Transformation of magnetite to hematite and its influence on the dissolution of iron oxide minerals

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Abstract
Deformed rocks of the Itabira Iron Formation (itabirites) in Brazil show microstructural evidence of pressure solution of quartz and iron oxides; it appears that magnetite was dissolved and hematite precipitated. The dissolution of magnetite seems to be related to its transformation to hematite by oxidation of Fe2+ to Fe3+. The transformation of magnetite to hematite occurs along {111} planes, and results in the development of hematite domains along {111} that are parallel to the foliation. The difference in volume created by the transformation of magnetite to hematite and the shear stress acting on the interphase boundaries allow fluids to migrate along these planes. The dissolution of magnetite involves the hydrolyzation of the Fe2+—O bonds at interphase boundaries of high normal stress. The high fugacity of oxygen in the fluid phase promotes the reaction of Fe2+ (in solution) with oxygen. Fe2+ ions oxidize to Fe3+ and precipitate as hematite platelets with their longest axes oriented parallel to the direction of maximum stretching. The transformation of magnetite to hematite during deformation plays an important role in the fabric evolution of the iron formation rocks. The transformation along {111} creates planes of weakness that facilitate fracturing. The fracturing plus the dissolution result in a reduction of magnetite grain size, and the oriented precipitation results in layers of hematite platelets. These processes produce a new fabric characterized by a penetrative foliation and lineation.

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