Occurrence and genesis of secondary magnesium-bearing calcites in soils

The presence of magnesium-bearing calcites is indicated in soil carbonate accumulation layers and concretions. A differential solubility method permitted estimation of the mol percent magnesium-carbonate content of calcites in the presence of dolomite. The relationship between diffraction spacings and mol percent MgCO₃, of selected samples is in good agreement with that depicted by other workers for biogenic magnesium-bearing calcites.Magnesium-containing calcites are concentrated in clay (< 2 μ) and fine silt (2–10 μ) sizes of Cca horizons. Their presence in the clay fraction of Ck horizons is an indication that carbonate mineralogy may change at profile depths often considered to be unaffected by pedogenic processes.The dissolution of existing carbonates within solum horizons, the translocation of ions in solution and the reprecipitation of calcite in Cca and underlying horizons are the main processes applicable to a variety of different natural situations. The Mg level of the secondary calcites is likely regulated by the partitioning effect during their precipitation. Periodic moisture movement to greater depths and lateral movement of soil solutions serve as a regulating factor in maintaining soluble Mg at levels favorable to the precipitation of low-magnesium calcites in upland chernozemic soils of central Saskatchewan; higher magnesium contents occur in calcites occurring in subsoils underlying solonetzic and depressional chernozemic soils of the same area. Formation of magnesium-bearing calcites within concretions in soils of the Rusizi basin in Africa were likely influenced by Mg-rich ground waters; associated dolomite may also have been of a secondary nature.