Neoformation of micas in soils surrounding an alkaline-saline lake of Pantanal wetland, Brazil

The mineralogy and origin of micas were investigated in incipient soils surrounding a modern alkaline-saline lake of Nhecolândia, a sub-region of Pantanal wetland. Soils were sampled along a toposequence and analyzed by XRD, TEM–EDS, and ICP-MS. The studied micas, mainly concentrated in a green horizon, are dioctahedral, strongly associated with Fe3+ and Al, and interstratified with smectite layers. Classification of individual crystals shows that glauconite and Fe-illite are the dominant micas, but one crystal of illite was recognized. Si-rich amorphous materials are associated with small crystallites in the mica-enriched horizon. A recent study shows that water samples from the studied lake and the surrounding water table have high pH, negative Eh, temperatures up to 40 °C, high concentration of K, and low concentration of Si(OH)4. Experimental studies of micas synthesis reported in the literature show that similar water conditions allow for dioctahedral mica crystallization from initial precipitation of amorphous hydroxides. Therefore, water characteristics combined with presence of Si-rich amorphous materials in the mica-enriched horizon suggest that the micas of the study area are neoformed. The alternated origin of illite, glauconite, and Fe-illite mixed-layer minerals probably occurs due to seasonal variations of pH, temperature, and chemical composition of waters in microenvironments, since the changes at this scale are possibly faster and more extreme.