Abstract
The Mossbauer spectra of an albite glass and melt doped with Fe-57 (bulk composition NaFe0.04AlSi3O8) were measured in situ to 1100 degrees C. A specially designed furnace was used where a disk of the sample was enclosed in a capsule of graphite. The furnace was purged with argon to avoid oxidation of the graphite, and shields of boron nitride (BN) were used to protect both the gamma-ray source and the detector from thermal radiation. Room-temperature spectra show contributions from paramagnetic Fe2+, paramagnetic Fe3+, and magnetic Fe3+. The signal of Fe2+ disappears around 400 degrees C, and the signal of Fe3+ disappears at about 1100 degrees C. This behavior cannot be due to changes in the oxidation state of iron because in situ visible and near infrared spectra of the same sample show that Fe2+ persists to at least 1000 degrees C and the quenched samples do not show any change in oxidation state. Rather, the disappearance of Fe2+ and Fe3+ in the spectra is likely clue to the structural relaxation time around these cations becoming comparable to the Mossbauer timescale (lifetime of excited state approximate to 10(-8) s). We show that the temperature at which the Fe3+ contribution disappears corresponds to the expected glass transition temperature of a "ferrialbite" NaFeSi3O8 melt for a timescale of 10(-8) s. Our data therefore confirm that viscous flow is due to the rearrangement of the chemical bonds between oxygen anions and network-forming cations. Furthermore, our data show that Mossbauer spectroscopy can be used to study the speciation and relaxation time around iron in silicate melts even at temperatures above the macroscopic glass transition temperature as defined from viscosimetry and calorimetry.