DIAGENETIC STABILIZATION PATHWAYS OF MAGNESIAN CALCITES

Stabilization of high magnesian calcites (>4 mole% MgCO3) to low magne sian calcite (0-4 mole% MgCO3) and dolomite involves a reduction in th e solubility of these phases during diagenetic alteration. The solubil ity of a magnesian calcite is controlled not only by the Mg concentrat ion, but also other chemical and physical properties of the solid. The se other properties include the amount of: 1) trace element diluents o ther than Mg (e.g., sodium, sulfate, adsorbed or structural water); 2) carbonate ion positional or cation ordering; 3) microstructural and s urface 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 conc entration, by loss of other trace elements and/or changes in its physi cal properties. Few studies exist of magnesian calcites in sediments a nd limestones undergoing diagenetic alteration that can be used to doc ument the typical stabilization pathways followed by magnesian calcite s. Several stabilization pathways are proposed, based mainly on experi mental and theoretical arguments, to encourage further investigation o f magnesian calcite diagenesis.