Biotite Crystal Chemistry Determined Non-Destructively by Raman Spectroscopy

Stylianos Aspiotis

In the present paper, Raman spectroscopy, electron microprobe analysis and Mössbauer spectroscopy were combined in a research to study a series of 18 samples covering the whole phlogopite (KMg₃AlSi₃O₁₀(OH)₂) – annite (KFeAlSi₃O₁₀(OH)₂) solid solution series. By this, the development of a non-invasive technique for understanding the crystallochemical properties of biotite (general formula is written as A₁M₃T₄O₁₀X₂, M₃ = M₁M₂M₂) has been achieved. Phlogopite and annite belong to the biotite-groups minerals, which as subgroup of phyllosilicates that are frequently present in a variety of cultural-heritage objects, for instance in clay tablets (Uchida et al. 2015), seals (Zazoff 1983), earth pigments (Hradil et al. 2011), inscribed gems (Mihailova et al. 2021), ceramics (Bersani and Lottici 2016), and as minor mineral phases or weathering related products in common rock-based written artefacts, were selected for a truly non-destructive crystallochemical characterization.

The study used the Raman scattering parameters (peak positions ω, integrated intensities I, and full widths at half maximum; FWHMs) of the framework (15–1215 cm^-1) and OH-stretching phonon modes (3000–3900 cm^-1) to establish correlations between these features and the crystal chemistry of biotite. The findings of the study highlight several important points: (a) the ability to quantify ^MMg, ^MFe²⁺, and ^MFe³⁺ contents with a relative error of around 6%, 6%, and 8%, respectively, through the combination of the integrated intensities of OH-stretching peaks associated with different local configurations and the wavenumber of the vibrational mode near 190 cm^-1; (b) the estimation of ^MTi content based on the peak position and FWHM of the second strongest TO₄-ring mode at approximately 680 cm^-1, with a precision of 2%; (c) the quantification of ^TSi content derived from the position of the second peak related to TO₄-ring vibrations near 650 cm^-1; (d) the indirect calculation of ^TAl content in phlogopite by knowing the ^TSi amount and the challenges in estimating it for annite due to the potential presence of ^TFe³⁺; (e) the quantification of ^AK content by analysing the peak position resulting from T-O_b-T bond-stretching-and-bending vibrations around 730 cm^-1; and (f) the identification of interlayer-deficient biotites and F-rich phlogopite via distinct OH-stretching Raman peaks at approximately 3570 cm^-1 and 3695 cm^-1, respectively.

The study suggested the potential of employing Raman spectroscopy as a non-destructive analytical method for quantitatively assessing major elements (Mg, Fe, Si, Al, K) and a minor element (Ti) in the crystal chemistry of the biotite mineral group. However, it recognized the lower sensitivity of Raman spectroscopy compared to electron microprobe analysis, limiting its ability to detect trace elements effectively. This non-destructive approach holds particular significance in fields such as cultural heritage where samples cannot be pulverised or disassembled, hence such a detailed material profile can advance provenance studies.