The genesis of the blue amphibole asbestos of the Union of South Africa.

Doctoral Thesis


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University of Cape Town

University of Cape Town

The blue amphibole asbestos, crocidolite, which occurs as interbedded seams in banded ironstones of the Lower Griquatown stage of the Transvaal System in the Northern Cape Province, is the finely fibrous form of the soda-amphibole riebeckite. Despite the widespread occurrence of the Precambrian type of banded ironstones, crocidolite is a mineral of rare occurrence and is only developed to a relatively minor extent in South Africa, Western Australia and Central China. The composition, structure and properties of riebeckite in general and of crocidolite in particular are discussed and four new chemical analyses are given. Particular attention is paid to the mode of occurrence of fibrous riebeckite and it is suggested that the name "crocidolite" be reserved for the asbestiform riebeckite which occurs interbedded with banded ironstones. The general geology, lithology and mineralogy of Precambrian banded ironstones are described and their distribution in space and time is discussed. It is found that no banded ironstones are known to be younger than 1000 million years. This fact is related to special conditions of atmosphere, surface temperature and biological development which existed during the so-called Primitive Period of the Precambrian, a period which lasted from approximately 3000 million years to 1000 million years ago. The banded ironstones of the Transvaal System are described in some detail and seven new chemical analyses as well as trace element data are given. The genesis of banded ironstones is discussed at some length and particular attention is paid to the authigenesis of riebeckite. It is concluded that banded ironstones were deposited in seasonally fluctuating, typically "non-aggressive", fresh to brackish water lakes which were fed by sluggish, mature rivers. The solutions of iron and silica, alkaline earths and clay colloids carried by these rivers were derived from basic igneous rocks by chemical weathering in a tropical, monsoon-type climate. Recognisable detrital material is virtually absent from the banded ironstones of the Transvaal System, but it is thought that the stilpnomelane layers are aeolian deposits and that their composition gives some indication of the material which remained behind as "lateritic" soils in the source area. It is suggested that both crocidolite and riebeckite were formed by the low temperature dehydration, in situ, of an ordered precursor which could have been a clay mineral similar in structure to attapulgite, but containing ferrous and ferric ions in the octahedral layer. This clay mineral acquired sodium by cation exchange during dry periods when the depositional lakes were enriched in sodium. It is found that the actual formation of crocidolite is completely unrelated to dynamic stress. It formed only where layers of proto-riebeckite were in close contact or traversed by magnetite layers. It grew by the diffusion of proto-riebeckite through the magnetite layers, its transformation to riebeckite during its passage, and final addition to fibre growth points in contact with the magnetite layer. This process was initiated by slight temperature gradients in the magnetite layers and was propagated by a type of thermal autocatalysis. The temperature gradients in the magnetite layers resulted from their superior heat conductivity combined with differences in depth of burial of the strata. Economic deposits of crocidolite formed where a sufficient number of layers of protoriebeckite were in contact with magnetite layers which maintained or repeatedly attained the requisite temperature levels. The superimposed economic deposits of the Kuruman area, which appear to be related to folding, are due to an early period of folding, possibly even slumping during deposition, which took place prior to the formation of crocidolite and caused the thickening of the protoriebeckite layers in the crests and troughs of folds by plastic flow from the flanks. Three appendices, giving details of chemical and spectrographic analyses and of chemical experiments carried out, are attached.