A multimodal microcharacterisation of trace-element zonation and crystallographic orientation in natural cassiterite by combining cathodoluminescence, EBSD, EPMA and contribution of confocal Raman-in-SEM imaging

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Abstract

Summary

In cassiterite, tin is associated with metals (titanium, niobium, tantalum, indium, tungsten, iron, manganese, mercury). Knowledge of mineral chemistry and trace-element distribution is essential for: the understanding of ore formation, the exploration phase, the feasibility of ore treatment, and disposal/treatment of tailings after the exploitation phase. However, the availability of analytical methods make these characterisations difficult.

Summary

We present a multitechnical approach to chemical and structural data that includes scanning electron microscopy (SEM)-based imaging and microanalysis techniques such as: secondary and backscattered electrons, cathodoluminescence (CL), electron probe microanalyser (EPMA), electron backscattered diffraction (EBSD) and confocal Raman-imaging integrated in a SEM (RISE). The presented results show the complementarity of the used analytical techniques. SEM, CL, EBSD, EPMA provide information from the interaction of an electron beam with minerals, leading to atomistic information about their composition, whereas RISE, Raman spectroscopy and imaging completes the studies with information about molecular vibrations, which are sensitive to structural modifications of the minerals. The correlation of Raman bands with the presence/absence of Nb, Ta, Fe (heterovalent substitution) and Ti (homovalent substitution) is established at a submicrometric scale. Combination of the different techniques makes it possible to establish a direct link between chemical and crystallographic data of cassiterite.

Lay description

Tin is a metal that has been used commercially since the Bronze Age. Cassiterite (SnO2), a natural tin oxide, has been the most important source of tin (Sn) since that time. Sn is also associated with other metals such as titanium (Ti), niobium (Nb), tantalum (Ta), indium (In), tungsten (W), iron (Fe), manganese (Mn), mercury (Hg) in cassiterite. A thorough knowledge of its mineral chemistry and trace element distribution is necessary in the exploration phase for a better understanding of ore formation, for ore treatment feasibility, and for the disposal of tailings after the exploitation phase. However, these characterisations remain a challenge due to numerous analytical insufficiencies.

Lay description

This paper presents a complementary multitechnical approach to chemical and structural data using a panel of imaging and microanalytical techniques based on scanning electron microscopy (SEM), chemical imaging, cathodoluminescence in the SEM (SEM-CL – internal structure, trace element distribution), electron probe microanalysis (EPMA – chemical analysis and imaging at the micrometric scale), electron backscattered diffraction (EBSD – crystallographic mapping) and RISE confocal Raman-in-SEM imaging (chemical imaging). The correlation of SEM-CL intensity with crystallographic orientation (EBSD) and trace element distribution (EPMA) can clearly be established and the influence of each element on luminescence intensity can be defined at the micrometric scale.

Lay description

In addition, the benefits of using the RISE microscope are highlighted. The analyses of the same areas of cassiterite grains show the complementarity of the different analyses and observations performed by SEM-CL imaging, crystallographic observations (EBSD), elemental analyses and trace elements distribution (EPMA), and confocal Raman spectroscopy and imaging in the SEM (RISE). The correlation of An and B2G Raman bands with the presence/absence of trace elements Nb, Ta, Fe (heterovalent substitution) and Ti (homovalent substitution) is established at a submicrometric scale. The combination of data collected from the different techniques makes it possible to link directly both chemical and crystallographic data of cassiterite.

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