Site occupancy of natural richterite-winchite amphiboles from Libby, Montana, USA: a comparison between FTIR OH-stretching spectroscopy and micro-chemical (EPMA), X-ray diffraction (SREF) and Mössbauer data

Three natural amphibole samples collected from the former vermiculite mine near Libby, Montana, USA, have been analysed by Rietveld X-ray powder diffraction (XRPD) refinement and Fourier transform infrared spectroscopy (FTIR) in the OH-stretching region. The same materials have been analysed previously by electron microprobe analysis (EMPA), Mossbauer spectroscopy and structure refinement (SREF) single crystal X-ray diffraction (SC-XRD), which revealed that these amphiboles have a crystal chemical formula very close to an intermediate composition between winchite and richterite, i.e. (A)A(0.5)(B)NaCa(C)Mg(4.5)M(0.5)(3+ T)Si(8)O(22)(OH)(2) (A = Na and/or K; M3+ = Fe3+ and/or Al). The Rietveld analysis showed the powder samples used for the experiments here to be composed only of amphibole. This in turn allowed us to use FTIR OH-stretching data to derive cation ordering on these powder samples. The three FTIR spectra are quite similar and up to four components can be fitted to the patterns. The two lower-frequency components (labelled A and B) can be attributed to a local O(3)-H dipole surrounded by Mg-M(1)M(3)(3) and (Mg2Fe2+)-Mg-M(1)M(3) (respectively), an empty A site and Si-T(8) environments; on the contrary, the higher-frequency C and D bands indicate the presence of an occupied A site. The FTIR OH-stretching data alone allow us to calculate the site occupancy of the A, M(1)-M(3) and T sites with confidence, as compared with previously published data. By contrast M(4)- and M(2)-site occupancies are more difficult to evaluate. This study takes advantage of the large database of well characterized synthetic amphiboles, built over the last two decades. The comparison of vibrational spectroscopy data with micro-chemical and crystallographic data reported in this study demonstrate that the FTIR OH-stretching method alone is a valuable and rapid method to derive or at least sensibly constrain site occupancy for natural amphiboles. A much more detailed cation site occupancy can be obtained by combining micro-chemical and FTIR OH-stretching data.