ABSTRACT

Skeletal elements originally composed of high Mg calcite are shown to be recrystallized to aragonite in Recent marine carbonate sediments. An explanation is proposed based on the stability of the CaCO₃ polymorphs. The stability fields defined for calcite and aragonite from experimentation with pure minerals in distilled water should not be projected unmodified into the marine environment. Ions other than calcium in seawater can substitute for calcium in the crystal structures of aragonite and calcite. The ratio of a foreign ion to calcium in the crystal structures is defined by a thermodynamic equilibrium constant, the distribution coefficient, specific for each ion and crystal structure at a given temperature and pressure. A condition of equilibrium between coexisting crys alline CaCO₃ and seawater can be reached only if the distribution of major and minor ions between the separate phases is in agreement with this coefficient. The distribution coefficient for partitioning Sr⁺⁺ between aragonite and aqueous phase has been shown by Holland and his co-workers to be about 1 at earth surface conditions. Aragonite in equilibrium with seawater should contain about 0.9 mol % SrCO₃ and have a free energy of formation of about -269.5 ±.5 kcal/mol at earth surface conditions. The distribution coefficient for partitioning Mg⁺⁺ between calcite and an aqueous phase is shown, as an approximation, to be 0.02 at earth surface conditions, although some data indicate the coefficient may be as high as 0.06. Using the lower va ue, calcite in equilibrium with seawater should contain about 10 mol % MgCO₃ and have a free energy of formation of about -267.0±.5 kcal/mol at earth surface conditions. Low Mg calcite is not in equilibrium with seawater in shallow marine environments and should not be considered the stable form of CaCO₃ under these conditions. Of the impure polymorphs that could exist in equilibrium with seawater as defined by partition coefficients, aragonite has the lower free energy of formation and should be considered the thermodynamically stable form of CaCO₃ in shallow marine environments. Partition and stability information shows that marine carbonate sediments cannot undergo diagenesis to normal limestones, consisting of calcite containing 1 to 3 mol % Mg O₃, in marine water.