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

Abstract

AbstractX-ray absorption spectroscopy (XAS) has been used to study the local structures of Fe, Co, Zn, and Mg in synthetic and natural staurolites. 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 Å, 3.5–4.1, 0.013–0.023 Å2; Co 1.97–1.98 Å, 3.5–3.9, 0.009–0.014 Å2; Zn 1.95–1.96 Å, 3.7–3.8, 0.008–0.009 Å2; and Mn 1.99 Å, 3.8, 0.012 Å2. No significant differences were found which depended on element concentration or whether the sampes are synthetic or natural. The refined bond length for Mg in end-member Mg-staurolite is 2.00 Å 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.