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Quantification and Compositional Characterization of Pedogenic Calcite and Dolomite in Calcic Horizons of Selected Aquolls¹

ABSTRACT

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 [(Ca_0.95Mg_0.05)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 [(Ca_0.99Mg_0.01)CO₃] with about 7% poorly crystalline high-Mg calcite [(Ca_0.90Mg_0.10)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.

¹ Contribution from the Dep. of Agronomy, Univ. of Kentucky, Agric. Exp. Stn., Lexington, KY 40546. The investigation reported in this paper (no. 86-3-193) is in connection with a project of the Kentucky Agric. Exp. Stn. and is published with approval of the Director.

² Graduate Research Assistant and Assistant Professor, respectively, Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546.

Received for publication September 17, 1986.

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