Experimental weathering of biotite, muscovite and vermiculite: a Mössbauer spectroscopy study

Biotite, muscovite and vermiculite were treated in HCl and H2SO4 solutions at pH ranging between 7 and 1. The solid residue was studied using Mossbauer spectroscopy at room temperature. The spectra of the untreated samples, D0Bio, D0Mus and DOVer were fitted assuming quadrupole splitting distribution, QSD. Different fitting models with different combinations of Gaussian components for the Fe sites were tested and the models with chi(2) near 1.0+/-0.1237 and with the least number of free parameters were adopted as working models. The models were then used to fit the experimental products of each group. If a model fails to describe a spectrum of a sample in the group then this is taken to signal a significant crystal chemical change induced by weathering. All three minerals are oxidized during weathering and the correlation between oxidation ratio and pH is modeled using a Weibull function. The rate of oxidation in biotite is higher than that in muscovite and vermiculite, The average CS and QS of the Fe sites in a group remain more or less constant during the experiment. However, the position of the peak QS of Fe-[6](2+) in biotite is shifted to low energies with decreasing pH, a measure of the decreasing covalence character of the Fe/Mg-O bonds. In muscovite the major feature observed is the enhancement of the bi-modal distribution of the QSD of Fe-[6](2+) with decreasing pH.