Raman spectroscopy for crystallochemical analysis of Mg‐rich layered silicates: Serpentine and talc

Abstract

A series of 10 talc (nominally MMg3TSi4O10X(OH)2) and 22 serpentine (nominally MMg3TSi2O5X(OH)4) minerals have been studied by Raman spectroscopy and electron microprobe analysis (EMPA) to establish quantitative relationships between the content of octahedrally coordinated M‐site cations and Raman peak parameters, which would allow for noninvasive material profiling. We show that in the case of talc, the position of the strong and well‐resolved peak near 360 cm−1, related to MO6 vibrations, as well as the integrated intensities of the OH‐stretching Raman signals (3600–3750 cm−1) can be used to determine the amount of MMg and M(Fe2+ + Mn2+) with a precision similar to that of EMPA. In the case of serpentine, the overall shape of the Raman scattering arising from OH‐stretching modes (3600–3750 cm−1) can be initially used to distinguish between the different polymorphs: antigorite, chrysotile, and lizardite, as well as the lizardite‐/chrysotile‐rich polygonal/polyhedral serpentine varieties and then, the full width at half maximum (FWHM) of the strongest MO6 peak at ~380 cm−1 can be used to estimate MMg content in each of the analyzed polymorphs/varieties and a relative error of 5%. In addition, MFe2+ amount in chrysotile and chrysotile‐rich polygonal serpentine can be quantified by FWHM of the peak near 230 cm−1 with a relative error of 5%, whereas MFe2+ amount in antigorite can be derived from FWHM of the peak near 130 cm−1 with similar relative uncertainties. We demonstrate the effectiveness of our approach on two types of rock‐based cultural‐heritage objects, cylinder seals and inscribed gems, where sampling is prohibitive.