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V.—The Residual Earths of British Guiana commonly termed ‘Laterite’

Published online by Cambridge University Press:  01 May 2009

J. B. Harrison  and
K. D. Reid
K. D. Reid
Affiliation:
Assistant Analyst British Guiana.
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Extract

The losses of their constituents during the decompositions of the rocks.— As shown in this paper the analyses of the rocks and of their decomposition-products do not indicate the extent of the degradation which has taken place. To ascertain this it is necessary to re-calculate the analytical figures so that their proportions are comparable to those of one of the constituents which is assumed to have remained unchanged. The structures of the Surinam and the British Gruiana bauxitic laterite show clearly that, contrary to what is not unfrequently assumed, alumina cannot be regarded as a static component in rocks and their decomposition-products. It is evidently under condition of lateritization capable of entering into solution, transference from place to place, and redeposition in the form of alumina hydrate where conditions are favourable. The constituent which appears to be the most stable under conditions of weathering is titanium oxide, especially that present as ilmenite. But unfortunately as a rule it is present in such low proportions in rocks and in their residuary products that, when taken as the static constituent, errors in analysis of low value are productive of variations of wide extent in the results calculated on them.

Type
Original Articles
Information
Geological Magazine , Volume 7 , Issue 12 , December 1910 , pp. 553 - 562
Copyright
Copyright © Cambridge University Press 1910

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References

page 558 note 1 In the Guianas the formation of masses of bauxite appears to be due to later segregation changes in the metasomatic residual earth rather than to the completeness of the original decompositions.

page 561 note 1 Some ten or eleven years ago, whilst lecturing on agricultural science, I used the following experiment to illustrate the action of pure water on rock. Rock powder was prepared from various types of rock by grinding on a bucking plate. About twenty grams of the rock powder was placed in a beaker, and from 100 to 150 cc. of cold, recently distilled water poured on it. To the water thus freed from carbonic acid a few drops of a solution of phenol-phthalin was added. In the course of a few minutes the water commenced to change to purple, and after ten minutes or so bad elapsed the depth of colour produced served as a measure of the rates of decomposition of the various rocks by the water. I found that the most readily decomposable rocks were felspar-porphyry and porphyrite, the next being granititegneiss, followed by granite and granitite. The basic rocks were more slowly attacked by the water. After standing for some time when the colour of the water had ceased to deepen it was then poured or filtered off completely, the rock powder again treated with fresh quantities of the boiled water or in phenol-phthalin, when the gradual colourization again ensued. This, if desired, could be repeated many times, using the same rock powder. The experiments well illustrated the action of water free from carbonic acid on the rocks, and its repetition with successive quantities of water show it to be a mass action. The rates of decomposition of the various rocks thus indicated were found to correspond with the extent of their denudation and degradation on the lower-lying lands of the colony.