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Hodgson, Alan Algarth (1963) The thermal decomposition of amphiboles, with special reference to South African crocidolites and amosites. Doctoral thesis, Sir John Cass College.
The asbestiform amphiboles have been examined by differential thermal and thermal gravimetric analysis; and by direct dehydration measurement in both oxidising and inert atmospheres. Sixty-three ore samples of crocidolite and amosite from asbestos deposits in South Africa have been examined together with some crocidolites, anthophyllites, actinolites and tremolites from various world-wide sources. Chemical analyses haw been obtained for most of these samples.
The thermal decomposition of the asbestiform amphiboles proceeds by three stages involving, in order, lose of physically combined water; loss of chemically combined (constitutional) water; and breakdown of the residual structure into simpler molecules, mainly comprising pyroxenes and cristobalite.
Experimental evidence indicates that the loss of constitutional water from these amphiboles may involve reactions other than a net dehydroxylation. The hydrogen ions from hydroxyl groups may be partially or wholly, oxidised at exposed surfaces, depending on the mobility of the protons through the structure. Further evidence suggests that amosite, and probably anthophyllite and actinolite, undergo in inert atmospheres an auto-oxidation reaction, causing the release of free hydrogen from hydroxyl groups. These complications emphasise the need for careful inspection of weight loss and dehydration curves in both inert and oxidising atmospheres before assessing the constitutional water content of amphiboles.
Although some of the properties of crocidolite differ distinctly from those of the other amphiboles, all these minerals form an approximate series, in the order crocidolite, amosite, anthophyllite, actinolite, tremolite, linked by such properties as decreasing proton mobility, interesting temperature of dehydration, increasing temper- ature of decomposition and Increasing magnesium content.
A petrological examination of a number of the crocidolite and amosite ore samples has been made. The findings, together with evidence from d.t.a. and chemical analysis, provide grounds for suggesting that the Bushveld igneous mass influenced the unique formation of amosite in the Transvaal.
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