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Written Artefact Profiling Guide
Written ArtefactProfiling Guide

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Written Artefact Profiling Guide

Serpentine and Talc Crystal Chemistry Determined Non-Destructively by Raman Spectroscopy

Stylianos Aspiotis

The present paper reports the results of comprehensive Raman spectroscopic analyses combined with electron microprobe analysis on a set of 10 talc (nominally MMg3TSi4O10X(OH)2) and 22 serpentine (nominally MMg3TSi2O5X(OH)4) minerals. The main objective of this research was to establish precise quantitative correlations between the presence of octahedrally coordinated cations at the M-site of the abovementioned minerals and specific Raman peak parameters (peak positions ω, integrated intensities I, and full widths at half maximum; FWHMs) by following the already established methodological approach in biotite-group minerals (Aspiotis et al. 2022). These correlation trends were crucial in facilitating the noninvasive characterization of materials. Talc and serpentine minerals were selected as they can be frequently found in rock-based cultural heritage objects, for instance in cylinder seals (Beran 1968; Zazoff 1983; Collon 2005) and inscribed gems (e.g. Mihailova et al. 2021).

The analysis of talc unveiled that a distinct peak located around 360 cm-1, corresponding to MO6 vibrations, together with the integrated intensities of the vibrational modes within the OH-stretching range (3600–3750 cm-1), effectively reflected the concentrations of MMg and M(Fe2+ + Mn2+) with a degree of accuracy comparable to EMPA measurements. In the case of serpentine, the study showed how the overall pattern of Raman scattering linked to OH-stretching modes (3600–3750 cm-1) could serve as an initial means of differentiation between the various serpentine polymorphs, i.e. antigorite, chrysotile, and lizardite, including also polygonal/polyhedral serpentine varieties dominated by lizardite or chrysotile. Furthermore, the FWHM of the predominant MO6 peak at approximately 380 cm-1 emerged as a reliable indicator of quantifying the MMg content within each polymorph/variety analysed, with a relative error of 5%. The quantification of MFe2+ amount within chrysotile and chrysotile-rich polygonal serpentine was correlated with the FWHM of the peak around 230 cm-1, exhibiting a similar relative error of 5%. Similarly, for antigorite, the FWHM of the peak close to 130 cm-1 was proven sensitive to Fe2+ content within the octahedral layers and was used for its quantification with comparable levels of uncertainty.

The effectiveness of this analytical methodology was successfully demonstrated through its application on valuable selected cultural-heritage artefacts belonging to the collections of Museum für Kunst und Gewerbe Hamburg (MKG), in particular on two Babylonian cylinder seals and two inscribed gems dated to the Late Antiquity (4th–3rd century BCE), where traditional sampling methods were unfeasible.