The Formation of Laterite

Extract

If the term “laterite” is restricted to apply only to hydrates of iron, aluminium, or in special cases manganese, formed by katamorphic processes, then obviously a great deal of what we commonly term “laterite” is incorrectly named. I am assuming that laterite is the end or apical product of a process of rock degradation which may stop short at the formation of the hydrated silicates—clays or lithomarges—or continue right on to hydrate according to chemical and physical environment and nature of the parent rock. Harrison’s studies of deposits in British Guiana show that lithomarge and bauxite are associated with each other in all possible proportions, with or without the hydrates of iron (which does not appear to pass through a secondary hydrated silicate or “kaolin” stage), and free silica, which on one occasion has been recorded in the form of tridymite. These substances are mixed in all degrees, and can only be distinguished by laboratory tests. The point which decides whether one of such rocks should be classed as a laterite or a lithomarge seems to be quite arbitrary, unless it is agreed that a laterite should contain an excess of hydrates over hydrated silicates.