Quantification and Compositional Characterization of Pedogenic Calcite and Dolomite in Calcic Horizons of Selected Aquolls
1987
A combination of thermogravimetric analysis (TGA), x-ray diffraction (XRD), and selective dissolution analysis (SD) was used to quantify and calculate structural formulae of carbonate mineral phases in silt fractions of calcic horizons in selected Texas Coastal Prairie Aquolls. The Algoa pedon (Aeric Calciaquolls) had about 11% poorly crystalline high-Mg calcite [(Ca.₈₇Mg.₁₃)CO₃], 12 to 20% dolomite, and approximately 25% low-Mg calcite [(Ca₀.₉₅Mg₀.₀₅)CO₃]. The dolomite was a Ca-rich variety that was previously interpreted to be pedogenic. The Addicks pedon (Aquic Haplustolls) lacked dolomite. The upper calcic horizon was dominated by low-Mg calcite [(Ca₀.₉₉Mg₀.₀₁)CO₃] with about 7% poorly crystalline high-Mg calcite [(Ca₀.₉₀Mg₀.₁₀)CO₃]. The presence of dolomite and high-Mg calcite in the Algoa pedon are related to elevated soluble Mg/Ca ratios. Thermodynamic relationships based on soil solution Mg/Ca activity ratios support these findings. It is suggested that poorly crystalline Ca-Mg carbonates, including high-Mg calcites, may be more abundant in calcic horizons than commonly believed. Their poor crystallinity makes them difficult to detect by standard XRD methods. However, TGA coupled with quantitative XRD and SD can be useful in detecting their presence and estimating their chemical composition.
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