THE ALTERATION OF PLAGIOCLASES AND AUGITES UNDER DIFFERING PEDO‐ENVIRONMENTAL CONDITIONS

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Abstract
Summary: The alteration of plagioclases and augites in lithorelicts were studied in thin sections of five selected profiles which differed in their mineralo‐chemical composition and stage of soil formation. The scanning electron microscope was employed to monitor the products of alteration of the pseudomorphs.In the base‐rich environment of the Eutropept and Tropudalf, both primary minerals alter to montmorillonite with small amounts of amorphous alumino‐silicates present on the plagioclases, and amorphous materials of undetermined composition on the augite. Local desiliciflcation of the feldspars results in gibbsite.In more acid environments, as in the Dystropept, the plagioclases alter to metahalloysite and some amorphous aluminosilicates, and the augite to amorphous iron hydrous‐oxide and goethite. In a freely drained environment as in the Tropudult, the alteration of plagioclases leads to kaolinite and locally to gibbsite; all the augite is converted to goethite. No rock relicts are present in the Oxisols.The differential alteration is a function of the pedo‐environment which is determined by the rate and amount of water percolating down the profile.