Orientation Averaged Visible/Near-Infrared and Mid-Infrared Optical Constants of Hydrous Ca-Sulfates: Gypsum and Bassanite

The hydrous Ca-sulfates, gypsum and bassanite, have been detected on Mars using both visible/near-infrared (VNIR) remote sensing measurements and in-situ rover-based mineralogical measurements. Understanding the distributions and abundances of minerals on the Martian surface from remote sensing data is important in evaluating ancient Martian environments. Optical constants are critical input parameters in radiative transfer theory, which allows for the extraction of mineral abundances from a mixture spectrum. We have derived VNIR (0.35–2.5 μm, at 10 nm spectral sampling) optical constants of gypsum and bassanite powders using radiative transfer theory and laboratory reflectance spectra of separated size fractions as well as multiple phase angles. In addition, the MIR (2.5–25 μm, or 4,000–400 cm⁻¹, at 2 cm⁻¹ spectral sampling) orientation averaged optical constants of gypsum and bassanite were generated using the classical Lorentz-Lorenz dispersion theory. We compare the optical constants of gypsum derived in this work with previously published estimates and note both similarities and differences. The optical constants derived in this work will assist in modeling VNIR reflectance spectra and quantifying mineral abundances from mineral mixtures.