Abstract

Alteration of clay minerals by weathering in Illinoian till of the Northwest Allegheny Plateau was determined by X-ray techniques. Analysis of closely spaced samples throughout the weathering profile from fresh till to the A-soil zone shows that each zone of the weathering profile is characterized by a specific clay mineral assemblage. Chlorite in the fresh till is degraded by hydration of the brucite sheets which leads to the development of vermiculite in the upper soil zone. Well-crystallized illite of the fresh till is degraded by hydration of the intermica sheets, and a random mixed layering of illite and montmorillonite is produced.

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