Some examples of the application of Mössbauer spectroscopy and transmission electron microscopy to micas

I discuss the weaknesses and strengths of Mössbauer spectroscopy and transmission electron microscopy and present the potential of combining them using experimental weathering of biotite as an example. A natural biotite was treated in acid at room temperature and the reaction products were removed by a continuous flow of an external eluent that was in contact with the mineral suspension through a dialysis membrane. The fractionation of Mg/Fe into the fluid was determined by wet chemical analysis, the state of Fe in the solid was monitored by Mössbauer spectroscopy, and the structural changes were characterized by transmission electron microscopy. Weathering is controlled primarily by pH, the type of acid used, and K selectivity of the solid. Once equilibrium has been reached, further weathering does not affect the oxidation state of iron. The tetrahedral layer collapses during the last phase of weathering leaving an amorphous phase rich in Si and Al. Moreover, transmission electron microscopy shows that weathering is a discontinuous processes resulting in quasi-periodic texture. The periodicity is controlled by the concentration of the K in solution.