Optical absorption spectra and Fe distribution in the structures of Li-Fe micas

• Published in: Geochemistry international Vol. 47; no. 2; pp. 174 - 185
• Main Authors: Platonov, A. N., Khomenko, V. M., Shuriga, T. N.
• Format: Journal Article
• Language: English
• Published: Dordrecht SP MAIK Nauka/Interperiodica 01.02.2009; Springer Nature B.V
• Subjects: Absorption; Crystal structure; Earth and Environmental Science; Earth Sciences; Geochemistry; Ions; Iron; X-ray diffraction; Charge Transfer Band; Optical Spectrum; Optical Absorption Spectrum; Octahedral Layer; Geochemistry International
• ISSN: 0016-7029; 1556-1968
• DOI: 10.1134/S0016702909020050

The paper proposes pioneering data on the polarized optical absorption spectra of Li-Fe micas: intermediate members of the siderophyllite-zinnwaldite-polylithionite and annite-protolithionite-zinnwaldite-trilithionite series with variable Fe and Li proportions and Li- and Fe-bearing muscovite (phengite). Based on the analysis of structural data, the complicated structure of the Fe 2+ → Fe 3+ charge transfer band in the mica structures is explained, and arguments are presented to justify the ascribing of its shortwave component (CTB-1, ν = 17200-14900 cm −1 ) to charge transfer in the pair Fe 2+ (M2) → Fe 3+ (M2) and the longwave component (CTB-2, ν = 14200-13600 cm −1 ) to charge transfer in the pair Fe 2+ (M1) Fe 3+ (M2). It is demonstrated that the anomalous shift of the superposition of two-component CTB toward the shortwave region, to 17000 cm −1 , results from a decrease in the length of oxygen edges between adjacent M2M2 and M1M2 tetrahedrons when Li is accommodated in the mica structure. The first data are presented on the spectrum of Fe 2+ ions in large distorted M3(M1) tetrahedrons (OAC Fe 2+ II) in hetero-octahedral Li micas (zinnwaldite), and the behavior of the corresponding absorption bands at 11400 and 8000 cm −1 is determined. It is proved that characteristics of the optical spectra of Fe 2+ ions can be used as an indicator of the structure of the octahedral layer in the mica structures. Results of the comparative analyses of spectral parameters of the Fe 2+ → Fe 3+ charge transfer, crystal field spectra of Fe 2+ ions, and the crystal-chemical characteristics controlling them in micas of the polylithionite-siderophyllite series are completely consistent with the character of cation ordering in the crystal structures of these micas determined by X-ray diffraction analysis.