Calcium Carbonate Concretions Formed by the Decomposition of Organic Matter

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Abstract
Bacterial decomposition of butterfish and smelts in small sealed jars containing seawater and other solutions, for periods ranging from 65 to 205 days, results in a large increase in concentrations of dissolved bicarbonate, carbonate, and ammonia (plus volatile amines). Accompanying this is a rise in pH and the precipitation of Ca ++ ion from solution. The Ca ++ is not precipitated as CaCO 3 but instead as a mixture of calcium fatty acid salts or soaps with from 14 to 18 carbon atoms. This can be explained by the thermodynamic instability of CaCO 3 relative to Ca soaps in the presence of excess free fatty acid. It is suggested that some ancient CaCO 3 concretions, especially those enclosing fossils of soft-bodied organisms, may have formed rapidly after death in the form of natural Ca soap (adipocere) which was later converted to CaCO 3 .