The standard thermodynamic properties of vermiculites and prediction of their occurrence during water–rock interaction

The standard thermodynamic properties ( Δ G f ° , Δ H f ° , S°, and V°), at 298.15 K, 1 bar, of 16 vermiculites and their heat-capacity coefficients were computed in this research following the approach of Wolery and Jove-Colon (2007). In this way, data consistent with those of other sheet silicates contained in the thermodynamic database data0.ymp.R5 of the EQ3/6 software package were obtained. gh the uncertainty of these data is too high to investigate exchange reactions involving vermiculites, they can be profitably used to predict the conditions of vermiculite formation during weathering. The shallow groundwaters interacting with granitoid and gneissic rocks and overlying soils, from an area of the Sila Massif (Calabria Region, Italy), were taken into account for an example of application. Results of speciation–saturation calculations for these waters show that: (i) in general, production of vermiculites hosting Mg2+ and Ca2+ ions in the interlayer sites is favoured with respect to generation of vermiculites whose interlayer sites are occupied by Na+ and K+ ions; (ii) the possibly forming solid phases (all metastable with respect to Mg–Fe–vermiculite) are kaolinite, Mg–Al–vermiculite, and Mg–Mg–Fe–vermiculite, in order of increasing pH values. In detail, kaolinite/Mg–Al–vermiculite coexistence occurs at pH close to 6.7, whereas Mg–Al–vermiculite/Mg–Mg–Fe–vermiculite coexistence occurs at pH close to 7.3.