Chapter 24 - Lateritic and Bauxitic Materials

Abstract

Lateritic profiles are very complex and mostly polygenetic. They have been widely studied but no generally accepted definition or classification of laterite exists. This chapter presents a review of the various micromorphological characteristics of lateritic and bauxitic materials, without following a specific genetic model. A standard lateritic profile includes several layers, from bottom to top: the soft laterite (mottled clay zone), the hard laterite (laterite sensu strictum for some authors), and the gravel-rich layer. The soft laterite is characterised by redistribution of hematite, mostly forming a network of hypocoatings surrounding and isolating kaolinitic cells. The hard laterite is either nodular or vermicular, with high concentrations of hematite and goethite and with common corrosion of quartz grains. Nodules and channels are generally coated by goethite-kaolinite coatings, whose genesis in not well understood. The gravel-rich layer is an accumulation of allochtonous iron oxide nodules with various internal fabrics. In bauxites and Al-rich laterites, several types of gibbsite enrichment, partly marked by different crystal size ranges, are observed in thin sections. Although many descriptions of laterites and bauxites have been published in different disciplines, such as geology, pedology and geomorphology, a lack of coherent concepts and terminology greatly complicates attaining a comprehensive view of their micromorphological characteristics.