Non-destructive determination of the biotite crystal chemistry using Raman spectroscopy: how far we can go?

Abstract

Abstract. Raman spectroscopy combined with electron microprobe analysis as well as Mössbauer spectroscopy was applied to a series of 18 samples along the phlogopite (KMg3AlSi3O10(OH)2)–annite (KFe32+AlSi3O10(OH)2) join to establish a truly non-destructive method for crystallochemical characterization of biotite (A1M3T4O10X2, M3 = M1M2M2). The Raman scattering arising from the framework (15–1215 cm−1) and OH-stretching phonon modes (3000–3900 cm−1) was used to build up correlation trends between the Raman spectral features and crystal chemistry of biotite. We show that (a) the contents of MMg, MFe2+, and MFe3+ contents can be quantified with a relative error of ∼ 6 %, ∼ 6 %, and ∼ 8 %, respectively, by combining the integrated intensities of the OH-stretching peaks assigned to various M1M2M2 local configurations with the wavenumber of the MO6 vibrational mode near 190 cm−1; (b) the MTi content can be estimated from the peak position and FWHM (full width at half maximum) of the second strongest TO4-ring mode at ∼ 680 cm−1, with a precision of 22 %; (c) the content of TSi can be estimated from the position of the second peak related to TO4-ring vibrations near 650 cm−1; (d) for phlogopite the TAl content can indirectly be calculated by knowing the amount of TSi, whereas for annite it is hindered by the plausible presence of TFe3+; (e) the AK content can be quantified by the position of the peak generated by T-Ob-T bond-stretching-and-bending vibration at ∼ 730 cm−1; and (f) interlayer-deficient biotites and F-rich phlogopite can be identified via their unique OH-stretching Raman peaks around 3570 cm−1 and 3695 cm−1, respectively. Our results show a potential tool for non-destructive quantitative estimations of the major (Mg, Fe, Si, Al, K) and minor (Ti) elements of the crystal chemistry of the biotite mineral group by using a non-destructive technique such as Raman spectroscopy, although its sensitivity is generally lower than that of electron microprobe analysis and therefore cannot detect trace elements. This is fundamental within the framework of cultural heritage where samples cannot be powdered or disassembled.