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Formation of Mica in Surface Horizons of Dryland Soils¹

ABSTRACT

A mineral with a 10Å X-ray spacing is the dominant clay mineral in the surface horizon of many dryland soils in which vermiculite or vermiculite-biotite is dominant in subjacent horizons. The mineral occurs in soils with aridic, torric, xeric, and ustic moisture regimes. To study its genesis we separated weathered mica flakes from soils developed in granitic-textured rocks. The mineral has a refractive index of 1.64 to 1.65 and an exchange capacity of 24.7 to 44.0 meq/100 g. Its chemical composition shows that, except for its K content, it is as weathered as the vermiculite or vermiculite-biotite of subjacent horizons. It has fixed NH₄⁺ (5.4 – 8.0 meq/100 g) as well as K⁺. Together K⁺ and NH₄⁺ amount to more than half of its total charge. Most of its charge comes from the tetrahedral sheet, and its tetrahedral charge is less than that of weathered mica flakes in B and C horizons. Since the weathered mica flakes in these soils contain relatively large amounts of K, small additions of K⁺ or NH₄⁺ released by plant decay will collapse the minerals to 10Å. The process reduces the loss of K⁺ through leaching and may supply important amounts of N in soils that contain little organic matter.

¹ Contribution from Soil Survey Investigations Div., SCS, USDA.

² Research Soil Scientists, SCS, USDA, P. O. Box 672, Riverside, California 92502, and Director, Soil Survey Investigations Div., SCS, USDA, Washington, D.C. 20250, respectively.

Received for publication July 31, 1972. Accepted for publication January 11, 1973.