Abstract

Bauxite capping has formed by alteration of shale. Because the shale is already hydrated and contains mostly tektosilicates and phyllosilicates, the sequence of alteration stages differs from that in bauxitization of igneous rocks. Stage 1 is characterized primarily by a decrease in Si/Al and Si/Al+Fe ratios. In stage 2, Na, K, Ca, and Mg decrease along with Si, and Ti; Al, Fe, Mn, and P show relative increase. The same changes persist in stage 3, but Si/Al+Fe and felsic/mafic ratios generally show an increase. The preliminary hydration stage, characteristic of igneous-rock bauxitization, is missing.

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