Bi-directional (normal incidence and observation angle of 30 °) infrared spectra of pure synthetic goethite (blue dots) and synthetic goethite/basalt mixtures (red and black dots). Spectra were measured at T = 80 °C and under vacuum conditions. Because the 3 μm band of pure goethite is close to saturation, we calculated the ratio of the spectra of the two mixtures (green dots, offset and rescaled for clarity) to retrieve the shape of a “diluted” goethite. Solids lines correspond to moving averages.