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 done 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 general reduction of the cell parameters that is ascribed mainly to the higher fluorine content of fibrous richterite. Possible site occupancies were obtained by combining chemical data and Rietveld refinement results
