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

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+MO 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.

Hybrid magnetic graphitic nanocomposites towards catalytic wet peroxide oxidation of the liquid effluent from a mechanical biological treatment plant for municipal solid waste

“…1a, some changes in the magnetite phase occurred, maghemite (a = 8.379 Å), iron (a = 2.868 Å) and traces of hematite (proto) being identified in the diffraction pattern of Fe 3 O 4 /MGNC, in addition to magnetite (a = 8.343 Å) and graphite. Since magnetite nanoparticles are very sensitive to oxidation in the presence of oxygen, these phase modifications can be ascribed to the progressive oxidation of Fe 2+ ions in the inverse spinel structure of magnetite to Fe 3+ -most likely during the Fe 3 O 4 /MGNC synthesis step performed in liquid phase (i.e., in oxidative media), resulting in its partial oxidation to maghemite and hematite [33,34].…”

Electron Backscattering Diffraction (EBSD) as a Tool to Evaluate the Topotactic and Epitactic Growth of Minerals: The Example of the Magnetite and Hematite

“…The coincidence of planes and poles during the growth of hematite in a magnetite substrate or through a magnetite transformation was systematically described in previous studies [e.g., 3,4]. We assume that the growing of hematite on the {111} Mgt substrate is an example of an epitaxy instead of topotaxy that is clearly present in the transformation of magnetite to hematite in the [110] Mgt and [100] Mgt .…”

Micro- to nano-scale characterization of martite from a banded iron formation in India and a lateritic soil in Brazil

“…The magnetite replacement by hematite can occur during oxidation via O 2− addition or Fe 2+ loss (Lepp 1957;Davis et al 1968). Alternatively, a dissolution-precipitation process (chemically or biochemically mediated) can be controlled by pH changes (Brown et al 1997;Ohmoto 2003;Otake et al 2007) or by deformation (Lagoeiro 1998). All these processes have been proposed for banded iron formations (BIFs), where the magnetite-hematite transformation is common (Banerji 1984;Morris 1985;Ohmoto 2003;Mücke and Cabral 2005;Beukes et al 2008).…”