Calibrations of phase abundance, composition, and particle size distribution for olivine‐orthopyroxene mixtures from reflectance spectra

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Abstract
Spectral reflectance measurements of characterized (phase abundance, particle size) mixtures of olivine and orthopyroxene were utilized to define the correlations between spectral and albedo parameters of such assemblages and their mineralogical or textural properties. Thirty‐three different spectral parameters falling into three general classes (relative or ratioed, absolute or albedo, and wavelength) were investigated for empirical sensitivity to one or more of the mixture properties. Theoretical considerations and previous experimental observations were utilized to understand their functional relationships. The ratio of areas for the 1‐ and 2‐μm absorption bands is shown to be a sensitive indicator of the olivine‐orthopyroxene abundance and is very nearly independent of particle size and mineral composition. In conjunction with an abundance determination, the wavelength position of the 1‐μm absorption feature can be utilized to determine the molar iron contents of the olivine and orthopyroxene phases. This calibration is insensitive to particle size but will produce systematic deviations if the phases have significantly different iron contents or if more than a few percent of a clinopyroxene component is present. The spectral albedo in the 0.6‐ to 0.7‐μm region is relatively insensitive to phase abundance and can be used to constrain particle size if phase composition has been determined.