Transformation of clay minerals in salt-affected soils, Pantanal wetland, Brazil

Abstract

Some of the saline lakes occurring in the Nhecolândia, a sub-region of the Pantanal wetland, have transformed into brackish lakes due to atypical freshwater input from seasonal flooding. Consequently, the Saline-Sodic soils formed around the saline lakes, previously submitted to salinization and solonization, have been converted into Sodic, Solodized-Solonetz and Solod soils around the brackish lakes, under the action of solonization and/or solodization. In this research, fine clay fractions (<0.2 μm size) of B natric horizons of Saline-Sodic soils surrounding a saline lake and Sodic and Solod soils surrounding brackish lakes were studied in order to understand the genesis of clay minerals with the gradual transformation of these salt-affected soils. Fine clay mineralogy was studied by experimental XRD, full XRD profile modelling using NEWMOD 3.2.1, STEM/HAADF and ICP-OES. In the Saline-Sodic soils, illitic mixed-layered R0 kaolinite-illite (K-I) and R0 illite-smectite (I-S) comprise most of the samples, the percentage of illite (in K-I and I-S), kaolinite in R0 kaolinite smectite (K-S and K-I), and smectite (in K-S and I-S) layers was 73–76%, 14–16% and 10–11%, respectively. In the Sodic soils, illitic K-I and/or I-S still dominate the samples, but the percentage of illite layers (in K-I, I-S and/or illite-vermiculite) is smaller (52–68%), with an increase of kaolinite (19–35% in end member kaolinite, K-S and K-I) and smectite (7–21% in K-S and I-S) layers. Finally, the Solod soil shows dominance of smectitic I-S and illitic I-S, with a significant decrease in illite layers (36–41% in K-I and I-S), the maintenance of kaolinite layers (21–31% in pure kaolinite, K-S and K-I) and significant increase in smectite layers (31–37% in K-S and I-S). These progressive changes in mineral assemblages from the most alkaline (Saline-Sodic) to the most acidic soil (Solod) is probably due to gradual transformations, especially from the illitic phases, neoformed around the saline lake, to other 2:1 (mixed-layered smectite) and 1:1 (mixed-layered kaolinite) clay minerals under the new geochemical conditions of the brackish lakes. The mineral range observed in the samples suggest that the transformation from one clay mineral into another takes place without the complete dissolution and consequent precipitation, but as progressive mixed-layering reactions. This model explains the existence of several mixed-layered minerals, which is in agreement with the geochemical evolution of soils under progressive solonization and solodization.