The geology, mineralogy and chemistry of the Grahamstown clay deposits

Smuts, Johann (1983) The geology, mineralogy and chemistry of the Grahamstown clay deposits. Masters thesis, Rhodes University.

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Abstract

The Grahamstown clay deposits extend in a broad belt from 26°23 to 26°50 East longitude and from 33°15 to 33°22 South latitude along two distinct geomorphological features, the Grahamstown Peneplane (650 m) and the Coastal Plain {520m}. The clay deposits traverse four different lithologies including the Bokkeveld Shale, Witteberg Shale, Owyka Tillite and Ecca Shale. The two planes invariably have a covering of silcrete which is also present over most of the clay deposits except where erosion has taken place. X-ray fluorescence analysis shows that chemically there is a fairly wide variation between and within the deposits. The greatest variation is in the Si0₂/A1₂0₃ ratio which appears to be controlled by the parent lithology and to some extent by the amount of leaching. K₂0 shows an increase in concentration with depth and therefore indicates the limits of hydrolysis and leaching and of the clay. X-ray diffraction study shows the Peneplane and Coastal Plain deposits to be quite distinct. The Peneplane deposits consist of kaolinite, illite and quartz and the Coastal Plain deposits of kaolinite, illite, quartz and pyrophyllite. The presence of pyrophyllite is not fully understood as there is no indication of major faulting, metamorphism or pyrophyllite in the parent rock. The pyrophyllite most probably represents a transformation product of kaolinite. The kaolinite from the various deposits shows a considerable variation in crystallinity in both the X-ray diffraction traces and electron photomicrographs. The most poorly crystalline kaolinites are from the Coastal Plain deposits and the difference in crystallinity is most probab1y due to differences in the degree of hydrolysis and the parent rock material in the case of the tillite. Genetically all of the deposits are residual types generated by hydrolysis and subsequent leaching of micas and feldspars. The principal elements leached are silicon, iron and potassium. The hydrolysis and leaching took place over a long period of time in the flat lying areas of the Peneplane and Coastal Plain. The deposits are exploited economically and the clay is used principally in the tile, pottery and whiteware industries with some usage in the paper, refractory and brickmaking industries. The price commanded by raw kaolin is not very high and as a result the clay industry in Grahamstown is not as viable economically as it could be.

Item Type:Thesis (Masters)
Uncontrolled Keywords:Grahamstown, South Africa, Clay deposits, Bokkeveld Shale, Witteberg Shale, Owyka Tillite, Ecca Shale, Silcrete, Kaolin, Lithology, Hydrolysis, Leaching
Subjects:Q Science > QE Geology
Divisions:Faculty > Faculty of Science > Geology
Supervisors:Eales, H.V.
ID Code:4108
Deposited By: Philip Clarke
Deposited On:19 Nov 2012 15:27
Last Modified:19 Nov 2012 15:27
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