Incipient vermiculitization processes in biotites from Italian Oligo-Miocene sediments
Morandi Noris; Minguzzi Vanna; Tateo Fabio; De Carlo M.
Mineralogica et Petrographica Acta 42: 29-40
Three purified biotite populations at an intermediate alteration stage (average K (sub 2) O% 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 Fe (super 2+) , K, Mn, Ti, Ba, Rb, Cl, Zr, Y and Sc and enrichments of Fe (super 3+) , Cr, Co, Cu and V. The transformation proceeds through the oxidation of some Fe (super 2+) that causes the loss of K and the increase of H (sub 2) O. The distribution of Fe (super 2+) and Mg is ordered on the XY plane, the Mg being more effective in increasing the a (sub 0) value and the Fe (super 2+) the b (sub 0) value. The oxidation of Fe (super 2+) into Fe (super 3+) caused a decrease of the OH-structural bond energy, changing the temperature of the dehydroxylation reaction at about 600 degrees 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 H (sub 2) O and other cations) during sea-water transport, sedimentation and burial.