The crystal chemistry of julgoldite-Fe3+ from Bombay, India, studied using synchrotron X-ray powder diffraction and 57Fe Mössbauer spectroscopy

G. Artioli 1 , C.A. Geiger 2  and M. Dapiaggi 1
  • 1 Dipartimento di Scienze della Terra, Università di Milano, I-20133 Milano, Italy
  • 2 Institut für Geowissenschaften, Christian-Albrechts-Universität, D-24098 Kiel, Germany

Abstract

The crystal structure of julgoldite-Fe3+ from Bombay, India, was investigated by 57Fe Mössbauer spectroscopy and synchrotron X-ray powder diffraction. Only ferric iron was detected in the Mössbauer measurements and it occurs at two different octahedral sites in the atomic ratio 20:80. Based on Rietveld refinements, the Fe3+ cations are located at the X- and Y-octahedral sites with atomic percentages of about 25% and 75%, respectively. The resulting chemical formula of the Bombay julgoldite sample is Ca8(Fe3+2.7Al1.1Mg0.2)(Fe3+8.0)Si12O42(OH)14. The oxidation state of Fe is not the same as that arrived at through simple crystal-chemical considerations. Such analysis cannot give quantitative results for the valence state of mixed-valence cations in pumpellyite-type minerals and their intracrystalline partitioning behavior. Assignments of the Mössbauer absorption doublets and an analysis of Fe-intracrystalline partitioning behavior are discussed with reference to previous works on different pumpellyite-type minerals.

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