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Carbonate Phases in Calcareous Soils of the Western United States

USDA-ARS, Southern Piedmont Conservation Res. Center, Watkinsville, GA 30677

R. H. Loeppert^* and L. P. Wilding

Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843

* Corresponding author.

ABSTRACT

The carbonate phases of 39 calcareous soil samples, representing 23 soil series from the western US, were studied by wet-chemical techniques, x-ray diffraction (XRD), and electron microscopy to determine the particle-size distribution, chemical composition, mineralogy, particle morphology, and Mg-substitution patterns of the carbonate phases. The multivariate relationship between carbonate mineralogy and saturated-paste extract composition was also investigated. Soil samples were fractionated into sand, coarse-silt (5–50 µm), fine-silt (2–5 µm) and clay particle-size fractions. The silt-size fraction accounted for a median proportion of 57% of the total soil carbonate. The percentage of the total carbonate in the clay-size fraction was variable among soils, ranging from <1 to 60%. Calcite, Mg-substituted calcite, and dolomite were identified by XRD, and each of these phases was identified in the clay-size fraction of selected soils. The occurrence of clay- and fine-silt-size carbonates is indicative of the presence of pedogenic secondary carbonates (including dolomite). For the clay-size fractions, the Mg/(Ca + Mg) molar ratios of the dissolution extracts of several of the samples were considerably higher than the theoretical ratios calculated from the experimentally determined calcite and dolomite contents, indicating the possible occurrence of poorly crystalline Mg-rich carbonate phases that were too disordered or too small in particle size to give distinct XRD reflections. Dolomite had a predominantly euhedral particle morphology, compared with calcite, which appeared as aggregates of smaller (<1 µm) particles. The mineralogy and morphology of the specific carbonate phases probably reflect the environment of precipitation and seasonal differences in soil-solution composition.

Contribution of the Texas Agric. Exp. Stn. as Journal Series no. 24640. This research was supported by a grant from BARD (Project no. 743-84).

Received for publication March 3, 1989.

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