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Biotite Kaolinization in Virginia Piedmont Soils: II. Zonation in Single Grains¹

ABSTRACT

Kaolinized zones in soil biotite grains were investigated by x-ray diffraction (XRD), microprobe, and optical methods. These zones displayed microcrenulations and thus a dulled luster on (001) cleavage surfaces. Compared to unaltered biotite, kaolinized zones were lighter in reflected light but darker in transmitted light, probably because the crenulated (001) surfaces diffuse light. Kaolinized/unkaolinized interfaces in some grains were roughly perpendicular to (001) cleavage. This suggests that kaolinization proceeds inward from the grain edge along a multilayered front by a topotaxial rather than epitaxial reaction. Kaolinized zones contained islands of residual biotite and had an optic axial angle (2V) and dispersion more consistent with biotite than kaolinite. Biotite-derived kaolinite grains contained Fe, which was either structural or occluded, and had Si/(Al + Fe + Mg + Ti) ratios >1.0. A significant vermiculite intermediate phase is not essential for kaolinization, as indicated by the lack of XRD evidence of vermiculite as well as by the simultaneous loss of K and divalent octahedral cations in examined biotite/kaolinite grains.

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

² Former Research Associate, Professor, and Distinguished 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.