Application of the improved regression method to derive Δ Gf0 of non-stoichiometric clay minerals and their correlations with compositional parameters

Abstract

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 (Δ G f 0) of a compositionally wide range of illites, smectites, vermiculites and chlorites. The Δ G f 0 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 Δ G f 0 data were compared with available Δ G f 0 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 Δ G f 0 and ionic contents for the different clay groups were done. Δ G f 0 of illites showed strong correlations with Fe 3+ and (VI) Al 3+ (octahedral Al); the values became more negative with increasing (VI) Al 3+ and less negative with increasing Fe 3+ in the structure. A regression equation involving Si 4+, (IV) Al 3+ (tetrahedral Al), (VI) Al 3+, Fe 3+, Fe 2+, Mg 2+ and K + gave a high regression coefficient, r 2=0.998. Δ G f 0 of smectites were markedly influenced by the Fe 3+ content (higher Fe 3+ contents lowered − Δ G f 0) and that of chlorites by the Fe 2+ and Mg 2+ contents (higher Fe 2+ lowered − Δ G f 0 and higher Mg 2+ increased it); vermiculites did not show strong influence of any of the structural ions on the Δ G f 0.