Near‐infrared optical constants of pyroxene as a function of iron and calcium content

We derive the imaginary coefficient of the complex index of refraction ( k ) from reflectance spectra of 30 pyroxenes using the method of Lucey (1998). We apply the modified Gaussian model of Sunshine et al. (1990) to these k spectra to obtain two continuum parameters and nine Gaussian parameters that describe the 1, 2, and 1.2 μ m crystal field absorptions. This method takes advantage of the redundancy in the data and minimizes the number of free parameters, resulting in a compact expression of optical constants that is less susceptible to noise in the data and contamination from terrestrial weathering. Multiple regression results indicate that the continuum and Gaussian parameters are well predicted by pyroxene FeO and CaO contents; thus a complete k spectrum for pyroxene can be derived from arbitrary FeO and CaO concentrations. The uncertainty is highest for the 1.2 μ m M1 crystal field absorption in both orthopyroxene and clinopyroxene. These k spectra can be used with radiative transfer theory to produce model reflectance spectra of particulate silicate surfaces at different viewing geometries, particle sizes, levels of space weathering, and mixtures with other minerals.