Optimal Raman-scattering signal for estimating the Fe³⁺ content on the clinozoisite–epidote join

Abstract

Abstract. To provide a tool for fast estimation of the Fe3+ content in Ca2(Al, Fe3+)3Si3O12(OH) epidote grains, including in thin sections and crude-rock samples, we applied Raman spectroscopy to 33 areas from 15 natural samples with Fe3+ ranging from 0.22 to 1.13 atoms per formula unit (apfu), the chemistry of which was independently determined by wavelength-dispersive electron microprobe analysis (WD-EPMA). The Raman spectra were collected from the very areas subjected to WD-EPMA. We have analysed both the OH-stretching region (3215–3615 cm−1) and the spectral range generated by the framework vibrations (15–1215 cm−1). Similarly to the IR spectra, the Raman peaks in the OH-stretching region shift toward higher wavenumbers with increasing Fe. However, the quantification of Fe3+ based on OH-stretching Raman peaks can be hindered by the multicomponent overlapping and significant intensity variations with the crystal orientation. Among the Raman signals generated by framework vibrations, the position of four peaks (near 250, 570, 600, and 1090 cm−1) exhibit a steady linear regression with the increase in Fe content (in apfu). However, the peak near 250 cm−1 attributed to MO6 vibrations also depends on the crystal orientation and therefore is not always well resolved, which worsens the accuracy in Fe-content determination based on its position. The peaks near 570, 600, and 1090 cm−1 arise from Si2O7 vibrational modes, and although their intensities also vary with the crystal orientation, all three signals are well resolved in a random orientation. However, among the three Si2O7-related signals, the 570 cm−1 peak is the sharpest (peak width <10 cm−1) and is easily recognized as a separate peak. Hence, we propose to use the position of this peak as a highly reliable parameter to estimate the Fe content, via the linear trend given as ω570=577.1(3)-12.7(4)x, where ω is the wavenumber (cm−1) and x is Fe content (apfu), with accuracy ± 0.04 Fe3+ apfu. The peaks near 600 and 1090 cm−1 may be complementarily used for the Fe estimate, based on the following relations: ω600=611.6(2)-13.8(4)x and ω1090=1098.8(3)-13.5(5)x. Analyses of the effect of Sr as a substitution for Ca and Cr at the octahedral sites indicate that contents of Sr <0.12 apfu do not interfere with the quantification of Fe via the ω570 (x) relation, whereas Cr >0.16 apfu leads to overestimation of Fe; Cr presence can be recognized however by the broadening of the peaks near 95 and 250 cm−1.

Heflikite, ideally Ca2(Al2Sc)(Si2O7)(SiO4)O(OH), the first scandium epidote-supergroup mineral from Jordanów Śląski, Lower Silesia, Poland and from Heftetjern, Tørdal, Norway

“…Some Raman bands correspond closely to the bands observed in the spectra of clinozoisite and epidote (e.g. Liebscher, 2004;Limonta et al, 2022;Nagashima and Mihailova, 2023, and references therein) but most of them occur at slightly different wavenumbers owing to differences in the site occupancies among these minerals. According to Nagashima et al (2021), the most intense Raman band at 975 cm -1 in the spectrum of heflikite can be assigned to the Si-O stretching mode, the band at 568 cm -1 to the Si-O-Si bending mode, and the features in the regions around 100 cm -1 and 240 cm -1 are most probably connected to heavy-cation vibrations and external silicate modes.…”

“…The positive relationship between the concentration of Fe in the clinozoisite-epidote solid-solution series and the wavenumber of the bands' maxima has been firmly established (e.g. Langer and Raith, 1974;Della Ventura et al, 1996;Liebscher, 2004;Gatta et al, 2012;Nagashima et al, 2021;Limonta et al, 2022;Nagashima and Mihailova, 2023). In the Raman spectra of clinozoisite-epidote minerals, the position of the O-H stretching peak typically varies from ∼3340 cm -1 in Fe-poor crystals to ∼3390 cm -1 in Fe-rich species (e.g.…”

“…In the Raman spectra of clinozoisite-epidote minerals, the position of the O-H stretching peak typically varies from ∼3340 cm -1 in Fe-poor crystals to ∼3390 cm -1 in Fe-rich species (e.g. Limonta et al, 2022;Nagashima and Mihailova, 2023). Aside from this shift, a split of the O-H stretching band in piemontite is observed, caused by the Jahn-Teller effect related to the presence of Mn 3+ (Della Ventura et al, 1996;Liebscher, 2004).…”