Fine-grained dust coatings on Martian rocks and soils obscure underlying surfaces and hinder mineralogic interpretations of both remote sensing and in situ observations. We investigate laboratory visible/near-infrared spectra of various thickness of palagonite coatings on basalt substrates. We develop a two-layer Hapke scattering model incorporating porosity, grain size, and derived absorption coefficients of palagonite and basalt that reproduces the observed spectra only when the single scattering particle phase function is varied with wavelength. ¿ 2001 American Geophysical Union