RECONCILING DATA ON THE IRON OXIDATION STATE OF ANHYDROUS AND HYDROUS ALUMINOSILICATE GLASSES AND MELTS: A POLYMERIC APPROACH

The oxidation state of iron has been the object of attentive investigations during the last four decades. A first class of investigation involves glasses synthesized under nominally anhydrous conditions at atmospheric pressure: interpretation of iron oxidation state Vs. composition has given rise to some contradictions in literature about the structural role played by this element in silicate melts. Controversies are particularly relevant in the case of divalent iron, as testified by many spectroscopic determinations. The second class of investigation concerns hydrous aluminosilicate glasses synthesized under different T-P conditions. Again, no unique redox pattern has been found so far in literature, the ferric to ferrous iron ratio depending in a complex fashion on composition, temperature, pressure and oxygen fugacity of synthesis. The present study aims at showing that it is possible to reconcile such data by accounting for the acid-base properties of studied melts/glasses in the framework of a polymeric approach based on the concept of optical basicity and considering water speciation. Useful insights may thus be given about the dissociation equilibria of water in aluminosilicate melts/glasses. It is concluded that the developed model may be usefully employed for studying the evolution of the oxidation state of degassing and erupting silicate melts, showing that redox variations may be more reasonably ascribed to the melt compositional control rather than to changes in oxygen fugacity during magma migration from depth to surface.