Original Articles

The mineralogy of five weathering profiles developed from Archaean granite in the eastern Transvaal, Republic of South Africa

Published in: South African Journal of Plant and Soil
Volume 8, issue 3, 1991 , pages: 146–152
DOI: 10.1080/02571862.1991.10634823
Author(s): C. Bühmann, Republic of South Africa, W., F.A. Kirsten, Republic of South Africa

Abstract

Mineralogical, chemical and electron microscopy studies were carried out on five weathering profiles developed on Archaean granites from the Nelspruit area to study changes in the distribution of primary minerals and their weathering products with profile development. The pedons differ significantly in their degree of pedogenesis. A gibbsite-kaolinite-quartz-hematite association has been established for the most highly-weathered and a feldspar-quartz-mica-kaolinite assemblage for the least-weathered surface horizon. The following observations apply to all profiles studied: Microcline is enriched in the sand—and plagioclase in the silt fraction. Plagioclase weathers much more readily than K-feldspar. Biotite is remarkably stable. Kaolinite and gibbsite contents increase towards the profile surface. Halloysite (7Å) is commonly associated with kaolinite, but decreases towards the top of each profile. Secondary iron phases comprise goethite and hematite. High gibbsite contents are associated predominantly with hematite as a secondary iron phase, while kaolinite is associated with goethite. Trends in major element composition with profile development mirror mineralogical findings: decreases in Na2O and CaO accompany a decrease in plagioclase contents and a decrease in K2O is consistent with decreasing proportions of microcline and/or biotite. Gradual increases in Al2O3 and Fe2O3 values reflect the formation of kaolinite/gibbsite and secondary iron phases, respectively. Mineral pangeneses are not related to elevation, topographic position or slope. Gibbsite is interpreted as being a relict of a more humid climate and bears evidence of palaeoclimatic changes in this part of South Africa.

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