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

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, 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 FeO_total 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.