Kinetics of iron oxidation in silicate melts: a preliminary XANES study

X-ray Absorption Near Edge Structure (XANES) experiments made between 600 and 700 T at the Fe K-edge have been used to study the kinetics of iron oxidation in a supercooled melt of Fe-bearing pyroxene composition. To provide a firmer basis to redox determinations, the redox state of a series of samples was first determined from wet chemical, Mossbauer spectroscopy and electron microprobe analyses. The XANES experiments show that variations in relative abundances of ferric and ferrous iron can be determined in situ, even just above the glass transition, and that some information can also be obtained on the structural environment around iron cations. The kinetics of iron oxidation do not vary much with temperature down to the glass transition. This observation suggests that the rate-limiting factor in this process is not oxygen diffusion, which is coupled to relaxation of the silicate network, but, as described by Cooper and coworkers, diffusion of network modifying cations along with a counter flux of electrons. (C) 2004 Elsevier B.V. All rights reserved.