Sequential mineral transformation from kaolinite to Fe-illite in two Brazilian mangrove soils

- Mangrove soils are dynamic systems between continental and marine environments.
- Crystal-chemical of clays reactions are important to understanding those ecosystems.
- The research indicates a transformation from kaolinite to Fe-illite in mangrove soils.
- Clay reactions observed may influence the cycle of major elements in coastal ecosystems.
- Interaction between minerals and biotic components in mangrove soils was discussed.

Mangrove soils have peculiar chemical and physical conditions generated by the variable water salinity, cyclic tidal changes and intense biological activity that produce complex clay suites. In this study, the crystal-chemical characteristics of clay minerals from two mangrove soils were investigated in detail to further our understanding of the processes taking place. The two Brazilian mangroves are from zones of contrasting climatic type and continental sediment character: (1) tropical wet climate area receiving highly weathered sediments (Bragança, Pará State) and (2) mild semiarid climate area supplied with a mixture of highly and moderately weathered sediments (Acaraú, Ceará State). The investigation of the 2-0.2 μm and < 0.2 μm size fractions of the soils using XRD, Mössbauer spectroscopy, chemical analysis of individual mineral grains (TEM-EDS) and bulk samples (XRF) showed mineral assemblages composed by endmember and mixed-layer clay minerals. Three to four mineral phases were detected by XRD in each sample such as kaolinite-smectite minerals (K-S), illite-smectite (I-S) and Fe-rich illite, suggesting a progressive transformation via mixed-layering from detrital kaolinite (unstable in mangrove soil conditions) to K-S and I-S minerals rich in smectite, and then to Fe-rich illite. The chemical characterization corroborates this gradual transformation: there is an enrichment in K, Fe and Mg at the expense of Al in all samples. A solid-state reaction explains the sequential transformation, where crystal-chemical changes take place without complete disruption of the lattice. There is evidence of octahedral Fe reduction during the illitization stage, showing the influence of a reducing environment, from decaying organic matter and bacterial activity, on the clay minerals. The increase in layer charge is balanced by K and NH4 ions, abundant in the interstitial water. Superior Fe-rich illite contents in Acaraú mangrove, compared to Bragança mangrove, are explained by high inputs of 2:1 clays from sediment source areas inland. The above clay mineral reactions shed light on the role of clays and the interaction between minerals and biotic components in mangrove soils. In addition, these reactions may influence the cycle of major elements in coastal ecosystem surrounding mangroves.