An EXAFS study of the local structural environments of Fe, Co, Zn and Mg in natural and synthetic staurolites

University of Manchester, Department of Earth Sciences, Manchester, United Kingdom

X-ray absorption spectroscopy (XAS) has been used to study the local structures of Fe, Co, Zn, and Mg in synthetic and natural stauraolites. Results for the near-edge features (XANES) and refined EXAFS were used to deduce the crystallographic site(s) occupied for each element. The least squares refined, mean first shell metal-O bond lengths, coordination numbers, and Debye-Waller factors are in the ranges: Fe 1.97-1.99 Aa, 3.5-4.1, 0.013-0.023 Aa ² ; Co 1.97-1.98 Aa, 3.5-3.9, 0.009-0.014 Aa ² ; Zn 1.95-1.96 Aa, 3.7-3.8, 0.008-0.009 Aa ² ; and Mn 1.99 Aa, 3.8, 0.012 Aa ² . No significant differences were found which depended on element concentration or whether the samples are synthetic or natural. The refined bond length for Mg in end-member Mg-staurolite is 2.00 Aa but EXAFS data did not allow refinement to give reliable coordination number and Debye-Waller factor estimates. The EXAFS data, together with staurolite and model compound XANES features, suggest that > about 90% of Fe, Co, Zn and Mn are concentrated in the tetrahedral T2 site, while in Mg-staurolite > about 75% of the total Mg is in T2 with the remainder in octahedral sites. In natural staurolite from Pizzo Forno (PF2), a greater proportion of Mg appears to be in octahedral coordination.

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