Observing defects in rocks with cathodoluminescence

Cathodoluminescence is commonly used to pre-screen geological samples and combined with other SEM techniques for in-depth study. Various CL imaging modes can provide information on the structure of a rock, including its defects.

Observing defects in rocks with CL 1

The defects that can be observed can be categorized in 2 groups:

Intrinsic defect states include:

  • vacancies, which happen when an atom is missing in the crystal lattice of the mineral
  • interstitials, which happen when an atom is in the wrong place in the crystal lattice
  • dislocations in the crystal structure
  • defects that arise from the growth
  • defects that arise from damage such as stress, compression from heating, tectonic movement or radioactive decay

Extrinsic defects are the second kind of defects, which are not inherent to the mineral. They include:

  • foreign dopants
  • rare-earths (Eur, Yb, Nd)
  • transitions metals (Mn, Fe, Cr, Ti)
  • sensitizers
  • quenchers

When interpreting the data, it is important to keep in mind a combination of intrinsic and extrinsic defects can be present in the rock, which can influence the cathodoluminescence emission. 

Various CL modes can be used to get different information about geological materials. CL intensity mapping is a fast (often video rate) technique which allows to collect the CL from every point with a single pixel detector. CL intensity mapping can be used on zircons, a mineral commonly used for geological dating. CL allows to observe zonations and bands, which are related to crystal growth and evolution of defects. Cathodoluminescence is usually used as a first step to select the most interesting crystals for further investigation with mass-spectroscopy. 

Hyperspectral imaging or spectroscopy mode is used to measure the full CL spectrum for different excitation positions. This mode allows the observation of the boundaries between zones/bands in the material, and can provide spectral data on each band. The characteristic peaks in the spectrum show the presence of intrinsic and extrinsic defects, and allow the identification of the elements present. 

Therefore, cathodoluminescence is a powerful technique that allows identifying defects and other parts of rock materials.

If you would like know more please have a look at the webinar which is downloadable from DELMIC’s website at https://request.delmic.com/en/cl-webinar-series-4th-session-geological-samples