Modeling of the impact of dolomite and biotite dissolution on vermiculite composition in a gneissic shallow aquifer of the Sila Massif (Calabria, Italy)

- First reaction path modeling exercise including the thermodynamic data for several vermiculite compositions.
- Prevalence of dolomite dissolution leads to Mg-Al- and Ca-Al-vermiculites.
- Prevalence of biotite dissolution leads to Mg-Mg-Fe- and Ca-Mg-Fe vermiculites.
- Reaction path modeling confirms that vermiculite composition is also a function of pH.

Reaction path modeling of water-rock interaction in a gneissic shallow aquifer of the Sila Massif was performed in kinetic (time) mode, under conditions of closed-system with secondary minerals and closed-system with CO2, to investigate the influence of both dolomite dissolution and biotite dissolution on the chemical characteristics of secondary vermiculites. Magnesium-Al- and calcium-Al-vermiculites are the major components of the vermiculite solid solution precipitated in the early stages of the process, which is dominated by dolomite dissolution. In contrast, Mg-Mg-Fe- and Ca-Mg-Fe vermiculites are important components of the vermiculite solid solution produced in the late stages of the process, where biotite dissolution prevails. Outcomes of this reaction-path-modeling exercise on vermiculite chemistry are fully consistent with the results obtained by Apollaro et al. (in press) through speciation-saturation calculations. In particular, Apollaro et al. (in press) showed that the pH of Mg-Al-vermiculite/Mg-Mg-Fe-vermiculite coexistence is 7.3. This value is virtually equal to the pH of Mg-Al-vermiculite/Mg-Mg-Fe-vermiculite iso-activity, 7.35, which is obtained from the results of reaction-path-modeling runs 3 and 4 carried out in this work.