A CRITIQUE OF THE SCHELLMANN DEFINITION AND CLASSIFICATION OF 'LATERITE'

Abstract

Schellman's definition and classification of 'laterite' are based on the SiO2, Al2O3 and Fe2O3 contents of weathered formations in comparison to the chemical composition of the underlying rocks, from which the weathered materials are assumed to be derived. This approach is open to misinterpretation because many regolith materials have formed by lateral transport, both physical and chemical; it ignores the role of absolute accumulation; it pays little attention to the morphological characteristics of 'laterites' that give clues to their origins; it ignores the detailed mineralogical compositions of weathered materials and 'laterite'; and it does not permit field identification. Understanding of geology, stratigraphy, geomorphic evolution, mineralogy and micromorphology are essential ingredients of regolith investigations. Chemical analysis alone is insufficient. Schellmann's chemical classification seems appropriate only to a small subset of potential 'laterites' in which the whole profile is of bedrock and saprolite, and where there has been no lateral movement of solids or solutions. Schellmann's definition demands formation by tropical weathering, which eliminates ferruginous duricrusts formed outside the tropics. His approach produces confusion by grouping disparate ferruginous/aluminous materials together as 'laterite.' The range of applications of the term 'laterite' is so broad that it has become meaningless and the Schellmann approach has not resolved this issue.