Midinfrared spectroscopy of synthetic olivines: Thermal emission, specular and diffuse reflectance, and attenuated total reflectance studies of forsterite to fayalite

Synthetic olivine samples ranging in composition from forsterite to fayalite are analyzed in the midinfrared using thermal emission, specular and diffuse reflectance, and attenuated total reflectance spectroscopies to study the spectral effects of Mg-Fe solid solution. For each method, fundamental bands gradually change in position and strength from Mg₂SiO₄ at larger wave numbers to Fe₂SiO₄ at smaller wave numbers. Each spectrum is diagnostic of chemistry within the continuum, as previously noted. In this study, 10 identified fundamental bands are traceable across the solid solution series for each technique. In pelletized sample spectra, the 10 bands shift approximately linearly in position by as little as 11 to as much as 64 cm^-1. In powdered sample spectra, the bands shift by as little as 12 to as much as 74 cm^-1 (disregarding one outlier point). Moreover, for every spectral technique, an even larger linear shift is identified of a specific emissivity maximum/reflectivity minimum (the flection position). From forsterite to fayalite, this flection position shifts by at least 88 cm ^-1, which is, on average, 48% more than the largest fundamental band shift within the same data set for the pelletized spectra and 44% more for the powdered spectra. Also the R² and 2 values of the best fit line for the flection position shift (versus Fo_#) generally were as good as or routinely better than those of the fundamental bands. Thus, the flection position should be considered as a means of determining Mg-Fe olivine composition when using thermal emission, specular reflectance, diffuse reflectance, or attenuated total reflectance spectroscopic data.