Thermal decomposition of amosite, crocidolite and biotite

SUMMARY. The oxidation of amosite, crocidolite, and biotite has been determined attemperatures up to nearly 9o0 ~ under both a vacuum (IO- ~ mm Hg) and oxygen (io and 600 mm Hg). Infrared spectra gave the loss of constitutional hydroxyl under these conditions. The loss of tensile strength of the amphiboles with increasing temperature seems to be due to thermal decomposition. For the three minerals oxidation takes place progressively over a broad temperature range. Under vacuum there is a certain temperature above which the ferric iron previously formed is reduced; this temperature corresponds to the completion of the loss of hydroxyl. The crocidolite anhydride in the literature is most probably an oxycrocidolite formed by dehydrogenation, the truly dehydroxylated zones being amorphous. W HEN hydrated silicates containing ferrous iron are heated the constitutional hydroxyl decomposes and the iron may also change its valence state. The oxidation may result from either the incorporation of oxygen into the material (oxygenation; Addison and Sharp, 1968) or a dehydrogenation (Brindley and Youell, 1953): Fe2++ OH---> Fe3++O--+H. This reaction combines oxidation and dehydroxylation and does not perturb the oxygen packing in the crystals, it involves a migration of proton