DIVISION S-9 SOIL MINERALOGY - IRON-OXIDES IN SELECTED BRAZILIAN OXISOLS .1. MINERALOGY

Twelve Oxisols from the Triangulo Mineiro region, Minas Gerais state, Brazil, derived from four different parent materials were studied to provide insight into their Fe-oxide mineralogy. The clay fraction ( < 2-mu-m) of all soils consisted of kaolinite and Fe oxides (hematite and/or goethite); gibbsite and anatase were found in most of the soils; maghemite was detected in several of them. Citrate-dithionite (CD) treatment of the soil clays showed hematite preferentially dissolved compared with goethite, and a higher dissolution rate for poorly crystalline than for well-crystalline goethite. The calculated values for Al substitution in the Fe oxides, based on the CD extracts of the total clay corrected for Al soluble in acid ammonium oxalate, and of clay treated for gibbsite removal gave fair to good agreement with Al substitution determined by differential x-ray diffraction (DXRD) for those samples in which the Fe-oxide fraction was dominated by either goethite or hematite. Aluminum-substituted maghemite, detected by DXRD, was present only in soils from mafic rocks, suggesting its formation through oxidation of the magnetite present in the parent material. Aluminum substitution, determined by DXRD, varied from 17 to 36 mol % for goethites, 6 to 15 mol % for hematites, and 16 to 26 mol % for maghemites. The mean crystallite dimension (MCD(hkl)) of some hematite samples determined from DXRD showed preferential crystal development in the X-Y direction, suggesting a platy nature.