase composition was observed, although all samples possessed monohydrocalcite, calcite, aragonite, and magnesian-calcite. The Rietveld refinement of the structure of monohydrocalcite showed significant changes in the orientation of water molecules and a major change in the orientation of a carbonate group with respect to the currently accepted structure. The current refinement shows that the hydrogen-bonding structure consists of three distinct networks, with each of the three carbonate groups being bonded to a single water molecule via one linear and one bifurcated hydrogen bond.