ABSTRACT

Drainage conditions ƒ When the weathering products obtained are soluble, they have to be eliminated for the reactions to proceed. Thus, well drained environments are favourable for a strong desilication going up to gibbsite. Gibbsite is produced at the top slope, for example, in toposequences. Poorly drained environments at the bottom of toposequences are, on the contrary, favourable for retention of silica and thus of 2/1 clay minerals, especially of the smectite type. However, bauxites are often found in basins. On the one hand they could have been transported there. On the other hand, under fully water-saturated conditions, the equilibria of the reactions presented in Table 3.3 (§ 3.3.1) are displaced to the right. This could be sufficient for bauxitization to occur (Tardy 1993).