DIAGENETIC STABILIZATION PATHWAYS OF MAGNESIAN CALCITES

Stabilization of high magnesian calcites (>4 mole% MgCO3) to low magnesian calcite (0-4 mole% MgCO3) and dolomite involves a reduction in the solubility of these phases during diagenetic alteration. The solubility of a magnesian calcite is controlled not only by the Mg concentration, but also other chemical and physical properties of the solid. These other properties include the amount of: 1) trace element diluents other than Mg (e.g., sodium, sulfate, adsorbed or structural water); 2) carbonate ion positional or cation ordering; 3) microstructural and surface defects; and 4) adhered small particles. Crystal size also may affect the solubility of a magnesian calcite. A magnesian calcite may become more stable in the natural environment by a decrease in Mg concentration, by loss of other trace elements and/or changes in its physical properties. Few studies exist of magnesian calcites in sediments and limestones undergoing diagenetic alteration that can be used to document the typical stabilization pathways followed by magnesian calcites. Several stabilization pathways are proposed, based mainly on experimental and theoretical arguments, to encourage further investigation of magnesian calcite diagenesis.