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Weathering Sequence of Clay-size Minerals in Soils and Sediments

II. Chemical Weathering of Layer Silicates¹

ABSTRACT

Studies were made of several clays of soils and other deposits to clarify the nature of weathering of layer silicates. Data obtained by X-ray diffraction, thermal, R(OH)n sorption, and elemental analyses lead to the proposal of general occurrence of interstratified X-amorphous zones in 2:1 layer silicates. As weathering of true micas proceeds through illite, intermediates, vermiculite, and montmorin, a given K interlayer is rapidly depleted of K along a preferential weathering plane, leading to some degree of expansion. Concurrent hydroxylation by H+ addition to the octohedral layer together with dealumination accounts for loss of some of the mica layer charge. Increasing proportions of such weathered planes leads to various combinations of X-amorphous and X-crystalline zones typical of illite, vermiculite, and montmorin found in soils and other deposits, accounting for their diffraction properties; internal surface; K, H₂O, and OH contents; and exchange properties. Functional continuity of mineral weathering in Great Soil Groups of China through layer silicates (including kaolinite), gibbsite, hematite, and anatase is demonstrated.

¹ Contribution from Department of Soils, University of Wisconsin, Madison, Wis. Published with the permission of the Director of the Wisconsin Agricultural Experiment Station. This work was supported in part by the University Research Committee through a grant of funds from the Wisconsin Alumni Research Foundation. Received for publication September 15, 1951.

² Professor of Soils, University of Wisconsin; Soil Technologist, National Geological Survey of China; and Graduate Assistants, respectively.