Incipient vermiculitization processes in biotites from italian oligo-miocene sediments

Three purified biotite populations at an intermediate alteration stage (average K2O% between 5.3 and 5.6) and a vermiculite sample have been studied in order to define the crystallographic parameters. Samples come from Cenozoic volcaniclastic sediments of Northern Apennines and Sardinia The purified samples have been analyzed by diffractometric, thermal, microscopic and chemical methods. The biotite populations show heterogeneous compositions within the same sample, and show different degree of vermiculitization marked by progressive depletion of Fe2+, K, Mn, Ti, Ba, Rb, Cl, Zr, Y and Sc and enrichments of Fe3-, Cr, Co, Cu and V. The transformation proceeds through the oxidation of some Fe2+ that causes the loss of K and the increase of H2O. The distribution of Fe2+ and Mg is ordered on the XY plane, the Mg being more effective in increasing the a(o) value and the Fe2+ the b(o) value. The oxidation of Fe2+ into Fe3+ caused a decrease of the OH-structural bond energy, changing the temperature of the dehydroxylation reaction at about 600°C. The vermiculitization process may be explained through a very early and fast contact of the biotites with the atmosphere. The consequent iron oxidation causes the vermiculitization process (losing K, changes of OH-cation bonding strength, income of H2O and other cations) during sea-water transport, sedimentation and burial.