The interpretation of reflectance spectra (though remote sensing or laboratory research) requires spectral reflectance models that can be inversed to determine the more fundamental physical properties of these surfaces. One of the main hypotheses in reflectance models is that a soil or a powder are generally considered as a collection of particles or aggregates with averaged optical properties. Verification of this hypothesis is of great interest. This paper deals with the study of the scattering and absorption of light by a single mineral particle where diameter ranges from 0.01 to 50 μm. The particles are assumed to be spherical, and their complex refractive index is assumed to be equal to that of hematite or goethite (ordinary indices). These two minerals are responsible for the nonlinear and wavelength-dependent behavior of lateritic soils reflectance spectra upon moistening. The absorption and scattering efficiencies, single scattering albedo, scattering diagram, and opening angle were computed for a single sphere. Different scenarios were considered: first, a sphere in the air, second, a sphere in water, and third, a sphere surrounded by a water layer of different thickness. All the computed spectra (400--700 nm) are classified into three diameter groups (0.01--0.1, 0.1--2, and 2--50 μm). The behavior of the calculated spectra (second and third scenarios) depends both on the particle diameter and the wavelength. A simplified Hapke's model was then used to simulate the reflectance spectra of powders of hematite and goethite. The main result is the existence of an optical elementary arrangement cell which is for the soil what a unit cell is for a crystal. ¿ American Geophysical Union 1993