Poorly-crystalline minerals and complex mineral assemblages

Minerals formed in the weathering environment are often characterized by effects of crystalline imperfection: superparamagnetism, foreign element substitution, and close intergrowths of different mineral species frequently complicate their identification by Moessbauer spectroscopy. Because of these effects, Moessbauer spectra taken at 77 K may not be sufficient to distinguish between iron oxides and silicates, and spectra may have to be taken at 4.2 K or even lower temperatures in order to fulfil this purpose. Chemical extraction procedures may help to separate the constituents of mineralogically complex samples, but some of these procedures can reduce structural iron (III) in clay minerals to iron (II), thus creating artefacts. (orig.)