• Lagoeiro, Leonardo
• Barbosa, Paola
• Nogueira, Victor Mota E.

Abstract

<jats:p>The transformation behavior between iron oxides and oxyhydroxides like magnetite, hematite and goethite is still not entirely understood. Crystallographic similarities allow one to predict topotactic relationships between them. The related crystallographic aspects have been explored by means of electron backscatter diffraction (EBSD). Samples from natural aggregates of magnetite, hematite and goethite were collected from iron-formation rocks that outcrop in the southeast of Brazil, in a region known for large deposits of iron and other ores. EBSD misorientation data indicate a pronounced relationship between these iron phases. The transformation seems to be related to the oxygen framework. The original close-packed cubic arrangement of O atoms in magnetite changes to a hexagonal close-packed framework in hematite, <jats:italic>i.e.</jats:italic> {111} of magnetite is parallel to {0001} of hematite. The matrix in which the magnetite grains are embedded is made of aggregates of hematite and goethite. In the matrix, the coincidence observed in the magnetite grains is not observed. On the other hand, the well known twin boundary relationships already described for these mineral phases can be observed here as an orientation relationship in three-dimensional misorientation space. The spatial data of axis–angle pairs suggest that the twin boundaries serve as facilitating surfaces for phase transformation in the Fe–O–OH system. The main conclusion of this study is that the transformation occurred in the solid state with the newly transformed goethite inheriting the crystallographic orientations of the former crystals and that this phenomenon is better recognized by combining observations of misorientation relationships. In the matrix, such a relation is not observed and, for that reason, the minerals in the matrix are thought to have been formed by a different process, which might have involved dissolution and precipitation of the phase newly out of solution.</jats:p>

Topics

• impedance spectroscopy
• mineral
• surface
• grain
• phase
• Oxygen
• precipitation
• iron
• electron backscatter diffraction
• twin boundary