Modeling Wavelength Dependent Mid‐Infrared (5.5–25 μm) Optical Constants of Silicate Glasses: A Genetic Algorithm Approach

Abstract

Wavelength‐dependent mid‐infrared (400–1,800 cm−1; 5.56–25 μm) 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 Kīlauea, 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.

Compositional Characterization of Glassy Volcanic Material From VSWIR and MIR Spectra Using Partial Least Squares Regression Models

“…In the VSWIR, glasses have shallower absorption features and lower albedo than crystalline material of the same composition (e.g., Minitti et al, 2002;Moroz et al, 2009). In the MIR, glasses display a shift in the position of the primary silicate stretching features to shorter wavelengths (longer wavenumbers) with increasing polymerization, similar to what is observed with minerals (e.g., Fu et al, 2017;King et al, 2004;Minitti et al, 2002;Pisello et al, 2019;Varatharajan et al, 2023). In spectra of intimate mixtures, low modal abundances of minerals (≥20% of the phase assemblage; Cannon et al, 2017;Horgan et al, 2014;Minitti et al, 2002) can obscure spectral features of a glass phase.…”