Exploring Raman spectroscopy for crystallochemical analysis of talc and serpentine: application beyond geosciences

Talc and serpentine-group minerals are Mg-dominant trioctahedral layered silicates that are common in igneous and metamorphic rocks and can be found in a wide range of geological conditions. Hence, a precise physicochemical characterization of these phyllosilicates in intact mineral assemblies, e.g. in thin sections as prepared for polarization microscopy, can provide a better insight into the processes of mineral formation, magma differentiation, and alteration. Moreover, talc and serpentines are common mineral components in a variety of cultural-heritage objects such as engraved gems and old Babylonian cylinder seals. Hence, material profiling of artefacts can help understand their origin through crystallographic and crystallochemical markers that may advance provenance studies. Since sampling of such objects is mostly prohibitive, the development of non-destructive, non-invasive, and preparation-free analytical methods is desired.

To address this quest, a series of 18 serpentine-group minerals (nominally Mg₃Si₂O₅OH₄) and 10 talc samples (nominally (Mg₃)Si₄O₁₀OH₂) with different contents of Fe as a minor element was selected and studied by Raman spectroscopy and wavelength-dispersive electron microprobe analysis (WD-EMPA) to explore the potential of Raman spectroscopy as a truly non-destructive method for quantitative compositional characterization of these groups of phyllosilicates. The methodological approach is based on the already established quantitative relationships between the crystallochemical composition and the Raman signals of biotites (Aspiotis et al., 2022). The goal was first to verify whether the Raman scattering arising from the framework vibrations (15-1215 cm^-1) and OH-bond stretching (3500-3900 cm^-1) can assist in the identification of serpentine-group minerals and talc samples with various cationic compositions at the octahedral site. Secondly to establish quantitative relationships between the Raman signals (peak positions, integrated intensities, and full widths at half maximum) and the crystal chemistry of these phyllosilicates. We demonstrate that the quantification of ^MMg and ^M(Fe²⁺+Mn) contents in talc from the Raman spectroscopic analysis is as accurate as from EMPA. Regarding serpentines, ^MMg and ^MFe²+ amounts can be determined as well with a relative precision of ~ 2 and 5%, respectively.