Polarized single-crystal FTIR-spectra of natural staurolite

Abstract

Polarized infrared absorption spectra of thin single-crystal slabs parallel to (010) and (001) of a staurolite from Pizzo Forno, Ticino, with analyzed composition (Fe_2.9Mg_0.9Zn_0.1Mn_0.1)Al_17.5Ti_0.1(Si_7.7Al_0.3)O₄₈H₃ have been measured in the range of 3000–4000 cm⁻¹. From the pleochroitic behaviour of the OH-vibrations three groups of bands can be distinguished: the bands of group I, a strong band at 3445 cm⁻¹ plus a weak shoulder at 3358 cm⁻¹, and the bands of group II, a weak band centered at 3677 cm⁻¹ plus a shoulder at 3635 cm⁻¹, are assigned to the H1 and H2 protons, respectively. The bands of group III, a weak band at 3577 cm⁻¹ plus a shoulder, cannot be interpreted on the basis of the proton positions known so far. We assign them to an additional proton H3, which is bonded to O1 and shows a bifurcated hydrogen bridge to two O5 in a vacant T2 site.