Mineralogy and chemistry of different fractions of some soil laterites from northeast Brazil

The mineralogy and chemical composition of different laterite samples from Paraíba (NE Brazil) have been studied in order to improve our understanding of the laterite forming process. Determinations on size-fractionated samples of concretions and fines show that kaolinite (b-axis disordered form), haematite, goethite and quartz are the dominating minerals.

Laterisation process leads to the propagation of small-sized fractions (fines) into larger-sized ones (concretions). The propagation only involves the cementation of quartz and clay minerals by goethite and/or haematite and no segregation of alumina is involved. The larger sizes of the fines are similar in composition to the concretions, of which they are small-scale expressions.