Characterization of the Crystallographic Preferred Orientation Relationships of the Magnetite-Hematite-Goethite Phase Transformation during Martitization

Abstract

The most frequent crystallographic preferred orientations developed during the progressive phase transformation of magnetite-hematite-goethite are described and analyzed in two natural samples of banded iron formations from Carajás Mineral Province. Microtextures of martitized grains containing the three phases and the microplaty matrix were analyzed in a scanning electron microscope equipped with a detector for electron backscatter diffraction. For identifying the correlation between magnetite, hematite and goethite lattice and topotaxity during transformation, multiple orientation relationships between the three phases were tested and verified using three-dimensional misorientation analysis. The results show that basal planes of goethite coincide with basal planes of hematite, which coincide with octahedral planes of magnetite. This indicates that transformation between the three minerals happens topotactically, and the oxygen lattice framework is preserved in all members of the reaction as a form of crystallographic memory. As a result of progressive and cyclical changes in oxidation/reduction conditions, an assemblage of high-order orientation relationships is observed and assigned to a complex process of transformation twinning in-between phase transformation of magnetite, hematite and goethite. In the N4WS iron ore deposit, iron oxides/hydroxides from martitized grains work as susceptible markers of environmental changes still in solid state during the diagenetic process.