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Occurrence of High-charge Beidellite in a Vertic Haplaquoll of Northwestern Minnesota¹

ABSTRACT

The 2:1 layer silicate mineralogy of a Vertic Haplaquoll of north-western Minnesota was studied using x-ray diffraction and determination of cation exchange capacity (CEC). The x-ray diffraction behavior of Li- and K-saturated samples demonstrated that a highcharge smectite with tetrahedral charge dominates the mineralogy of both the coarse and the fine clay fractions. The high CEC and the large difference between Li-CEC and K-CEC confirmed the x-ray results. Alkylammonium-clay complex basal spacings show two phases of 2:1 clays; a high charge (about 0.9 mol_c per unit cell formula) phase concentrated in the coarse clay and an Fe-rich beidellite with a charge of 0.4 to 0.5 mol_c per unit cell formula. The high charge phase largely represents K⁺ -depleted mica formed during the analytical procedure. This phase may also contain some soil vermiculite. The results presented in this study suggest that beidellite is a product of mica weathering, and that during weathering soil mica is transformed first to a vermiculite then to a beidellite, preserving the tetrahedral character of the charge.

¹ Scientific Journal Series Paper no. 14 974. Minnesota Agric. Exp. Stn., Univ. of Minnesota, St. Paul, MN 55108.

² Graduate Research Assistant, Associate Professor, and Professor, respectively, Dep. of Soil Science, Univ. of Minnesota, St. Paul, MN 55108.

Received for publication February 26, 1986.