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Laterite

rock, iron, ferric, weathering, chemical, conditions, rocks, tropical, water and various

LATERITE. The name laterite (later, a brick) was given by F. Buchanan-Hamilton (1807) to the peculiar mantle of ferru ginous rock which covers large areas in southern India. Similar material from the Futah Jallon territory of west Africa, analysed by P. Berthier (182o), yielded 2.8% silica, 8.6% alumina, 77.2% ferric oxide and '1.4% combined water. Laterite is recognized to day as a curious residual weathering product of such rocks as basalts, granites and shales. Its formation appears to have been remarkably restricted to a relatively recent geological period— subsequent to Cretaceous times. Laterite is in process of forma tion in many tropical countries subject to monsoon conditions.

Normal laterite is a porous clay-like rock largely impregnated with ferric hydroxide. Not infrequently this ferric hydrate occurs as small pisolitic nodules. Exposed surfaces of laterite are of a blackish brown colour and often have a scoriaceous, lava-like ap pearance. When freshly broken, laterite shows a vermicular structure, has a porous texture and is mottled with various tints of brown, red and yellow. Creamy aluminous hydroxide often fills the tubular cavities. When first quarried this rock is generally soft enough to be cut with a pick. After exposure it hardens— the result of dehydration. Lateritic debris possesses the property of re-cementing into masses which resemble the primary material. Two types of laterite are generally recognized—a high-level, primary or in situ type, and low-level, secondary or detrital (Buchanan's) laterite. There is a curious association of litho marge beneath laterite clearly suggestive of a genetic relationship.

Laterite occurs widely in Peninsular India, in Malaya and the East Indies, in the Northern Territories and Western Australia, in the equatorial regions of Africa, and in various parts of South America and Cuba. There is little doubt that J. D. Falconer's (191I) iron clay from Nigeria is laterite, and J. H. Goodchild (1913) has shown that the conga of Brazil has the same corn position as laterite. The variable chemical composition of primary laterite depends largely on relative proportions of the two lateritic constituents—the hydroxides of ferric iron and aluminium. When these two constituents are present in equal amounts—e.g., so% limonitic matter H20 32.7o%) and 5o% tri hydrate of aluminium 32.70%, substance represents typical laterite. If the ferruginous matter exceeds so% the rock would be limonitic laterite (hematitic if dehydrated). When the aluminous proportion exceeds 5o% the rock is bauxitic. (See BAUXITE.) The nature of its constituents makes laterite greatly resistant to atmospheric weathering. It makes a fair build ing stone. Ferruginous laterite has been used as iron ore, and aluminous laterite (bauxite) is aluminium ore.

W. G. McGee

(1879) considered certain upper Mississippi fer ruginous deposits were similar to laterite. F. R. Mallet 0880 thought the same of the iron clays of Ulster and suggested a lacustrine origin for both. J. Walther (1889) was of the opinion that nitric acid, introduced during thunderstorms with tropical rain, interacted to produce easily hydrolysed iron salts which effected the lateritization of the rock in situ. Philip Lake (1890)

suggested that laterite was the direct weathering product of various rocks. This opinion was supported by R. D. Oldham (1893). Solfataric action was advanced by C. W. Hayes (1895), and by G. A. J. Cole (1896) for the formation of the bauxites of Arkansas (U.S.A.) and Antrim (Ireland) respectively. The idea of laterite in association with pyritiferous rocks, and resulting from the re actions of free sulphuric acid, was put forward by G. C. Dubois (1903) to account for the laterite of Surinam. T. H. Holland (1903) elaborated the conception of micro-organisms breaking up rock silicates, taking the silica necessary for their existence and leaving the alumina they did not want—the silica being sub sequently removed by alkaline solutions. J. M. Maclaren (1903) and J. Morrow Campbell (1910) independently arrived at the conclusion that laterite was a sub-soil replacement product re sulting from the action of ascending, heated, mineralized waters. C. K. Leith and W. J. Mead have produced evidence to show that the laterite of Cuba (1912) and the bauxite of Arkansas (1915) are the residual weathering products of serpentine (peridotite) and syenite, respectively. F. W. Clarke (192o) concludes that "Bauxite, like laterite, occurs under a variety of conditions, which suggest a dissimilarity of origin." W. A. K. Christie, on the field evidence obtained by C. S. Fox (1923), expressed the opinion that capillary pressures, dialysis and electrolytic migration must play an important role in the formation of so obvious a mixture of colloidal substance as Indian laterite. These data, this suggestion of electro-kinetic phenomena, and the field observations of various investigators have been incorporated (1927) in the tropical residual, weathering-product theory of laterite formation. The conditions stipulated in the revised theory are briefly : (I) A tropical climate subject to alternations of dry or wet seasons or monsoons.

(2) A level, or very gently sloping, elevated land surface which is not subject to appreciable mechanical erosion (abrasion by rain and wind).

(3) The chemical and mineralogical composition of the exposed rocks to be suitable for a supply of the lateritic constituents— alumina and ferric oxide.

(4) The texture of the rock to be (or rapidly become during weathering) sufficiently porous for the entry of percolating water, SO that the conditions for chemical action will be at a maximum.

(5) The infiltrating water to remain in the interstices of the rock for long periods annually; i.e., during the wet monsoon, but eventually to drain away in the dry period, thus giving maximum play to chemical erosion.

(6) The infiltrating water to contain either an acid or alkaline substance with which to react on the rock components as well as to constitute an electrolyte and allow electro-kinetic phenomena to operate.

(7) These annual processes to be in operation continuously for at least a geological epoch of roughly a million years. (C. S. F.)