Quantitative Determination of Calcite and Dolomite in Soils

Summary Soil carbonates were differentiated into calcite and dolomite and determined quantitatively by a method based on their differential solubilities in 0.30 M Na citrate—0.14 M Na dithionite. Calcite dissolution was favoured by the acidic and chelating nature of the citrate. Prior destruction of iron‐oxide coatings on soil carbonates and/or iron oxide‐carbonate complexes by the citrate‐dithionite iron‐removal process was necessary for complete calcite dissolution. This method consisted of a total carbonate determination, selective dissolution of calcite with a citrate‐dithionite extraction, and determination of dolomite by difference. Citrate‐dithionite soluble calcite and dolomite are calculated from the quantities of extracted Ca and Mg. Soil exchangeable Ca and Mg may interfere and should be removed prior to citrate‐dithionite extraction on soils of high exchange capacity and/or low carbonate content. Interferences arising from dissolution of non‐carbonate Ca‐ and Mg‐containing soil minerals were considered negligible. Replicate analyses of a broad spectrum of samples suggest that the method is highly satisfactory for the determination of calcite and dolomite contents of soils and sediments of very variable composition. The occurrence of dolomite appears to be related to dolomitic source areas and is detritial whereas calcite is primarily reprecipitated (secondary) carbonates, although some calcite may be of detritial nature, e.g. shell materials or calcitic limestone drift.