No CrossRef data available.
Article contents
IV.—The Work of Professor Lacroix on the Laterites of French Guinea
Published online by Cambridge University Press: 01 May 2009
Extract
In this zone the phenomenon of départ or leaching is carried to a finish, and though of little significance in the case of the gibbsitic laterites of the diabases, gabbros, and syenites, since these from the beginning of alteration in the zone of leaching have been deprived of the greater part of the elements to be removed, yet in the case of the mica-schists it is of great importance. But in every case there is an accentuation of a phenomenon already evident in the preceding zone, namely the emigration of iron towards the surface, where it becomes concentrated. At the same time concretionary phenomena, often resulting in the separation of the hydrates of iron and aluminium one from the other, become increasingly important and reach their maximum development quite close to the surface, where they lead to the formation of a resistant crust (cuirasse), in which hydroxide of iron acts as cement; this hydroxide may even become sufficiently abundant to constitute an ore of iron, particularly in the case of certain diabases, and above all of peridotites.
- Type
- Original Articles
- Information
- Copyright
- Copyright © Cambridge University Press 1915
References
page 77 note 1 Lacroix notices that this ascension of iron towards the surface was observed long ago by Hislop (1857, Lacroix gives 1863), and later by Maclaren (1906), in the laterite of India, and by Arsandaux in that of Western Africa and of the Congo, and by many other authors.
page 77 note 2 While this is doubtless true from the point of view of the actual result, Professor Lacroix overlooks the circumstance that on steep slopes the products of weathering are rapidly removed by erosion, so that lateritic products have no chance of accumulating.
page 77 note 3 Lacroix remarks that this theory has already been proposed by Holland (Geol. Mag., 1903, p. 62) to explain the formation of the low-level laterites of India, to which Holland attributes a detrital origin. Lacroix does not, however, regard the two cases as comparable.
page 81 note 1 Name in the Foula language (p. 273) for vast horizontal or undulating tracts in Guinea, either quite bare, or covered with a meagre vegetation of grasses, Cyperus, and semi-aquatic plants.
page 81 note 2 Lacroix notes that Maclaren (Geol. Mag., 1906, p. 546) supposes that in India gibbsite results from hydration, contrarily to the opinion of Holland, who regards laterite as characterized by a dehydration (Geol. Mag., 1903, p. 65). Judging from Lacroix' researches it will be seen that there is probably little change in the state of hydration of the oxides of aluminium and iron once they have been deposited, except in the cuirass, where the limonite and stilpnosiderite tend to lose their water with formation of hematite, whilst the aluminium hydrates are unaffected.
page 82 note 1 Lacroix was able to visit the bauxite deposits near Little Rock last summer, and concludes that they must be regarded as laterites formed from alkaline-syenites, with kaolin characterizing the zone of leaching and bauxite the zone of concretion.
page 82 note 2 It is interesting to recall here Holland's suggestion that the loss of water accompanying the hardening of laterite is due to the crystalline affinity of Fe2 O3 in two molecules of limonite leading to the formation of crystalline hematite with rejection of water.