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Scanning electron microscopy (SEM) is a powerful tool for characterizing metal ores. This method provides far greater resolution than optical microscopy and can provide vital information on elemental contents. Modes of operation such as Backscattered Electron (BSE) imaging and Energy Dispersive X-ray Spectroscopy (EDX) can provide valuable insights into an ore’s geochemical composition, its mineralogy and textural relationships between mineral phases such as intergrowths and grain-size.
BSE imaging relies on interactions between electrons from an electron beam and a specimen. Electrons incident on a sample, normally prepared as a thin section or polished block, are subsequently backscattered, i.e. scattered through angles more than 90O to the incoming beam, and are detected by a BSE detector. The beam is deflected by atomic nuclei or outer shell electrons. Stronger scattering occurs through interactions of the beam with heavier elements with higher positive nuclear charge and minerals containing heavier elements, therefore, show up brighter on a BSE image.
This imaging technique can be useful for differentiating between minerals of different compositions. For example, in an epithermal Au gold deposit, native gold (atomic mass 196.97) will appear much brighter than surrounding sulfide minerals such as pyrite, FeS2 (atomic masses, Fe 55.85, S 32.07) in a BSE image. The image will also reveal the grain size of the metals and minerals and whether the gold can be easily extracted from the surrounding phases of less value, which is crucial to developing efficient ore beneficiation processes.
EDX operates by generating X-rays from the sample. The electron beam excites an atom out of its ground state, i.e. it causes expulsion of a core electron with subsequent relaxation of an outer shell electron into the core hole. The energy surplus between the two shells is emitted as an X-ray of a wavelength characteristic to the element from which the X-ray was generated.
An elemental map of a sample can be generated with spatially resolved data indicating which elements occur where, and what other elements they are associated with. This is vital to understanding the ore with regards to textural relationships, distribution of metals across the deposit and on a micro-scale and helping identify any penalty elements that reduce the value of the ore, such as arsenic in arsenian pyrite. A somewhat reliable evaluation of exact minerals present can be done on point analyses where ratios of metal to sulfur on the EDX spectra could be indicative of certain phases; however, this is not entirely reliable and should be backed up with additional analyses such as X-ray Diffraction.
QEMScan®, developed by the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia and now sold by FEI Company, is an integrated and automated micro-analysis system that allows for quantitative chemical analysis of samples analysed using an SEM. In addition to elemental distributions and textural relationships, each pixel of an elemental map has an associated spectrum that can be assigned to a mineral depending on its composition. This allows for a better understanding of both elemental and mineralogical associations, can help resolve grain boundaries and provide surface area data and can improve understanding of economic/gangue mineral intergrowths and inclusions. The resolution of the image can be changed to gain either a quick idea of what the sample contains with a pixel size of 100s microns, or using a finer pixel size of 10s microns a far more detailed image can be generated.
Broader mineral groups, or those minerals consisting of solid solution series, can be grouped or defined into more detailed subgroups. For example, magnetite-chromite minerals could be spatially distinguished in a QEMScan image based on several different Fe:Cr ratios.
With portable benchtop SEMs now available and fully capable of being used in field studies or at mine sites, geologists can gain almost instantaneous insight into the mineralogy, geochemistry and quality of ore on-site. The implications for such a powerful tool and associated programmes are that targeting select areas for exploitation should be easier and that the development of optimised beneficiation and extraction techniques will be greatly improved.
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