Modeling Wavelength Dependent Mid-Infrared (5.5-25 m) Optical Constants of Silicate Glasses: A Genetic Algorithm Approach

Wavelength-dependent mid-infrared (400-1,800 cm(-1); 5.56-25 mu m) optical constants (real and imaginary indices of refraction; n and k) are determined using reflectance spectra at a spectral sampling of 4 cm(-1) for several silicate glasses of varying SiO2 wt% which include (a) basaltic volcanic glass from Kilauea, Hawaii, (b) synthetic andesite, (c) two synthetic dacites, (d) obsidian volcanic glass from Mount Lassen, California, (e) synthetic rhyodacite, and (f) rhyolitic volcanic glass from Mexico. Because glasses are optically isotropic, no specific orientation was required for spectral measurements, and polished glass samples were measured at random orientations using micro-FTIR spectrometer. Lorentz-Lorenz dispersion theory and Fresnel reflectance model for high symmetry materials were used to model the optical constants by optimizing oscillator parameters in modeled spectra to match laboratory spectra. A genetic algorithm (GA) approach automatically finds the natural oscillators and their parameters for each spectrum, and then uses these parameters in a non-linear least squares optimization routine. The study compared spectral parameters such as Christiansen feature, reststrahlen bands (RBs), and the peaks centered around 860-1,100 (peak 1) and 400-480 cm(-1) (peak 2) of n and k to their respective SiO2 wt% of the glasses. CF, RBs, peak 1 of n and k, and peak 2 of n shift to higher wavenumbers with increased SiO2 wt%, whereas peak 2 of k shifts to lower wavenumbers with increased SiO2 wt%. Derived optical constants of these glasses will improve quantitative abundance mapping of volcanic materials on the surfaces of silicate targets in the Solar System. Plain Language Summary The effective mapping of minerals and their abundances on a planet's surface from orbit is enabled by understanding the nature of the interaction of light of varying wavelengths with the planetary surface. The reflected/emitted light from the surface directly depends on the optical properties of the materials and their interaction with each other. In this study, we numerically modeled the mid-infrared optical constants of silicate glasses using a genetic algorithm (GA) approach. The GA allows the model to automatically locate the natural harmonic oscillators and their parameters that are responsible for creating the spectral signals that can be detected by spectrometers on orbital spacecraft. Derived optical constants of the glasses could be used in radiative transfer models for quantitative mineral mapping applications. Glasses on planetary surfaces are common in volcanic and/or impact materials and the mid-infrared spectral range has been widely used to map the surface mineralogy of planets including the Moon and Mars. Mapping the composition and abundance of these glasses therefore will help us to understand the thermal evolution of the planets themselves.