Nondestructive determination of the amphibole crystal-chemical formulae by Raman spectroscopy: One step closer

We present the results of a comprehensive study linking the crystal-chemical formulae of amphiboles, a series of extremely complex silicates with the general formula (ABCTOW, C = M1M2M3) to variations in the peak positions, widths, and intensities of the Raman-active modes. To this purpose, we have analyzed the Raman scattering generated by the framework vibrations (15-1,215 cm) and by the OH-stretching modes (3,000-4,000 cm) of 44 samples, spanning all six major subgroups. We show that, in addition to the information that can be derived from the OH-stretching range (Leissner et al., Am. Mineral. 2015, 100, 2682), further important features of the amphibole structure, composition, and cationic site population can be directly extracted from the framework Raman spectrum, namely, (a) the distinction between the monoclinic and orthorhombic symmetries; (b) the estimation of Al content, when Al > 0.5 apfu; (c) the estimation of Ti content, when Ti > 0.3 apfu; (d) the estimation of Li content, when Li > 0.3 apfu; (e) the detection of Al, when Al > 0.7 apfu; (f) an estimate of Mg; (g) the estimation of Fe in the case of Na amphiboles; and (h) the estimation of the Fe content at the M2 site in the case of Mg-Fe-Mn amphiboles. Additionally, we point out that the TO-ring-breathing mode near 670 cm, which is commonly used to fingerprint various amphibole species, has to be handled with a great care, because it is sensitive to the site population at all crystallographic sites.