Iron in silicate glasses of granitic composition: a Mössbauer spectroscopic study |
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| Authors: | Beate Spiering Friedrich A Seifert |
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| Institution: | (1) Mineralogisch-Petrographisches Institut, Universität Kiel, Olshausenstraße 40-60, D-2300 Kiel, Federal Republic of Germany;(2) Present address: Mineralogisch-Petrologisches Institut, der Universität Bonn, Poppelsdorfer Schloß, 5300 Bonn |
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| Abstract: | 57Fe Mössbauer 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/r
2 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. |
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