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 Si0 2 , FeO total , Fe 3+ /Fe 2+ , and alkali contents were synthesized in a gas-mixing furnace and analyzed using electron probe microanalysis, Mossbauer 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 SiO 2 content increases (1031-1054 cm -1 with Si0 2 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 Na 2 O+K 2 O 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 Fe 3+ /Fe 2+ (0.05-1.17). Our results show that KK transformed mid-infrared spectra of basaltic glasses may be used to determine the Si0 2 contents in basaltic glasses, irrespective of FeO total and Fe 3+ /Fe 2+ , and the alkali contents if Al/(Al+Si) is known. These observations will aid in the interpretation of laboratory and remotely sensed IR spectra.