Influence of pedogenetic processes on the validity of kaolinite crystallinity indices: A case study of an Amazonian Ferralsol-Podzol soil system with white kaolin

Structural defects of kaolinite could be associated with its genetic (weathering, pedogenetic, and hydrothermal) processes. Kaolinite crystallinity indices were evaluated in the context of pedogenetic processes associated with a representative Amazonian Ferralsol-Podzol soil system. This soil system, which is comprised of various types of kaolinitic materials and has a soil dynamics characterized by the progressive replacement of Ferralsols by Podzols, has been evaluated by Hinckley (HI), Aparicio-Galán-Ferrell (AGFI), Liètard (R2), and Amigó (A001) “crystallinity indices” and the “expert system” of Plançon and Zacharie (PZ). The results indicate that the complexity of the kaolinite population, which is due to pedogenetic processes, makes it difficult to interpret the usual kaolinite crystallinity indices. In areas where kaolinites are formed under low aggressive conditions (higher pH and lower dissolved organic carbon, DOC) and in equilibrium with the soil solutions, low-defect phase becomes dominant and there is a good consistency between the “crystallinities” provided by each index. However, in areas where kaolinites are formed under highly aggressive conditions (lower pH and higher DOC), the proportion of low-defect phase decreases. This induces changes in the values of the indices, which do not have the same weight from one index to another.