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Soil Mineralogy

Color Attributes and Mineralogical Characteristics, Evaluated by Radiometry, of Highly Weathered Tropical Soils

^a Dep. de Solos, Univ. Federal de Viçosa, 36570-000, Viçosa, MG, Brazil
^b Faculdade Politécnica de Uberlândia, 38400-436, Uberlândia, MG, Brazil

^* Corresponding author (mpfontes{at}ufv.br)

Highly weathered soils are important around the world. Determination of their observed colors and acquisition of spectral curves were used to study their mineralogy. Color was determined through visual comparison with Munsell Soil Color Charts, and soils spectral characterization was made with an infrared intelligent spectroradiometer (IRIS) covering a spectral window from 300 to 3000 nm. The variables of color behaved differently among soils. The Munsell hue was related to hematite content and the hematite/goethite ratio (Hm/Gt), and the Munsell value to organic content of goethitic soils. The Munsell chroma was related to the presence of Fe oxides, giving an idea of relative amounts of either hematite or goethite. Regression equations relating color indices showed that the redness factor (RF) gave the highest determination coefficients to estimate hematite content and Hm/Gt, and reconfirmed that redness saturation occurs with increasing hematite amounts. The typical minerals of these soils showed characteristic absorption bands in their spectral curves. Absorption bands of 450 and 900 nm for Fe oxides, 2200 nm for kaolinite and 2280 for gibbsite, were distinctive features of the main minerals present. Organic matter decreased the intensity of reflectance for most soils and, under its influence, the characteristic bands of goethite practically disappeared while the ones for hematite, kaolinite, and gibbsite were attenuated. High amounts of hematite decreased the reflectance factor strongly as compared with the soil organic matter (SOM). Qualitative mineralogy of highly weathered soils was assessed by radiometry using measurements in the visible and near infrared regions of the electromagnetic spectrum.

Abbreviations: BRF, bidirectional reflectance factor • HH, Humic Hapludox • Hm/Gt, hematite/goethite ratio • IRIS, infrared intelligent spectroradiometer • PT, Plinthaquox • RA, Rhodic Acrudox • RE, Rhodic Eutrudox • RF, redness factor • RH, Rhodic Hapludox • RR, redness rating • SOM, soil organic matter • TH, Typic Hapludox • XH, Xanthic Hapludox