Near-infrared (0.67–4.7 μm) optical constants estimated for montmorillonite

Optical constants are estimated from near-infrared reflectance spectra of the smectite clay montmorillonite using an iterative combination of particulate scattering theory and Kramers–Kronig analysis. The final results provide wavelength dependent imaginary indices of refraction similar to previous studies, but in addition produce wavelength dependent real indices of refraction, unlike most previous studies. A comparison of the values derived here to those previously reported for montmorillonite shows that the maxima absorption coefficient values from the two studies are in excellent agreement, and in other wavelength regions the two studies agree to within about a factor of two. This suggests that the approach presented can provide a reliable mechanism of estimating the wavelength dependence of both the real and imaginary indices for materials at visible and near-infrared wavelengths. The resulting near-infrared values are combined with mid-infrared values of the same sample to provide optical constants of montmorillonite over the 0.66–25 μm wavelength region.