AN EXAFS STUDY OF THE LOCAL STRUCTURAL ENVIRONMENTS OF FE, CO, ZN ANDMG IN NATURAL AND SYNTHETIC STAUROLITES

X-ray absorption spectroscopy (XAS) has been used to study the local s tructures 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, coordin ation numbers, and Debye-Waller factors are in the ranges: Fe 1.97-1.9 9 Angstrom, 3.5-4.1, 0.013-0.023 Angstrom(2); Co 1.97-1.98 Angstrom, 3 .5-3.9, 0.009-0.014 Angstrom(2); Zn 1.95-1.96 Angstrom, 3.7-3.8, 0.008 -0.009 Angstrom(2); and Mn 1.99 Angstrom, 3.8, 0.012 Angstrom(2). No s ignificant differences were found which depended on element concentrat ion or whether the sampes are synthetic or natural. The refined bond l ength for Mg in end-member Mg-staurolite is 2.00 Angstrom but EXAFS da ta did not allow refinement to give reliable coordination number and D ebye-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-s taurolite > about 75% of the total Mg is in T2 with the remainder in o ctahedral sites. In natural staurolite from Pizzo Forno (PF2), a great er proportion of Mg appears to be in octahedral coordination.