The role of very fine particle sizes in the reflectance spectroscopy of plagioclase-bearing mixtures: New understanding for the interpretation of the finest sizes of the lunar regolith

•5–20% of lunar regolith is very fine grained (<10µm).•Very fine sizes increase the albedo but reduce the spectral contrast.•Very fine plagioclase has featureless spectrum.•Mafic materials are affected by fine sizes.