Spectroradiometric discrimination of laterites with principal components analysis and additive modeling

Bidirectional reflectance factor determinations in the visible and near-infrared of mineralized laterite samples collected at the surface of the alkaline / ultramafic rocks-carbonatite complex of Maicuru, northern Brazil, were evaluated for iron, phosphate, and titanium content. Nineteen spectra were subjected to principal components analysis in order to get a better insight into the factors affecting spectral discrimination. The spatial distribution in a PC1 vs. PC2 plot is related to the overall reflectance (albedo) along the PO axis and to the spectral inversion around 1100 nm along the PC2 axis. The latter can be characterized by any spectral ratio of reflectance values taken before and after the inversion wavelength. Two major groups were thus identified: titanium-rich (anatase, ilmenite) laterites and aluminum phosphate laterites. The former presented spectra with low to intermediate albedos and lower near-infrared / visible and very near-infrared reflectance ratio values. The phosphate-rich samples presented intermediate to high albedos and higher reflectance ratios. Titaniferous laterites have a greater content of opaques than the phosphates which are usually richer in iron oxides. The presence of titanium lowers the albedo whereas iron oxides absorb only in the visible and very near-infrared. Binary mixtures of Fe 20 3 and carbon provided spectral variations analogous to the ones observed in the titaniferous laterites. Such modeling helped in evaluating the effects of opaques. As a practical result, a band radiometer could conceivably identify the titanium mineralized areas in the field.