02370nam a2200217 a 450000100080000000500110000800800410001910000150006024500800007526000160015530000110017150000650018252018200024765000120206765000160207965000110209565000090210665300110211565300130212665300130213915677922004-09-29       1988    bl uuuu  m   00u1 u    #d1 aMACEDO, J.  aPreferential reduction of hematite over goethite in some oxisols in Brazil.  a1988.c1988  a43 f.   aThesis (Doctor of Philosophy) - Cornell University, [S. l.].  aResearch was conducted to evaluate iron transformations in a hydrosequence of oxisols from the Central Plateau of Brazil. The best-drained member of the hydrosequence of soils selected for the study classifies as a Typic Acrustox (Dark-Red Latosol) and has codominant hematite and goethite iron oxide mineralogy and reddish hues (2.5 YR). X-ray diffraction and chemical extraction techniques were used to make detailed analyses of the iron oxides. Peak positions for hematite are coincident with those reported for normal hematite. Peak positions for goethite are shifted toward a lower dspacing, which corresponds to 38 mole % of Al00H substituting for Fe00H. Soil samples from the Bo horizon were subjected to reducing conditions in the laboratory over a range of timeperiods, using sucrose as a source of energy to speed yp the activity of the soil microorganisms. Reducing conditions in other samples were induced by citrate-bicarbonate-dithionite treatment at room temperature, using increasing additions of dithionite to achieve a range in reduction of these soils. In both experiments, the observed changes in soil minerologies confirm the preferential reduction and removal of hematite over goethite. The dithionite reduction of aluminum-substituted goethite is temperature dependent. The electrode potential of hematite and aluminum-substituted goethite are 98 and -2 mV respectively. Mineralogies and soil colors after reduction treatments are comparable to that of the Typic Acrustox (Red-Yellow Latosol) found in the wetter section of the hydrosequence. The results of this study support the hypothesis that differences in colors and iron mineralogies of soils in this hydrosequence are the result of the preferential reduction of hematite over goethite in the presence of a seasonally high water table.  aCerrado  aMineralogia  aOxisol  aSolo  aBrasil  aGoethita  aHematita