Application of the improved regression method to derive ΔGf0 of non-stoichiometric clay minerals and their correlations with compositional parameters

A curvilinear regression method earlier developed by us [Varadachari, C., Kudrat, M., Ghosh, K., 1994. Evaluation of standard free energies of formation of clay minerals by an improved regression method. Clays Clay Miner. 42, 298–307] was utilised here for deriving standard free energies of formation (ΔGf0) of a compositionally wide range of illites, smectites, vermiculites and chlorites. The ΔGf0 values ranged from around −1131 to −1321 kcal/mol for illites, −1097 to −1328 kcal/mol for smectites, −1295 to −1351 kcal/mol for vermiculites and −1596 to −1983 kcal/mol for chlorites. Derived ΔGf0 data were compared with available ΔGf0 data for compositionally similar minerals. In spite of some differences in their compositions, the values were within a close range (about ±0.3–0.5%). Statistical analysis of the relationships between ΔGf0 and ionic contents for the different clay groups were done. ΔGf0 of illites showed strong correlations with Fe3+ and (VI) Al3+ (octahedral Al); the values became more negative with increasing (VI) Al3+ and less negative with increasing Fe3+ in the structure. A regression equation involving Si4+, (IV) Al3+ (tetrahedral Al), (VI) Al3+, Fe3+, Fe2+, Mg2+ and K+ gave a high regression coefficient, r2=0.998. ΔGf0 of smectites were markedly influenced by the Fe3+ content (higher Fe3+ contents lowered − ΔGf0) and that of chlorites by the Fe2+ and Mg2+ contents (higher Fe2+ lowered − ΔGf0 and higher Mg2+ increased it); vermiculites did not show strong influence of any of the structural ions on the ΔGf0.