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V.—What is Laterite?

Published online by Cambridge University Press:  01 May 2009

L. Leigh Fermor
L. Leigh Fermor
Affiliation:
Geological Survey of India.
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Extract

1. The term laterite is used in two ways, namely, stratigraphically as the name of a geological formation, and petrographically as the name of a tropical superficial rock. This discussion relates only to the use of the term as a rock name.

2. Laterite (or rather some varieties of it) is formed by a process, the modus operandi of which is not discussed here, by which certain rocks undergo superficial decomposition, with the removal in solution of combined silica, lime, magnesia, soda, and potash, and with the residual accumulation, assisted, no doubt, by capillary action, metasomatic replacement, and segregative changes of a hydrated mixture of oxides of iron, aluminium, and titanium, with, more rarely, manganese. These oxides and hydroxides of iron, aluminium, titanium, and manganese are designated the lateritic constituents.

3. This residual rock is true laterite, and the presence of any considerable proportion (> x10 per cent) of non-lateritic constituents requires expression in the mime, as it always indicates want of completion in the process of lateritization. True laterite contains, then, 90 to 100 per cent of lateritic constituents.

Type
Original Articles
Information
Geological Magazine , Volume 8 , Issue 12 , December 1911 , pp. 559 - 566
Copyright
Copyright © Cambridge University Press 1911

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References

page 559 note 1 Manual of the Geology of India, 1893, p. 369.Google Scholar On p. 385 the use of this word as a lithologieal and a chronological term is discussed, and the former use preferred.

page 559 note 2 Ann. des Mines, vi, pp. 531–4.Google Scholar

page 560 note 1 Ann. Chim. Phy., ser. HI, lxi, pp. 309–42.Google Scholar

page 560 note 2 Since the above was written we have received in the library of the Geological Survey of India, vol. cxlviii of the Comptes Rendus (1909), in which there are two notes, on pp. 936–8 and 1115–18 respectivelyGoogle Scholar, by M. H. Arsandaux, “Surla composition de la auxite.” In the earlier note it is shown by treating certain French bauxites with concentrated H Cl, and then attacking the insoluble residue with concentrated H2SO4, that nearly all the alumina is present as Al2 O3. H2 O, thus corresponding, although the rocks were found in thin sections to be completely isotropic, to the composition of diaspore. The iron is shown to be present as anhydrous Fe2 O3. The titania is believed to be present as metatitanic acid, TiO2. H2O, and the small quantities of silica as 2 H2xO. Al2O3. 2SiO2 (the kaolin or lithomarge formula). The second note deals with the more siliceous bauxites (also completely isotropie), and indicates the existence of every gradation between clay and AI2O3. H2O.

page 563 note 1 Tschermak's, Mittheilungen, 1903, p. 18.Google Scholar

page 564 note 1 The same remark probably applies to many another tropical area in which lateritic rocks occur. Thus, I have quoted with, approval the work of Arsandaux, M. on the French bauxites (see note, p. 561).Google Scholar But I do not agree with his two later notes (pp. 682–5 and 1082–4 respectively, of C.R., vol. cxlix, 1909Google Scholar), in which he gives and discusses analyses of ‘laterites’ from the French Congo and the Soudan. As before, he extracts the Fe2O3, and in this case the free AI2O3 as well, with concentrated H Cl, and decomposes the insoluble residue by means of concentrated H2 S O4. To the former fraction (my lateritic constituents) he pays no attention, but shows that the portion insoluble in H Cl, which amounts to 68–99 per cent of the whole rock, varies in composition from micaceous alumino-potassic silicates allied to muscovite to kaolin practically free from alkalies. Most of these rocks have been formed in situ by the weathering of crystalline rocks (granites, schists, etc.), and it is evident from tBe author's analyses and descriptions that his so-called laterites are really clays containing a certain, usually small, proportion of lateritic material. The very uppermost crust may be laterite, properly so called, but no analyses of this are given.

Reference may also be made to the analyses of ‘laterites’ (derived from basic rocks—diabase and ophite), from French Guinea, by Chateaud, J. and Lemoine, P. (C.E., vol. cxlvi, pp. 239–42, 1908).Google Scholar The analyses show, in most cases, a considerable amount of silica, but it is apparently (p. 241) almost entirely in the free condition. On this assumption, No. 11, with only 5¶52 per cent of Si O2, is a true laterite; Nos. 136, 179, 180, 181, and 185, containing 12 to 28 per cent of Si O2, are quartzose laterites, whilst No. 195 (derived from phyllite) with 62¶30 per cent of Si O2 cannot be regarded as a laterite at all. That the authors do not regard these rocks as completely formed laterites (my true laterites) is indicated by the following passage: “La silice est presque complètement rendue libre; les ⅘ environ ont été entraînés; le reste, qui aurait probablement disparu si la latéritisation avait été complète, ne joue qu'un rô le insignificant et est, en majeure partie, à l'état de silice libre.” A diagram illustrates the chemical losses involved in the formation of laterite No. 11 from diabase No. 2, on the assumption of constant Ti O2.

page 564 note 2 Blanford, W. T., in his account of the Laterite of Orissa (Mem. Geol. Surv. India, i, p. 283, 1859),Google Scholar referring to the lithomarge underlying the laterite, says that on exposure it becomes hard like laterite.