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

Abstract

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, FeOtotal, Fe 3+/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/(A1+Si), the full-width half maximum of the KK abs. peak decreases as alkali content increases (235-188 cnr-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 FeOtotal and Fe3+/ Fe2+, and the alkali contents if Al/(A1+Si) is known. These observations will aid in the interpretation of laboratory and remotely sensed IR spectra.