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Clay Mineral Formation in Different Rock Types of a Weathering Boulder Conglomerate¹

ABSTRACT

A study was made of the weathering products from individual boulders from a conglomerate composed of boulders having differing geologic origin. A detailed study was made of three of these rocks, namely: granite and granite-gneiss rocks; basic igneous rocks (gabbro); and single feldspar crystals from a pegmatite formation. Clay mineral formation was dependent on the chemical nature of these rock materials and the immediate ionic environment within the weathering boulders. The sodium and potassium feldspar crystals of granitic and gneissic rocks weathered to kaolin minerals, whereas the primary minerals of the gabbro boulders weathered to montmorillonite. Significant differences in cation exchange capacities (CEC) and exchangeable cations were observed for the various weathered boulders. These differences were related to the original parent rock materials and to the present clay mineralogy of the weathered boulders. The dominant cation in all systems was aluminum, which comprised up to 83% of the exchange capacity of the weathered rock that contained a large amount of montmorillonite. Although the amounts of all basic cations in these systems were small, the weathered boulder material in which kaolin clays were predominant contained more Na than did the montmorillonite systems. The reverse was true for exchangeable Mg and K. Exchangeable Ca was small in all samples. The weathered boulders were extremely acid with pH values near 4.3.

Key Words: montmorillonite formation • kaolinite formation • soil formation

¹ Contribution of the Agronomy Department, University of Kentucky, Lexington, and the Agronomy Department, Virginia Polytechnic Institute, Blacksburg. Published (as journal article No. 66-3-15) with permission of the Director of the Kentucky Agricultural Experiment Station. Presented before a joint session of Div. S-2 and S-5, Soil Science Society of America Columbus, Ohio, Nov. 1, 1965.

² Assistant Professor of Agronomy, University of Kentucky and Professor of Agronomy, Virginia Polytechnic Institute, respectively.

Received for publication February 16, 1967. Accepted for publication June 6, 1967.