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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

Detrital Laterite or Lateritite.—Many of the so-called low-level laterites and some of the high-level laterites consist of admixtures of detritus from the various varieties of laterite already mentioned, with quartz and argillaceous detritus from granites and gneisses, the whole cemented together by the action of water on the lateritic constituents of the mixture. For such rocks the name detrital laterite is available, and could be used in all cases in which the reconstructed rock contained, say, at least 40 per cent of lateritic constituents. Theoretically, detrital laterites may contain, as an upper limit, 100 per cent of lateritic constituents, corresponding to the case of a detrital rock composed entirely of fragments of a pure laterite, without admixture of extraneous constituents. From the above it will be seen that I would allow greater elasticity in using the term laterite for the detrital forms than for those formed in situ (see next paragraph), provided the adjective detrital be carefully prefixed; for this word seems to me to imply the great variability of composition that must characterize these secondary laterites. But if it were considered desirable to avoid the use of the word laterite altogether in this connexion, the obvious term to use instead would seem to be lateritite, implying a rock the individual constituents of which are pieces of laterite. This term would be of great utility, and could be applied to all detrital and reconstructed lateritic rocks. The word is put forward here merely as a suggestion, but if geologists were to distinguish, in writing, between laterite, lateritoid, and lateritite, great clarity could be introduced into the literature of these substances.

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Type
Original Articles
Information
Geological Magazine , Volume 8 , Issue 11 , November 1911, pp. 507 - 516
Copyright
Copyright © Cambridge University Press 1911

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References

page 508 note 1 It is interesting to note that as long ago as 1838 Dr. J. Clark divided lateritie rocks into the three following classes: (1) lithomargie, (2) quartzy, (3) detrital. See Madras Journ. Lit. and Sci., viii, pp. 338 344,Google Scholar.

page 508 note 2 GEOL, . MAG., 1903, pp. 154–9Google Scholar.

page 511 note 1 As seen under the microscope in thin sections, the aluminous silicate is isotropic wherever it shows any translucency or transparency, and it cannot therefore be designated kuolinite, which is a crystalline mineral. Substances corresponding to Dana's description of lithomarge, which he gives as a variety of kaolin, are invariably, in my experience, amorphous and non-crystalline, and consequently I think that lithomarge is to be regarded as the colloidal variety of kaolinite.

page 511 note 2 Some authors assume its presence as ilmenite, e.g. Professor Harrison in his paper on the lateritic earths of British Guiana; but it cannot be present in this form in many of the Indian laterites. Beference to the analyses of aluminous laterites given in Rec. Geol. Surv. India, xxxii, p. 179, and xxxvii, p. 215Google Scholar, shows that in the majority of these laterites there is insufficient iron present for all the titanium to be in the form of ilmenite. Further, I have seen no evidence under the microscope of the existence of ilmenite in laterites, and, in fact, as far as I have seen, the high amounts of titanic oxide in somelaterites do not make their presence evident in any form in thin sections of the corresponding rocks. Consequently I am inclined to think that the titania is present as a collodial constituent, either as a hydrated oxide of titanium, or even possibly as a hydrated aluminium titanate, of composition parallel to that of kaolin and lithomarge, although this latter alternative is put forward as a mere suggestion, for which there is at present no evidence. Whether the titania be present as a hydroxide or as a collodial titanate does not matter for our present purpose, as the above analyses are susceptible of arrangement to agree with either alternative. I find that M. Arsandaux (note, infra) supposes the titania of French bauxites to be present as metatitanic acid, Ti O2, H2O.

page 512 note 1 Mem. Geol. Surv. India, xxxvii, p. 378Google Scholar.

page 513 note 1 GEOL. MAG., 1909, p. 52–5.Google Scholar

page 512 note 2 Loc. cit., 1910, p. 381Google Scholar.

page 513 note 3 Résumés of this work and of that of other previous writers are to be found in the following works: Lake, P., “Geology of South Malabar,” Appendix (Mem. Geol. Surv. India, xxiv, pp. 239–46, 1891)Google Scholar; Bois, G. C. Du, “Beitragzur Kenntnis der surinamischen Laterit, etc.” (Tschermak's Mitt., xxii, pp. 4–18, 1903)Google Scholar; Guillemain, C., “Beiträge zur Geologie von Kamerun” (Abh. König. Preuss. Geol. Landesanstalt, N.S., vol. lxii, pt. viii, on Laterite, pp. 242–323)Google Scholar.

page 513 note 4 Mem. Geol. Surv. India, xxiv, p. 239, 1891Google Scholar.

page 513 note 5 The italics are mine.

page 515 note 1 Mem. Geol. Surv. India, xxxvii, pp. 381–3, 1909Google Scholar.