Crystal chemistry of the clinochlore – chamosite solid-solution series quantified by Raman spectroscopy

Chlorites are rock-forming layered silicates with a predominantly trioctahedral cation arrangement. The clinochlore (nominally Mg₅Al(Si₃Al)O₁₀(OH)₈) – chamosite (nominally Fe²⁺₅Al(Si₃Al)O₁₀(OH)₈) series are the most widespread chlorite solid solution, which occurs in diverse geological settings. These can range from sedimentary rocks and low- to medium-grade metapelites and metagreywackes in oceanic crust, to hydrothermal settings as the typical alteration products of ferromagnesian magmatic minerals and deep-seated ultramafic hydrated peridotite mantle wedge reaching depths of 120 km ^[1-3]. Chlorite commonly contains up to 13 weight percent H₂O, contributing therefore in the volatile cycling and mass transport in subducting lithosphere. Thus, the accurate crystallochemical characterization of chlorites while they are still intact in the original mineral assemblages, like in thin sections prepared for polarization microscopy, will provide a better insight into how these layered silicates formed and changed over time. Furthermore, the determination of the crystallochemical composition of chlorites can broaden the knowledge in fields where sampling is either too complicated, e.g. extraterrestrial missions on Mars, or entirely prohibitive, for instance in cultural heritage ^[4]. For the latter, a non-destructive, non-invasive (i.e. preparation-free) and non-destructive (object remains intact during the measurement) analytical approach is needed, to establish quantitative relationships between the crystallochemical composition and the Raman signals of chlorite, similar to the strategies developed for other complex hydrous silicates ^[5-6]

 

This study focuses on a series of 11 chlorite-group minerals from the collection of the Mineralogical Museum, Hamburg, covering the whole clinochlore – chamosite solid-solution series, which were analyzed by Raman spectroscopy, wavelength-dispersive electron microprobe analysis (WD-EMPA), and Mössbauer spectroscopy. The goals were (i) to build up quantitative correlations between the Raman scattering of both framework (15-1215 cm^-1) and OH-bond stretching (3200-3800 cm^-1) vibrations with the chemical composition of chlorites, particularly when partitioning of Fe²⁺ and Fe³⁺ over the tetrahedral and octahedral sites is known., and (ii) to understand how the Raman spectral pattern depends on chlorite orientation. We demonstrate that tetrahedrally coordinated Si and Al can be quantified from the position of the strongest Raman peak at ~ 675 cm^-1 , arising from the TO₄-ring mode, whereas the amounts of octahedrally coordinated Mg, Fe²⁺ and Fe³⁺ can be quantitatively estimated through the fractional intensities of the the multi-component Raman scattering generated by the OH stretching, with typical peaks at ~ 3678, 3655, 3625, 3587, and 3570 cm^-1 assigned to specific local chemical configurations.

 

References

 

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[2] G. Manthilake, N. Bolfan-Casanova, D. Novella, M. Mookherjee, D. Andrault, Sci. Adv. 2016, 2, e1501631

[3] A. Steudel, R. Kleeberg, C. Bender Koch, F. Friedrich, K. Emmerich, Appl. Clay Sci. 2016, 132-133, 626.

[4] S. Aspiotis, A. Dietz, Z. Földi, F. Hildebrandt, J. Schlüter, B. Mihailova, J. Raman Spectrosc. 2025, 56, 228.

[5] S. Aspiotis, J. Schlüter, G.J. Redhammer, B. Mihailova, Eur. J. Mineral. 2022, 34, 573.

[6] N. Waeselmann, J. Schlüter, T. Malcherek, G. Della Ventura, R. Oberti, B. Mihailova, J. Raman Spectrosc. 2020, 51, 1530.