VNIR reflectance spectra of silicate-graphite mixtures: The effect of graphite content and particle size

We report the VNIR (350-2500 nm) reflectance spectra of a suite of silicic samples mixed with graphite. We used three end-member materials with different chemical composition in order to have a comprehensive understanding of the effect of opaque minerals on reflectance spectra of silicic rocks. To decouple the effect of granulometry and graphite content on reflectance properties, we first measured the reflectance spectra for each end member at different grainsizes (between similar to 20 to 250 mu m). Three selected grainsizes for each end-member were then mixed with graphite in the graphite/end-member weight ratio 1 to 5%. For each spectrum we evaluated the main band parameters (position, area, depth). Moreover, we also proposed and discussed the use of an additional parameter, the band centroid. Our results confirm that graphite mixed with silicate materials reduces albedo and decreases the spectral slopes and the spectral contrast of the mixtures. We discussed the subtle interplay between grain size, graphite content and chemical composition of the mixtures. We showed how graphite decreases the spectral slopes of the graphite-silicate mixtures proportionally to iron content (albedo) of the mixtures: low iron (bright) materials are more sensitive to spectral slope variation (decrease) as a function of graphite content. Finally we show how spectral slope influences the measured band parameters.