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Effect of SiO₂, total FeO, Fe³⁺/Fe²⁺, and alkali elements in basaltic glasses on mid-infrared spectra

Céleste D.M. Dufresne^1,*, Penelope L. King^1,2, M. Darby Dyar³ and Kim N. Dalby^1,

¹ Department of Earth Sciences, The University of Western Ontario, London, Ontario N6A 5B7, Canada
² Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
³ Department of Astronomy, Mount Holyoke College, South Hadley, Massachusetts, 01075, U.S.A.

Correspondence: ^* E-mail: celeste.dufresne{at}gmail.com

A suite of basaltic glasses were examined to determine how subtle compositional changes affect mid-infrared spectra (650 to 5400 cm^–1). Glasses with different SiO₂, FeO_total, Fe³⁺/Fe²⁺, and alkali contents were synthesized in a gas-mixing furnace and analyzed using electron probe microanalysis, Mössbauer spectroscopy, and micro-reflectance Fourier transform infrared spectroscopy. The major mid-infrared spectral feature in silicate glasses is a broad peak located at ~900 to 1100 cm^–1 arising from Si-(Al-)O asymmetric stretching vibrational modes. To accurately compare spectra of different glass compositions, we have applied the Kramers-Kronig (KK) transform to our spectra and examined the resulting absorption peak (KK abs. peak). The location of the KK abs. peak shifts to higher wavenumbers as SiO₂ content increases (1031–1054 cm^–1 with SiO₂ from 47.18 to 55.57 wt%). For basaltic glasses with near-constant Al/(Al+Si), the full-width half maximum of the KK abs. peak decreases as alkali content increases (235–188 cm^–1 with Na₂O+K₂O contents from 0.07 to 3.74 wt%). In contrast, the location and shape of the KK abs. peak are not affected by variations in total FeO (6.06–16.30 wt%) and Fe³⁺/Fe²⁺ (0.05–1.17). Our results show that KK transformed mid-infrared spectra of basaltic glasses may be used to determine the SiO₂ contents in basaltic glasses, irrespective of FeO_total and Fe³⁺/Fe²⁺, and the alkali contents if Al/(Al+Si) is known. These observations will aid in the interpretation of laboratory and remotely sensed IR spectra.

Key Words: IR peak location and shape • glass composition • Mössbauer spectroscopy • oxygen fugacity