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

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.
