The crystal chemistry and Fe-II-site properties of aluminosilicate garnet solid solutions as revealed by Mossbauer spectroscopy and electronic structure calculations

Geiger, C. A., Grodzicki, M. and Amthauer, G. (2003) The crystal chemistry and Fe-II-site properties of aluminosilicate garnet solid solutions as revealed by Mossbauer spectroscopy and electronic structure calculations Physics and Chemistry of Minerals, 30 (5). pp. 280-292. DOI 10.1007/s00269-003-0319-1.

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Abstract

The three binary garnet solid solutions (Fe3Al2Si3O12)-Al-II-(X3Al2Si3O12)-Al-II (X-II= Mg-II, Mn-II, Ca-II) have been investigated by Fe-57 Mossbauer spectroscopy at 298 and 77 K and by electronic structure calculations in the local spin density approximation. The spectra yield isomer shifts and quadrupole splittings that are typical for Fe-II in the dodecahedral X-site of 222 point symmetry and are similar for each of the three binaries recorded. Conversely, electronic structure calculations based on the experimental crystal structure of the different end-member garnets exhibit pronounced variations in some of the electronic properties of Fe-II that are not reflected in the spectroscopic data. These results are interpreted as indicating that the different X-O bonds in garnet solid solutions retain to a large degree the intrinsic lengths that they possess in their respective end members, and that the Fe-O bond does not change greatly as a function of composition. This is evidence for the state of alternating bonds and not for the virtual crystal approximation in describing the X-O bond types or lengths in aluminosilicate garnet solid solutions. The observed degree and behavior of the Fe-II doublet asymmetry in the Mossbauer spectra for the three solid solution series do not indicate major variations in the anisotropic recoil-free fraction of Fe-II. Variations in doublet asymmetry are more likely a result of complex next-nearest X-site neighbor interactions and/or some degree of short-range cation ordering, though doublets representing different local X-site cation configurations cannot be resolved or fitted to the experimental spectra.

Document Type: Article
Keywords: garnet Mossbauer spectroscopy solid solutions electronic structure calculations site relaxation thermodynamic-mixing properties molecular-orbital calculations almandine-spessartine garnets si-29 mas nmr silicate garnets synthetic almandine grossular garnet fe-57 mossbauer isomer-shifts pyrope
Research affiliation: Kiel University
Refereed: No
DOI etc.: 10.1007/s00269-003-0319-1
ISSN: 0342-1791
Date Deposited: 24 Jan 2012 06:55
Last Modified: 08 Oct 2012 10:35
URI: http://eprints.uni-kiel.de/id/eprint/16189

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