Chemical and structural characterization of fibrous richterite with high environmental and health relevance from Libby, Montana (USA)

Abstract

This study reports new structural and spectroscopic data of a sample of fibrous richterite from Libby, Montana (USA). The OH-stretching region was investigated by FT-IR. The spectrum showed, except for the typical absorption band at 3671 cm^-1 assigned to the vibration of the O-H dipole bonded to three ^[6]Mg cations, a well developed band at 3658 cm^-1 attributed to the ^M(1)+M(3)Fe²⁺ environment. The ^M(1)+M(3)Fe²⁺ occupancy calculated using the FT-IR data is in very good  agreement with that obtained combining Mössbauer and EMP data. Fe³⁺ was only assigned at M(2) owing to the absence in FT-IR spectrum of absorption bands at Δ=-50 cm^-1 from the tremolite reference band. Structural investigation was carried out by X-ray powder-diffraction using the Rietveld method. Cell parameters, fractional coordinates for all non-hydrogen atoms, and site scattering for M(1), M(2), M(3), M(4) and A were refined. The most relevant difference with respect to prismatic winchite is a generalized reduction of the cell parameters mainly ascribed to the higher fluorine content of fibrous richterite. Possible site occupancy was obtained by combining chemical data and Rietveld refinement results.