Crystallographic and magnetic preferred orientation of hematite in banded iron ores

doi:10.1016/S0191-8141(00)00085-7

Journal of Structural Geology

Volume 22, Issues 11–12, November 2000, Pages 1747-1759

https://doi.org/10.1016/S0191-8141(00)00085-7 Get rights and content

Abstract

The crystallographic preferred orientation of hematite in banded iron ores and the orientation of both the measured and the calculated principal susceptibility axes are strongly related. The maximum susceptibility is aligned with the lineation and the pole of the foliation coincides with the minimum susceptibility, although there are often distinct differences between the measured and calculated values of the susceptibilities. A wide variety of configurations of c-axis pole figures modeled by varying the parameters of the Bingham distribution and Bingham–Mardia-distribution reveal that quite different c-axis patterns of hematite ores may have the same anisotropy of the magnetic susceptibility (AMS) parameters. Large deviations between calculated and experimental AMS-data should initiate further investigations to resolve a probably unnoticed heterogeneity of the fabric. The present investigations show that the structural analysis of the preferred orientation of hematite ores by means of the rather inexpensive and fast magnetic method must be accompanied by the more expensive but unambiguous determination of preferred orientation by x-ray and neutron diffraction experiments in order to accomplish a complete and sound interpretation.

Introduction

The relationship between the c-axis fabric of hematite ores measured by means of optical and X-ray-methods and their magnetic fabric measured by means of a torque magnetometer was investigated and described by Hrouda et al. (1985). Since then the preferred orientation of a large series of different hematite ores from several localities and different metamorphic terrains of the Quadrilátero Ferrı́fero, Minas Gerais (Dorr, 1969) (Fig. 1) was measured by means of neutron diffraction (Brokmeier, 1989; Will et al., 1989) and the magnetic fabric by means of the Kappabridge KLY-2 of the same specimens. Three typical examples are presented in Fig. 2, which are interpreted as c-axis preferred orientations with a rotational degree of freedom of the a-axes around the c-axis (Siemes and Hennig-Michaeli, 1985; Will et al. 1990; Wenk, 1998). The minimum susceptibility is located in the center of the c-axis maximum, the maximum susceptibility in the center of the (110)-maximum or in the center of (100)-maximum (not shown in Fig. 2). The relationship between the crystallographic preferred orientation and the microfabric is described in Rosière et al., 1998, Rosière et al., 1999 and Quade et al. (2000). The abundance of recently acquired data initiated this new analysis of the relationship between the measured anisotrophy of the magnetic susceptibility (AMS)-data, the neutron measured pole figures, and AMS-data calculated from the (003)-pole figures.

Section snippets

Calculation and presentation of AMS-data

The AMS of hematite single crystals is characterized by a very small susceptibility K_c parallel to the c-axis and a much larger and isotropic susceptibility K_ab in the basal plane resulting in a ratio of P_c=K_ab/K_c>100 (Hrouda, 1980). In this case the anisotropy is controlled only by the intensity of the c-axis orientation; for example, at a constant c-axis concentration, P_c=100, P_c=1000, and P_c=10 000 give rise to virtually the same values of ore anisotropy degree. In accordance with Hrouda et

Numerical simulation of the c-axis distribution

In order to get a deeper insight into the relationship between the preferred orientation of hematite and its magnetic fabric a wide range of simulated c-axis distributions was evaluated. These mathematical c-axis pole figures were generated by a computer code featuring the versatile Bingham (1974) and Bingham and Mardia (1978) distribution, respectively. Both distributions are controlled by location and shape parameters (for details see Appendix A The Bingham distribution (, Appendix B The

Numerical modeling of c-axis pole figures

The AMS-diagram of Fig. 9(a) shows the principal susceptibilities measured for 44 naturally deformed samples from 10 different mines at the Quadrilátero Ferrı́fero, Minas Gerais, Brazil, located at different tectonic settings (Rosière and Chemale Jr., 1991), while Fig. 9(b) depicts the values calculated from the basal pole figures of the same samples. Obviously there are differences between them. Three sample points are numbered, and will be used to model appropriate Bingham distributions. The

Conclusions

c-Axis pole figures of hematite ores reveal approximately circular, to elliptical, to great circle configurations. They are favorably modeled by means of the versatile Bingham distribution.

A wide variety of configurations has been modeled by varying the parameters of the Bingham distribution in order to visualize the relationship between c-axis distributions of hematite ores and calculated susceptibilities.

They reveal that quite different c-axis patterns of hematite ores may have the same

Acknowledgements

Thanks are due to H.-G. Brokmeier and E.M. Jansen for the neutron measurements of the hematite textures at the Forschungszentrum Geesthacht and W. Schäfer and E. Jansen at the Forschungszentrum Jülich. Financial support by the PROBRAL (CAPES/DAAD)- and FINEP/PADCT project ‘Texture, physical anisotropy and metallurgy of iron ores of the Iron Quadrangle (Minas Gerais, Brazil)’ is gratefully acknowledged. Financial support by the DFG permitted the presentation of preliminary versions of this

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Cited by (20)

Texture development during progressive deformation of hematite aggregates: Constraints from VPSC models and naturally deformed iron oxides from Minas Gerais, Brazil
2016, Journal of Structural Geology
Citation Excerpt :
The way that iron ores accommodate deformation, at increasing temperature and strain, is of great interest since it involves important changes in the physical properties and, consequently, the economic potential of the ore bodies. Hematite, the main worldwide iron ore forming mineral, has been experimentally deformed in compression and torsion experiments, both as single crystals (Veit, 1921; Hennig-Michaeli and Siemes, 1982; Siemes et al., 2008) and aggregates (Hennig-Michaeli, 1977; Siemes et al., 2000, 2003, 2008, 2011; Rosière et al., 2001), together with analyses of the resulting microstructures by different approaches provided extensive evidence on the main deformation mechanisms operating in naturally deformed aggregates. The hematite twin and slip system modes activated at various conditions are summarized (Siemes et al., 2008) in Tables 1 and 2, respectively.
We show that naturally-deformed hematite from the Quadrilátero Ferrífero Province, Minas Gerais, Brazil, develops CPOs by dislocation creep, strongly influenced by basal plane parallel glide, even when this is not the favored slip system. Characterization of microstructure and texture, particularly intragranular misorientations, of naturally deformed hematite aggregates by EBSD allowed us to determine the importance of different slip systems, and confirm dislocation creep as the dominant deformation mechanism. Viscoplastic self-consistent (VPSC) models were constructed to constrain the slip systems required to operate for the observed CPO to develop, and its rheological implications. Changes in the CRSS ratio of hematite prism and basal slip systems and deformation regime lead to the development of distinct patterns of hematite crystallographic orientations. The basal slip-dominated simple shear model is the only one that can develop quasi-single-crystal CPO of the kind observed in highly deformed rocks from Quadrilátero Ferrífero. Comparison between naturally deformed hematite aggregates and VPSC models shows that CPO development of hematite is strongly influenced by a highly viscoplastic anisotropy through dislocation creep on hematite basal plane. Nonetheless, our results demonstrate that even the unfavorable slip systems should be regarded when the bulk rheology of mineral aggregates is evaluated.
Microstructures, crystallographic fabric development and deformation mechanisms in natural hematite aggregates deformed under varied metamorphic conditions
2012, Journal of Structural Geology
Citation Excerpt :
The authors verified that the CPO patterns vary with the increasing of the deformation and concluded that the deformation occurred by basal slip, diffusive processes and grain growth. Even though the neutron diffraction technique (Siemes et al., 2000; Rosière et al., 2001) gives a good insight to the CPO of the analyzed sample, a correlation with microstructures cannot be done. Since the EBSD technique permits the control of the acquisition grid in a selected area of the sample, it is possible to make a direct correlation of the crystallographic orientation and its corresponding microstructure.
Naturally deformed hematite aggregates from 15 different iron ore mines located in Quadrilátero Ferrífero region, Brazil, were analyzed in order to verify the influence of increasing temperature and deformation intensity on their microstructural and textural aspects as well as the deformation mechanisms associated with the metamorphic conditions. The electron backscattered diffraction (EBSD) technique was applied in order to get qualitative and quantitative data concerning with microstructural parameters, crystallographic preferred orientation (CPO) and misorientation between hematite grains. The microstructures of these aggregates vary from randomly oriented hematite grains with approximately equant grains, to strongly oriented and elongated grains following the increase in deformational and metamorphic polarity toward east in the region. In the low deformation domain (western region) the deformation mechanisms are typically microfracturing and dissolution precipitation creep for magnetite rich aggregates. In the high-strain domain (eastern region), the deformation is accommodated by a combination of basal intracrystalline slip (c) (<a>) and grain boundary sliding, with rotation around hematite [c] axis. No evidences for recrystallization processes in these aggregates can be supported by our results, probably due to the superposition of subsequent processes.
Crystallographic fabric development along a folded polycrystalline hematite
2008, Journal of Structural Geology
Citation Excerpt :
In the case of hematite-bearing rocks, the development of CPO seems to be related to two different processes. The first is dislocation-related process (e.g. Siemes et al., 2000), which for low metamorphic grade is dominated by basal <a> slip (Hennig-Michaeli, 1977; Siemes and Hennig-Michaeli, 1985; Siemes et al., 2000; Rosière et al., 2001). The resulting CPO can be intensified in shear zones or in folds by bulk rotation of hematite crystals (e.g. Cogné and Gapais, 1986).
Detailed analyses of microstructures and crystallographic preferred orientations (CPOs) of hematite rocks were conducted in samples of polycrystalline hematite around a tight fold in the Quadrilátero Ferrífero region, southeastern Brazil. Grain size is dominantly very fine in all samples (≈15–30 μm), with a slight increase toward the hinge. Grain aspect ratio increases substantially toward the hinge, from an average of 2 in the limb to 9 in the hinge. Distribution patterns of [0001] axes and poles of planes {10 $\bar{1}$ 0}, {11 $\bar{2}$ 0} and {10 $\bar{1}$ 4} suggest that intracrystalline slip operate on the basal plane along the <a> direction. The distribution of the poles of prismatic planes parallel to the foliation of reference frame indicate that the all the symmetric <a>-slip directions of hematite crystals were equally efficient during activation of basal intracrystalline slip. Increasing aspect ratio is accompanied by CPO intensification toward the hinge, where intensification was aided during deformation by bulk rotation of the hematitic plates and possibly by some grain boundary sliding. Such CPO intensification with the increase of aspect ratio toward the hinge is used to infer that the fold developed by flexural flow.
Neutron pole figures compared with magnetic preferred orientations of different rock types
2004, Physica B: Condensed Matter
Neutron diffraction is an excellent tool for pole figure measurement of rock samples. Due to high penetration depth of neutrons for most materials neutron diffraction represents an efficient tool to measure complete pole figures with reliable grain statistics even in coarse grained or inequi-granular materials. In the field of structural geology, the measurement of anisotropy of magnetic susceptibility is a standard technique to reveal the tectonic history of deformed rocks. The application of both techniques on still ongoing studies of Precambrian, Carboniferous and Quaternary rocks which are characterised by fundamental different tectonic evolutions and mineralogical compositions shows the wide field of relevance and importance of these methods in understanding tectonic processes in detail.
Texture, microstructure, and strength of hematite ores experimentally deformed in the temperature range 600-1100 °C and at strain rates between 10-4 and 10-6s-1
2003, Journal of Structural Geology
Polycrystalline hematite samples cored perpendicular and parallel to the foliation, i.e. perpendicular and parallel to a weak c-axis maximum, respectively, were deformed in triaxial compression experiments at confining pressures of 300 and 400 MPa, temperatures T between 600 and 1100 °C, and strain rates between 10⁻⁴ and 10⁻⁶ s⁻¹. The measured strength of hematite ranges from 890 to 67 MPa.
The grain size of the main starting material is up to 175 μm. Grain boundaries are intensely serrated. At T≤800 °C the grain boundaries become increasingly lobate and the number of r-twins decreases. Dynamic recrystallization starts above 800 °C and a foam structure with grain sizes up to 150 μm develops at T≥900 °C.
Neutron diffraction measurements show a distinct change of the preferred orientation (texture) in compression parallel to the foliation. Below 800 °C a {300}-maximum developed due to {a}〈m〉 slip. At 800 and 900 °C a c-axis maximum evolved, probably due to (c)〈a〉 slip. The original texture is preferentially preserved but with lower densities at T≥1000 °C, presumably caused by increasing diffusional flow processes. Perpendicular to the foliation only minor changes of the texture occurred.
The experimentally determined textures allow a better understanding of the natural preferred orientation of hematite ores. The extrapolation of experimental strength data of hematite, quartz, and carbonates to geological conditions are compatible with field observations.
Magnetic fabrics and fluid flow directions in hydrothermal systems. A case study in the Chaillac Ba-F-Fe deposits (France)
2003, Earth and Planetary Science Letters
This study presents a possible use of anisotropy of magnetic susceptibility (AMS) to describe the mineralizing process in hydrothermal systems. Ba–F–Fe-rich deposits within the Chaillac Basin are on the southern border of the Paris Basin. In these deposits hydrothermal textures and tectonic structures have been described in veins, sinters, and sandstone cemented by hydrothermal goethite. 278 oriented cores from 24 sites have been collected in these formations. In addition, a lateritic duricrust superimposed on the hydrothermal formation has been sampled. Rock magnetic investigations show that the principal magnetic carrier is goethite for the hydrothermal mineralization and for the laterite level. The AMS measurements show distinguishable behaviors in the different mineralogical and geological contexts. The K₁ magnetic lineation (maximum axis) is strongly inclined for the vertical veins. For the horizontally mineralized sinters, the magnetic lineation is almost horizontal with an azimuth similar to the sedimentary flow direction. The AMS of goethite-rich sandstone close to the veins shows strongly inclined K₁ as they are probably influenced by the vertical veins; however, when the distance from the vein is larger than 1 m, the AMS presents rather horizontal K₁ directions, parallel to the sedimentary flow. The laterite has a foliation dominance of AMS with vertically well-grouped K₃ axes and scattered K₁ and K₂ axes. Field structural observations suggest that the ore deposit is mainly controlled by EW extension tectonics associated with NS trending normal faults. Combining the AMS results on the deposit with vein textures and field data a model is proposed in which AMS results are interpreted in terms of hydrothermal fluid flow. This work opens a new investigation field to constrain hydrodynamic models using the AMS method. Textural study combined with efficient AMS fabric measurements should be used for systematic investigation to trace flow direction in fissures and in sand porosity.

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Journal of Structural Geology

Abstract

Introduction

Section snippets

Calculation and presentation of AMS-data

Numerical simulation of the c-axis distribution

Numerical modeling of c-axis pole figures

Conclusions

Acknowledgements

References (21)

Symposia on magnetic fabrics: introductory comments

Physics of the Earth and Planetary Interiors

Magnetocrystalline anisotropy of rocks and massive ores: a mathematical model study and its fabric implications

Journal of Structural Geology

Characterization of the magnetic fabric of rocks

Tectonophysics

Ore Minerals

Magnetic susceptibility anisotropy and fabric of some Adirondack granites and orthogneisses

American Journal of Science

An antipodally symmetric distribution on the sphere

The Annals of Statistics

A small circle distribution on the sphere

Biometrika

Neutron diffraction texture analysis of multi-phase systems

Textures and Microstructures

Cited by (20)

Texture development during progressive deformation of hematite aggregates: Constraints from VPSC models and naturally deformed iron oxides from Minas Gerais, Brazil

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

Crystallographic fabric development along a folded polycrystalline hematite

Neutron pole figures compared with magnetic preferred orientations of different rock types

Texture, microstructure, and strength of hematite ores experimentally deformed in the temperature range 600-1100 °C and at strain rates between 10<sup>-4</sup> and 10<sup>-6</sup>s<sup>-1</sup>

Magnetic fabrics and fluid flow directions in hydrothermal systems. A case study in the Chaillac Ba-F-Fe deposits (France)