Ferric iron in silicate melts in the system Na2O-Fe2O3 -SiO2 at high pressure

Abstract

To assess the effect of pressure on the coordination state of Fe3+, experiments have been conducted on silicate melts in the Na2O-Fe2O3-SiO2 system in the presence of O2 to pressures of 4 GPa and temperatures of 1700°C. Quenched melts (glasses) have been analyzed using 57Fe Mossbauer spectroscopy at 298K. Two peaks in the spectrum are symmetric at low pressure and are attributed to Fe3+(IV). The isomer shift of the Fe3+ doublet increases ~0.10 mm/s from 1 bar to 4 GPa. Above 1.5 GPa in iron-rich compositions, the spectra become asymmetric with the low-velocity peak becoming less intense and broader suggesting the presence of a second component. This component can be either a result of increasing distortion of Fe3+(IV) or a result of the gradual coordination change of Fe3+ from IV to VI. The effects of pressure on the coordination state of Fe3+ and the Fe3+/ΣFe affect the compressibility of a magma at depth. -from Author