Optical constants of powdered limestone obtained by taking into account the grain shapes: Applicability to Martian studies

The modelling and the interpretation of infrared spectra exhibited by astronomical dusty objects require fair acquaintance with complex refractive indices, the so-called "optical constants", of cosmic analog materials. It turns out that the spectra of the latter, in case of a crystalline granular material, depend on the size and the shape of the grains and may differ from the spectra of the same material but in bulk form. This phenomenon can be very elegantly accounted for by considering optical lattice excitations specific to small particles, the so-called "surface modes". We present a study of the spectral behaviour, in the 1.5-62.5  (mu)m range, of the optical constants of a particulate sample of limestone, a typical carbonate material mainly composed of calcite (CaCO 3). Shape effects have been accounted for by considering a collection of randomly oriented ellipsoids with various continuous distributions of shape parameters. It is shown that in the spectral region around the bands at 32  (mu)m and 44  (mu)m, whose assignment to surface modes raises no doubt, the optical constants derived for various shape distributions are markedly different from each other. We find that the best agreement between laboratory and theoretical spectra is obtained for spheres while for two continuous distributions of ellipsoids the fits are slightly worse. In other words the optical constants, that describe best the interaction between electromagnetic radiation and our limestone sample, are those derived by using Mie theory (valid for spheres); this is in agreement with SEM analysis which indicates a spheroidal shape of the particles. Such conclusions, valid for limestone particles, cannot be extrapolated directly to other particles and/or materials, since every case has to be treated independently. They should nevertheless be helpful in avoiding the possible error of interpreting absorption spectra of particulate crystalline stuffs without taking into account the effects of particle size and shape.