Cathodoluminescence can be observed when an high-energy electron beam penetrates a geological material to a certain depth (less or more, depending on the energy of an electron beam). Typically, rocks are insulators which have wide band gaps between 5 and 15 eV. This means that direct transitions in rocks would emit either DUV or EUV. However, visible range emission can also be often observed, and it is often caused by the defect states. Defect states are located between the ground state and the excited state, forming intermediate states. Defect states play an important role in CL emission from rocks as they cause the emission to move from DUV or EUV to visible range emission.
There are many different types of defects in crystals, and some defects have very distinct CL signature, which helps to identify them in rock materials. The defects can be divided into intrinsic and extrinsic.
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
All or several of these defects can be present in the crystal structure at the same time and can be linked to each other.
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)
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 even more about cathodoluminescence imaging for geological materials, please watch our recent webinar, which is available below.
Sample and images courtesy of Bjørgunn H. Dalslåen and Siri Simonsen (University of Oslo).