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

Abstract

AbstractWe present the results of a comprehensive study linking the crystal‐chemical formulae of amphiboles, a series of extremely complex silicates with the general formula (AB2C5T8O22W2, C5 = M12M22M3) 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−1) and by the OH‐stretching modes (3,000–4,000 cm−1) 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 TAl content, when TAl > 0.5 apfu; (c) the estimation of CTi content, when CTi > 0.3 apfu; (d) the estimation of CLi content, when CLi > 0.3 apfu; (e) the detection of CAl, when CAl > 0.7 apfu; (f) an estimate of CMg; (g) the estimation of CFe3+ in the case of Na amphiboles; and (h) the estimation of the Fe2+ content at the M2 site in the case of Mg–Fe–Mn amphiboles. Additionally, we point out that the TO4‐ring‐breathing mode near 670 cm−1, 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.