Characterization of granitoid profiles in the Sila Massif (Calabria, southern Italy) and reconstruction of weathering processes by mineralogy, chemistry, and reaction path modeling

Purpose This paper concerns the development of a multidisciplinaryresearch on weathering profiles of granitoid rocks inthe Sila Massif (Calabria, southern Italy), based on fieldinvestigations, chemical and minero-petrographic data, andgeochemical modeling. It aims at evaluating modes and degreeof weathering and focuses on the main transformationprocesses of the parent rock, integrating information obtainedby direct observations/analyses on samples with results ofsimulations of pertinent water–rock interaction processes, carriedout through the reaction-path-modeling approach.Materials and methods Physical responses to geological hammerhitting and morphological weathering features were usedto characterize increasing weathering grade (I–VI weatheringclasses) of a representative weathering profile. Also, the mainmicrofabric and minero-petrographic features were observedin thin sections; XRPD and SEM-EDS were used to identifyprimary minerals and neoformed products. Progressive dissolutionof the granitoid rock was simulated using the softwarepackage EQ3/6, 8.0 and a modified version of the thermodynamicdatabase data0.ymp.R5. A kinetic mode under closedsystemconditions was applied with respect to a defined set ofsecondary solid phases, which are allowed to precipitate.Results and discussion The partial transformation of biotiteand the partial destruction of feldspars, associated with theneoformation of secondary minerals and a substitution of the original rock fabric are the main mineralogical changes observed.Neoformed clay minerals and ferric products replacedfeldspars and biotite during the most advanced weatheringstage. Microfractures and morphological variations occur onthe original rock. The final result of weathering process is asoil-like rock characterized by sand-gravel grain-size fraction.Results of geochemical modeling suggest that the chemicalweathering processes occurring in the study area are relativelyclose to an iso-chemical transformation of the original granitoidrock, based on the small predicted changes in the concentrationsof solutes from the initial to the final state of thesimulation. The CO2-controlled dissolution of albite-rich plagioclaseis the most important reaction, followed by the dissolutionof K-feldspar, biotite, chlorite, and muscovite, inorder of decreasing importance. The precipitating secondaryminerals are ferrihydrite, saponite, vermiculite, illite, and kaolinitein order of decreasing pH values. The order of appearanceand the quantities of secondary (product) minerals dependon the setup of geochemical modeling.Conclusions Based on the standard thermodynamic propertiesof different illite endmembers, calculated in this work, itwas possible to predict the varying compositions of the illitesolid solution, which resulted to be similar to those observedin natural systems.