Iron in silicate glasses of granitic composition: a M�ssbauer spectroscopic study

Abstract

57Fe Mossbauer spectra of natural glasses (pumices and obsidians) and of synthetic glasses of granitic composition have been analyzed. — Ferric iron is found in tetrahedral coordination if enough M+-cations are available to balance the charge of both M+Fe3+O2 and M+AlO2 complexes. In other compositions the ratio of tetrahedrally to octahedrally coordinated Fe3+ depends on the ratio of mono-to divalent cations. — Ferrous iron occurs in two distinctly different octahedral sites. The existence of these sites can be attributed to different anionic units adjacent to Fe2+. The degree of polymerization of these units is reflected in the quadrupole splitting. The anionic units adjacent to Fe2+ are depolymerized for increasing mean Z/r2 of the network modifiers, which do not stabilize M3+ in the tetrahedra by local charge balance. — Increasing pressure diminishes the geometric differences between these types of ferrous iron-oxygen-octahedra, which gives rise to a more even distribution of Fe2+ among these sites and thereby to an ordering in the network of melts.