Weathering of biotite to vermiculite in Quaternary lahars from Monti Ernici, Central Italy

Abstract

Abstract The weathering of biotite in Quaternary lahars from Monti Ernici (Central Italy) via interstratification stages has been studied by X-ray diffraction of both the interstratified minerals and the initial (biotite) and final (vermiculite) end-members. The structure of the biotite and vermiculite is described by means of monodimensional Fourier series. The interstratified phases have been studied using the INTER program which permits an analysis of two interstratified components by Fourier transform methods. It is shown that in the alteration range for biotite to vermiculite even small samples have different spatial concentrations of biotite, vermiculite and an intermediate biotite-vermiculite-type phase. Hydrobiotite and a rare biotite-hydrobiotite with a tendency to regularity have also been documented. Chemically, the process involves considerable oxidation of Fe 2+ and the removal of most of the elements, corresponding to a 34% loss in matter. The sequence of the element losses is: K + > Fe 2+ > F − > Mn 2+ > Si 4+ > Mg 2+ > Al 3+ > Ti 4+ . There are also some gains in the order: H + > Ca 2+ > Fe 3+ > Na + .