Microstructures, crystallographic fabric development and deformation mechanisms in natural hematite aggregates deformed under varied metamorphic conditions

Mendes, M. A. S.; Lagoeiro, Leonardo Evangelista

doi:10.1016/j.jsg.2012.04.005

Cited by 15 publications

(17 citation statements)

References 22 publications

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“…Mineralogical data from the surrounding rocks allowed the temperature and the pressure for the formation of the hematite aggregates to be deduced at around 673 K and at a pressure not greater than 5 kbar (1 bar = 10 5 Pa). The crystallographic data obtained in the hematite samples are compatible with a pervasive deformation by intracrystalline plasticity in the basal hai system (Mendes & Lagoeiro, 2012;Rosiè re et al, 2001). In addition to the crystal plastic deformation, mechanical twinning by both basal (K 1 = {0001}) and rhombohedral laws played a significant role in accommodating the deformation, and twin boundaries are found ubiquitously.…”

Section: Sample Descriptionsupporting

confidence: 58%

EBSD analysis of rhombohedral twinning in hematite crystals of naturally deformed iron formations

et al. 2015

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The rhombohedral twinning in hematite has an important role in the accommodation of the deformation of hematite single crystals and hematite aggregates. It is a contact twinning and occurs as lamellae parallel to the planes of hematite as a result of twin gliding on such planes. On account of the recent applications of electron backscatter diffraction (EBSD) techniques in a wide range of microstructural studies, the determination of symmetry operations that relate crystals in a deformed crystalline aggregate is crucial for the full textural characterization. This study presents an EBSD‐based crystallographic analysis of the rhombohedral twinning on hematite crystals of a naturally deformed banded iron formation. Manipulations of theoretical pole figures depicting the symmetry relation of the rhombohedral twinning and misorientation and crystallographic data obtained by EBSD are used to establish the rotational relationship between twin and parent crystals. A method for determining pairs of axes and angles of rotation was developed which can be extended to any other twin laws or misorientation patterns in any other crystal system. It was found that the hematite rhombohedral twins are related to the parent crystal by an approximately 85° rotation about the 〈〉 directions. Hence it could be determined that this consists of a macroscopic twinning element which is an alternative to the conventional ones used to describe the symmetry of the twin. It also matches microscopic twinning elements for the rhomb twinning law. Additionally, this method allows the determination of the crystallographic orientation of the twin lamellae and which particular 〈〉 axis satisfies the 85°〈〉 pair of rotation. The use of an unambiguous angle–axis pair of rotation allows the identification of twin boundaries in complex and finely grained aggregates and the distinction of twinning laws in a particular crystal.

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Section: Sample Descriptionsupporting

confidence: 58%

EBSD analysis of rhombohedral twinning in hematite crystals of naturally deformed iron formations

et al. 2015

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“…The tabular-granular type (sample A 1 ) is attributed to areas of highly deformed rocks (Rosiere et al 2001;Mendes and Lagoeiro 2012). On the other hand, the sample C 2 is of tabular type.…”

Section: Discussionmentioning

confidence: 99%

“…The working distance used was of 32 mm and the step size for all samples of 4 mm (Mendes and Lagoeiro 2012;Vaudin 2005) and the mean angle of deviation was set to 1 . For acquisition and postprocessing of the data, a suite of applications from Channel 5 (Oxford-HKL Instruments) was used.…”

Section: Characterization Of the Samplesmentioning

confidence: 99%

“…The metamorphic and deformation conditions for the Iron Quadrangle region have been studied for a quite long time. Nevertheless, studies based on morphological types and textural evolution of iron formation rocks in the region have been only accomplished recently by the works of Rosière et al (2001), Barbosa and Lagoeiro (2010), Mendes and Lagoeiro (2012). They have a common sense that metamorphic and deformation intensity increase from west to east and from south to north in the region.…”

Section: Introductionmentioning

confidence: 99%

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Effect of the Morphological Types in Grinding of Iron-Ore Products

Graça

Lagoeiro

Lima

et al. 2015

Mineral Processing and Extractive Metallurgy Review

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The iron-ore products are characterized in order to investigate the effect of the mineralogical features on the grinding results into pelletizing plant. The studied materials are from four mines of the different regions of Iron Quadrangle, processed by flotation, magnetic separator and produced from screening with P 80 in 8 mm. In this study, we apply for the first time the novelty of electron backscatter diffraction technique for the iron ore phase identification and quantification of the microstructural parameters. The mineralogical and microstructural parameters were introduced as a base in the specific grinding test applied in the industrial routine. Each sample was characterized based on the mineral shape, pores, fractures, polycrystalline hematite, and aggregate of hematite with goethite. The test results demonstrate that there is a strong relationship between the gain of the specific surface area, represented by the K-factor, and the morphological types.

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“…The Casa de Pedra deposit is a folded Paleoproterozoic banded iron formation in the Quadrilatero Ferrifero. Low grade regional metamorphism has modified the chemical and structural properties of iron oxides in this region (Mendes and Lagoeiro, 2012).…”

Section: Samples and Methodsmentioning

confidence: 99%

Helium diffusion parameters of hematite from a single-diffusion-domain crystal

Farley

2018

Geochimica et Cosmochimica Acta

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This contribution reports new parameters for helium diffusion in hematite useful for interpretation of cosmogenic 3 He and radiogenic 4 He chronometry. Fragments of a coarse, euhedral single crystal of hematite from Minas Gerais, Brazil were subjected to bulk step-heating helium diffusion experiments after proton irradiation to make a uniform distribution of 3 He. Aliquots of three different grain sizes ranging from ~300 to ~700 μm in equivalent-sphere radius yielded helium diffusion activation energies Ea~170 kJ/mol, very similar to previous estimates for Ea in hematite. Uniquely in this specimen, diffusivity varies with the dimensions of the analyzed fragments in precisely the fashion expected if the diffusion domain corresponds to the physical grain. This contrasts with previous studies that concluded that the analyzed hematites consist of polycrystalline aggregates in which helium migration is governed by the size distribution of the constituent crystallites. These new data permit a direct estimate of the helium diffusivity at infinite temperature for hematite of ln(Do) =-0.66 ± 0.35 cm 2 /sec. The major implication of the new diffusion parameters is that hematite is very retentive of helium even at very small crystal sizes. For example, a 20 nm radius hematite crystal, at the smallest end of the size range so far described in dated polycrystalline hematite specimens, will retain more than 99% of its ingrown He over 1 Myr at 30 o C, and more than 90% over 100 Myr. Under most conditions, hematite is close to quantitatively helium-retentive on the Earth's surface, simplifying radiogenic and cosmogenic helium dating of this phase. In a system cooling at 10 o C/Myr, the 20 nm hematite crystal has a He closure temperature of ~70 o C, similar to a typical ~100 μm apatite crystal. Helium is likely held tightly in hematite owing to its dense hexagonal closest packing structure and absence of migration-enhancing channels. The isostructural minerals corundum and sapphire are likely to be similarly helium retentive.

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Microstructures, crystallographic fabric development and deformation mechanisms in natural hematite aggregates deformed under varied metamorphic conditions

Cited by 15 publications

References 22 publications

EBSD analysis of rhombohedral twinning in hematite crystals of naturally deformed iron formations

EBSD analysis of rhombohedral twinning in hematite crystals of naturally deformed iron formations

Effect of the Morphological Types in Grinding of Iron-Ore Products

Helium diffusion parameters of hematite from a single-diffusion-domain crystal

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