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Biotite Kaolinization in Virginia Piedmont Soils: I. Extent, Profile Trends, and Grain Morphological Effects¹

ABSTRACT

Biotite kaolinization was examined for acidic, well-drained Virginia Piedmont soils with respect to extent, profile trends, and grain morphological effects. Fifty-eight samples from 16 profiles were treated to remove Fe-oxides, fractionated by particle size, and examined mineralogically using optical microscopy and x-ray diffraction (XRD). Selected samples were examined by infrared (IR) spectroscopy, differential scanning calorimetric (DSC), scanning electron microscopy (SEM), and Hg-intrusion porosimetry. All profiles contained kaolinized biotite sand grains and had kaolinite-dominated clay fractions. Vermiculite was a minor component of most samples. Kaolinization increased with decreasing particle size and, for some profiles, with decreasing depth. Individual sand-sized grains commonly contained both biotite and kaolinite. Kaolinized grains had lighter colored, duller (001) cleavage surfaces, and more pores than unaltered biotite grains. Pores extended from the edges inward, exposing new surfaces to soil solution thereby fostering continued kaolinite encroachment. The pore volume difference between kaolinite and biotite specimens was only slightly greater than the calculated difference based on a 1:1 layer-for-layer replacement of biotite by kaolinite.

¹ Contribution of the Dep. of Agronomy, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061.

² Former Research Associate, Professor, Associate Professor, and Professor, respectively. Senior author is now Assistant Professor of Soil Mineralogy/Genesis in the Soil Science Dep. at the Univ. of Florida, Gainesville.

Received for publication July 9, 1984. Accepted for publication March 27, 1985.

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