The imaginary coefficients of the complex index of refraction of olivine, low-Ca orthopyroxene, and high-Ca clinopyroxene are derived as a function of iron content and wavelength over the spectral region 0.4 to 2.5 μm from reflectance spectra of powders and Hapke radiative transfer theory. Reflectance spectra as a function of composition and grain size are computed and compared. I find that while spectral effects of variations in these parameters are similar, they are distinguishable. Variations in iron content do not explain all the observed spectral variations in the spectra of the samples investigated. ¿ 1998 American Geophysical Union