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Mineralogy and Acidity of an Inland Acid Sulfate Soil of Texas¹

ABSTRACT

The Aubrey soil occurs in the East Cross Timbers Region of north-central Texas. It has developed in an acid clayey shale of the Lewisville member of the Woodbine geologic formation of Upper Cretaceous age. Jarosite [KFe₃(OH)₆(SO₄)₂] occurs in localized concentrations in the lower horizons. Selected particles form the C horizon were 87.6% jarosite and served as a reference mineral for the soil. Jarosite content of the B3 and C horizons was 1.6 to 6.7% and was detectable by thermal and x-ray diffraction analyses of the whole soil. Scanning electron microscope examination indicated that the jarosite occurred as cubes and octahedra. These isometric forms suggest jarosite formation from pyrite present in the parent material. Smectite and kaolinite are the major clay constituents and they are both more poorly crystalline in the major A and B horizons than in the lower three horizons. The pH in water ranges from 5.0 in the upper B horizon at the 8-cm depth to 4.0 in the C horizon to the 152-cm depth. Exchangeable and nonexchangeable acidity increase with depth into the C horizon. The Aubrey soil has developed an accumulation of free Fe oxides above the jarosite zone, which is characteristic of an acid sulfate soil.

¹ Contribution from the Texas Agric. Exp. Stn., College Station.

² Associate Professor of Agriculture at Southwest Texas State Univ., San Marcos, TX 77866; Professor of Soil Mineralogy, Texas A&M Univ., College Station, TX 77843.

Received for publication March 28, 1980. Accepted for publication February 17, 1983.