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Effects of Weathering on Glauconite: Evidence from The Abu Tartur Plateau, Egypt

Published online by Cambridge University Press:  01 January 2024

Brigitte Pestitschek ,
Susanne Gier ,
Mahmoud Essa  and
Hans Kurzweil
Brigitte Pestitschek
Affiliation:
Department of Geodynamics and Sedimentology, University of Vienna, 1090 Vienna, Austria
Susanne Gier*
Affiliation:
Department of Geodynamics and Sedimentology, University of Vienna, 1090 Vienna, Austria
Mahmoud Essa
Affiliation:
Department of Geology, University of Assiut, Assiut 71516, Egypt
Hans Kurzweil
Affiliation:
Department of Geodynamics and Sedimentology, University of Vienna, 1090 Vienna, Austria
*
*E-mail address of corresponding author: [email protected]
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Abstract

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Recognizing weathering effects is significant for any work carried out on glauconites at the surface. The mineralogy and chemistry of glauconite grains exposed to weathering in a hot arid climate for a maximum of 42 y were studied here. The objective of the study was to find the mineralogical and chemical differences between weathered glauconite from the surface and fresh glauconite from the subsurface.

One specific glauconite-bearing layer at the surface (Layer 16) of the Abu Tartur phosphate mine, located in the Western Desert of Egypt, was studied in detail and compared to a fresh, subsurface glauconitic sandstone from the underground mine.

Even within this single surface layer, the brownish-green glauconite grains vary in color and chemical composition. From top to bottom, the grains show an increase in Fe and K and a decrease in Al and S. In addition, the grains show an internal color zonation caused by variation of Fe and K contents between the center and rim of the grains. The differences in color and chemical composition are even more pronounced between the weathered-glauconite grains from the surface and the fresh glauconite grains from the subsurface which are dark green and enriched in Fe and K.

The clay fractions consisted of mixed-layer glauconite (illite)-smectite, with the surface samples containing more expandable smectite (50%) than the subsurface samples (20%). In the charge-distribution diagram for muscovite-pyrophyllite-celadonite, the weathered glauconites at the surface showed a clear trend from smectitic glauconite at the top to illitic glauconite at the bottom of the layer, whereas the fresh subsurface sample plotted exactly in the glauconite field.

The color, mineralogy, and chemistry indicate that the surface samples were strongly altered by weathering processes and that glauconite transformed progressively into Fe-rich mixed-layer illite-smectite and then into smectites.Weathering can thus completely reverse the glauconitization process. For any chemical and mineralogical characterization of glauconites at the surface, these weathering effects must be taken into consideration.

Keywords

Abu Tartur MineEgyptGlauconiteSmectiteWeathering
Type
Article
Information
Clays and Clay Minerals , Volume 60 , Issue 1 , February 2012 , pp. 76 - 88
Copyright
Copyright © Clay Minerals Society 2012

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