Fast Characterisation of Minerals with Similar Crystal Structures

Summary

A large amount of valuable information is held in geological materials. The phases present, the grain texture and the orientation of those grains all provide important clues on how a rock was formed [1]. As such, an understanding of the crystalline structure and properties of geological materials provides a powerful insight. In the Scanning Electron Microscope (SEM), electron backscatter diffraction (EBSD) can be used to gather this information.

Here we present an example demonstrating the combined power of the Oxford Instruments Symmetry® EBSD detector and the Ultim® Max 170 Energy Dispersive X-ray Spectroscopy (EDS) detector coupled with AZtec TruPhase for the successful fast characterisation of a mineral sample containing phases with very similar crystal structures.

SEM Based Analysis

The interaction between the sample and the electron beam in the SEM generates a cascade of signals which can be used to extract information about the sample. By controlling the position of the electron beam it is possible to get different types of information from the same spatial location on the sample. Emitted x-rays can be detected using an EDS detector, which will provide information about the chemical composition.

EBSD is a SEM based diffraction technique which gives crystallographic information from the interaction between the sample and electron beam. At each beam position an EBSP (Electron Backscatter Pattern) is collected and analysed automatically, thereby providing information about the crystal structure and orientation at that specific location.

Combining these two techniques while scanning the beam over the sample surface means that both chemical and crystallographic information can be collected simultaneously in a single map acquisition over the entire sample surface.

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Fig. 1 - (a) An example of an electron image an iron ore sample and the simultaneously acquired (b) EBSP and (c) EDS spectrum from a hercynite grain at the beam location marked with a cross.

Analysing Geological Materials

EBSD analysis of geological samples can be extremely challenging due to the potentially relatively weak diffracted signal and low symmetry of many mineral phases which can result in a limited data acquisition speed. For faster analysis the EBSD detector needs to be sensitive enough to acquire good, high resolution EBSPs in a short time period without requiring the use of very high probe currents. The extreme sensitivity of the Symmetry CMOS-based EBSD detector, primarily due to its unique fibre-optic lens system, enables high quality diffraction patterns to be collected in just a few milliseconds and has resulted in a significant improvement in acquisition speeds.

In addition, mineral phases with very similar crystal structures can be challenging to differentiate using EBSD alone. In this instance combining EBSD with chemical differences revealed by EDS can deliver a powerful solution. AZtec TruPhase enables phases with very similar crystal structures to be differentiated based on a combination of chemistry and crystal structure. This technique simultaneously collects EBSD and EDS data from the same point on the sample. When there is more than one possible solution to the electron backscattered pattern (EBSP), the EDS information from the same point is used to rank the EBSD solution.

Sample

Here, we show the use of EBSD to gain microstructural information about an iron ore sample from Hebei, China. This sample contains Ilmenite (FeTiO₃), Magnetite (Fe₃O₄) and Hercynite (Fe₂AlO₄). Magnetite and Hercynite are both types of spinel, with very similar crystal structures: both are cubic, belonging to the space group 227 and have similar unit-cell dimensions (Table 1). This makes it difficult to distinguish them using EBSD alone (Fig. 2a). However, they have important chemical differences: Hercynite contains Al which is absent in Magnetite. This makes it possible to differentiate them using AZtec TruPhase.

Table 1 - Crystallographic properties of the phases in the iron ore sample.

AZtec TruPhase

When using AZtec TruPhase both an electron backscatter pattern (EBSP) and an EDS spectrum are simultaneously acquired from each analysed point. When the EBSD indexing of a diffraction pattern offers more than one viable phase, the EDS spectrum is compared with reference spectra for each phase and a ranking of possible solutions is generated. This approach enables an accurate differentiation of Hercynite and Magnetite – an example of which is shown in Fig. 2b.

(a) Without TruPhase

(b) With TruPhase

Fig. 2 - Two phase maps acquired from the same area. (a) uses traditional EBSD indexing, demonstrating that it is difficult to distinguish Magnetite and Hercynite – the majority of the field of view is a mix of yellow and red indicating that both Magnetite and Hercynite are frequently being mis-identified. (b) collected using AZtec TruPhase showing the successful differentiation between Magnetite and Hercynite – the separate phases are clearly identified.

High Speed Simultaneous EBSD & EDS Analysis

This TruPhase analysis was performed over a large area (see Fig. 3) of 420×315 µm, at an accelerating voltage of 20kV and a probe current of 17nA, using a measurement step size of 0.23 µm with a total of ~2.5 million analysis points. The data were collected in only 50 minutes at almost 900 patterns per second (pps), with a 99% indexing success rate. This acquisition speed using Symmetry is over 10 times faster than equivalent analyses using a conventional charge coupled device (CCD) based EBSD detector at the same level of data quality.

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Fig. 3 - (a) An Electron image of the analysed large area. (b) X-ray maps show relative concentration of Ti, Fe, O and Al. (c) EBSD IPF maps show orientations. (d) EBSD phase map.

In this sample, the three phases have a distinct orientation relationship. It can be seen from the pole figures (Fig. 4) that Magnetite and Hercynite have a completely coherent orientation, while Ilmenite and Magnetite have the following orientation relationship: Ilmenite {0001}//Magnetite {111}, Ilmenite {10-10}//Magnetite {110}. This particular orientation relationship suggests that the mineral forms a solid solution during the crystallization process due to the isomorphous replacement of the elements, and the solid solution decomposes and precipitates different phases during cooling.

Fig. 4 - Contoured pole figures showing the orientation relationship between the three phases.

Conclusions

The Symmetry EBSD detector provides an excellent combination of speed and sensitivity, making it the ideal detector for analysing mineral samples. AZtec TruPhase, combining crystallographic information with compositional data from simultaneous EDS analyses using Ultim Max detectors, enables reliable phase discrimination between phases with similar crystal structures. In this example a large area iron ore sample, showing a complex mixture of different spinel and oxide phases, has been effectively analysed in 50 minutes using Symmetry, Ultim Max 170 and AZtec TruPhase. These results are of direct value in the determination of the structural evolution of these minerals in iron ores.

Acknowledgements

Dr. Wei Tan, Guangzhou Institute of Geochemistry, China, is thanked for providing the magnetite sample.

Reference

[1] Vernon, R.H. (2004) A practical guide to rock microstructure, Cambridge University Press.