VISIBLE SPECTROMETRIC INDEXES OF HEMATITE (HM) AND GOETHITE (GT) CONTENT IN LATERITIC SOILS - THE APPLICATION OF A THEMATIC-MAPPER (TM) IMAGE FOR SOIL-MAPPING IN BRASILIA, BRAZIL

Data on mineralogy and diffuse reflectance spectra, obtained from 56 s amples of Brazilian lateritic soils, were used to establish quantitati ve relationships between spectral parameters and iron oxides contents (hematite and goethite). A redness index RIH calculated from Helmholtz chromatic co-ordinates (lambda(d), P-e, Y) and correlated to hematite content is found. A good estimate of the ratio Hematite/(Hematite + G oethite) is given by the dominant wavelength lambda(d) which takes int o account a large proportion of the tint detected on the soil sample. Based on these laboratory results, radiometric indices were elaborated by combining only those parts of the visible spectrum that correspond to Thematic Mapper (TM) channels: (i) a hematite index I-Hm to estima te hematite concentration; and (ii) a ferric index I-Fe to estimate th e ratio hematite/(hematite + goethite). The hematite index, applied to a TM image of the area around Brasilia, allowed hematite contents in the surface horizons of lateritic soils to be mapped accurately.