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Iron Oxide Mineralogy in Some Brazilian Oxisols

出版	North Carolina State University, 1988
URL	http://books.google.com.hk/books?id=GsoPywAACAAJ&hl=&source=gbs_api

註釋Twelve Brazilian Oxisols from the Triangulo Mineiro region, in the state of Minas Gerais, derived from four different parent meterials were sampled and studied to provide insight into their iron mineralogy. This area was chosen because the climatic conditions are homogeneous but there is a good deversity of soil parent materials. The general mineralogy of all soils consisted of kaolinite and iron most of the soils and maghemite was detected in several of them. Citrate-dithionite (CD) treatment of the soil clays showed hematite preferentially dissolved compared to goethite and a higher dissolution rate for poorly crystalline than for weell crystalline goethite. The clculated values for Al-substitution in the Fe oxides based on the CD extracts of the total claycorrected for Al soluble in acid ammonium oxalate gave good agreement with Al substitution determined by differential x-ray diffraction and Al substitution calculated for the samples treated for gibbsite removal. Aluminium-substituted maghemite, detected by DXRD, was present only in soils from mafic rocks, suggesting its formation through a slow oxidation of the magnetite present in the parent material. Aluminium substitution determined by DXRD varied from 17-36 mole % for goethites; 6-14.5% for hematites, and 16-26% for maghemites. The mean crystallite dimension (MCD) of hematite and goethite also determined from the DXRD showed a preferential crystal development in the X-Y direction of some hematite samples indicating its platy nature. (...).