Introduction: Visible to near-infrared (VNIR) spectroscopy is an incredibly important and useful tool for the identification and characterization of minerals and mineral assemblages across planetary bodies. Of particular interest is the potential to quantitatively analyze remote sensing VNIR reflectance data that cover a wide area (e.g., the 18 m/pixel spatial resolution of the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) targeted images [1]) and therefore likely contain a combination of minerals over that area. Thus, the spectra gathered by remote sensing instruments reflect mixtures of minerals that we would like to deconvolve into its endmember abundances. This spectral unmixing is often done by employing radiative transfer theory, which requires knowledge of the real and imaginary indices of refraction (n and k, respectively) for the minerals of interest, which are also often referred to as optical constants. Using these optical constants for a mineral, paired with a chosen grain size, the bulk single scattering albedo (SSA) can be determined. From there, SSA spectra can be combined linearly to produce spectra that match the remote sensing data. Unfortunately, optical constants have not yet been derived for many minerals. To facilitate more accurate modeling of carbonatebearing areas on Mars, we are building on the library of optical constants presented in [2 and 3] to include a suite of carbonate minerals: calcite (Ca-carbonate), aragonite (Ca-carbonate), magnesite (Mg-carbonate), siderite (Fe-carbonate), and rhodochrosite (Mncarbonate). We are using the MATLAB code documented in [4]. In this abstract, we present preliminary n and k results for calcite. Samples and Data Collection: Sample preparation: Each of the samples was sieved into three size fractions: 45-63 μm, 63-90 μm, and 90-125 μm. The samples were crushed into powder using a mortar and pestle and then dry-sieved into the three size fractions. After the initial sieving, the samples were dry-sieved once more before being washed with pure ethanol to remove residual fines. After drying the samples were dry-sieved one last time to verify the size fractionation. VNIR data collection: VNIR spectra were collected using the ASD FieldSpec3 Max spectrometer at Stony Brook University. The incidence angle of the light (i) was held constant at 45°, while the emergence angle (e) varied from 0° to -60° at 10° intervals, resulting in the following phase angles (g = i + |e|): 45°, 55°, 65°, 75°, 85°, 95°, and 105°. The sample was held a constant distance away from both the light source and detector at every angle. We optimized the detector and gathered a white reference for every angle and sample. As an improvement on the setup described in [2 and 3], we have added a small table that spins at a constant rate of 4 revolutions per minute to acquire a more accurate mean sample spectrum compared to the previous protocol of manually rotating the sample cup four times during data collection. Data calibration: After collection, the data were corrected to account for changes in the absolute reflectance of the Spectralon standard over time and according to [5] to correct for photometric effects of the Spectralon standard.

Vnir optical constants for a diverse suite of carbonate minerals

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Vnir optical constants for a diverse suite of carbonate minerals