Integrating Research and Education > Geochemical Instrumentation and Analysis > Optical Cathodoluminescence (Optical CL)

Optical Cathodoluminesence (Optical-CL)

Darrell Henry, Louisiana State University

What is Optical-CL?


Figure 1. Comparison of plane-polarized, crossed-nicols and optical-CL images of granitic gneiss from the Beartooth Mountains in Wyoming. Images by Darrell Henry.
This photo is an image from a granitic gneiss that exhibits relict myrmekite as well as recrystallized myrmekite textures. In the optical-CL image, the blue grains are K-feldspar, the yellow-brown with reddish rims are plagioclase with albitic margins, the bright yellow grain included in the opaque grain is apatite and the dark (but plane-polarized colorless grains) are quartz. The CL image was obtained with a Technosyn optical-CL system at Louisiana State University.

Optical cathodoluminescence (optical-CL) permits optical examination or imaging of a sample through the attachment of an evacuated CL-stage to an optical microscope. The CL-stage attachment uses a cathode gun to bombard a sample with a beam of high-energy electrons. The resulting luminescence in minerals allows us to see textures and compositional variations that are not otherwise evident using light microscopy (see Figure 1). The stage allows X-Y movement of the sample to examine much of the surface area in the sample.

Fundamental Principles of Optical-CL


The theory behind the production of the luminescent response by SEM-CL is the same as that for (SEM-CL) instrumentation (see CL theory).

Optical-CL Instrumentation - How Does It Work?


Optical-CL attachments can be categorized in two types: cold-cathode CL and hot-cathode CL.

Cold-cathode CL. The most commonly used optical-CL system is termed a cold-cathode CL. It is an attachment to a microscope that allows the sample to be examined optically with the microscope and with CL in the same area. In a cold-cathode CL system the electron beam is generated by the discharge that takes place between the cathode at negative high voltage and anode at ground potential in ionized gas at a moderate vacuum of ~10-2 Torr. The result is relatively low-intensity CL in most CL-active silicate minerals. Because some of the electrical charges at the discharge may reach the sample and neutralize the static charge built up, it is not necessary to coat the sample with a conductive coating. The CL response in the sample can be viewed through the objective lens of the microscope or the image can be recorded with high-speed film or with a digital camera (Figure 2).

Figure 2. Top and cross-sectional views of a Luminoscope optical-CL system that is an attachment to an optical microscope. Modified after Marshall (1988).
The electron beam is generated at the cold cathode discharge point, is deflected by a magnet and encounters the sample in the evacuated stage. The Luminoscope stage was formerly manufactured by Premier American Technologies.
There are several commercial manufacturers of the cold-cathode CL stage with somewhat different designs. The more commonly used stages include:


Hot-cathode CL.A hot-CL instrument generates the electrons by a heated filament and the electrons are directed at lower voltage toward the anode. The vacuum is generally lower (<10-5 Torr) and the accelerating potential is about 14 keV. This stage has a more stable beam and has greater CL intensity. There are several non-commercial versions of the hot-CL instrument. These instruments have higher sensitivity and the ability detect short-lived luminescence in minerals. Another form of the hot-CL instrument is the SEM-CL and will be discussed further.

Applications


CL emissions can provide general information on the trace elements contained in minerals or the production of mechanically induced defects in the crystals. Perhaps more importantly for the geologic context, the distribution of the CL in a material gives fundamental insights into such processes as crystal growth, replacement, deformation and provenance. These applications include:

  • investigations of cementation and diagenesis processes in sedimentary rocks
  • provenance of clastic material in sedimentary and metasedimentary rocks
  • details of internal structures of fossils
  • growth/dissolution features in igneous and metamorphic minerals
  • deformation mechanisms in metamorphic rocks.


Strengths and Limitations of Optical-CL


Strengths of acquisition of CL images with the Optical-CL relative to the SEM-CL include:

  • Attainment of real-time true-color CL images
  • Relatively inexpensive
  • Simple to use
  • Conductive coating of the sample in a cold-cathode stage is unnecessary
  • Acquisition of optical images from the petrographic microscope to place the minerals in spatial context
  • Integration of light over the time of exposure or digital acquisition such that it works well for phosphorescent minerals (e.g. calcite/dolomite and apatite)

Limitations of acquisition of CL images with the Optical-CL relative to the SEM-CL include:

  • Magnifications of the images are limited
  • CL images are restricted to the visible range of the spectrum
  • Lower resolution of images
  • Timing of acquisition of the image is dependent on the film speed
  • Additional processing required for analog film and the different color sensitivities of the film


Literature


For more detailed information regarding the theory and practice of Optical-CL, please see:

  • Boggs, S.,Jr. and Krinsley, D. (2006) Application of Cathodoluminescence Imaging to the Study of Sedimentary Rocks. New York, Cambridge University Press, 165 p.

  • Gotze, J., Plotze, M., and Habermann, D. (2001) Origin, spectral characteristics and practical applications of the cathodolumimescence (CL) of quartz - a review. Mineralogy and Petrology, 71, 225-250.

  • Marshall, D. J. (1988) Cathodoluminescence of Geological Materials. Boston, Unwin Hyman, 146 p.

  • Pagel, M., Barbin, V., Blanc, P. and Ohnenstatter, D. (2000) Cathodoluminescence in Geosciences. Berlin, Springer-Verlag.

  • Remond, G., Balk, L. and Marshall, D. J. (1995) Luminescence, Scanning Microscopy 9, Proceedings of the 13th Pfefferkorn Conference, Scanning Microscopy International, Chicago.

  • Barker, Charles E., 1986, Notes on cathodoluminescence microscopy using the technosyn stage, and a bibliography of applied cathodoluminescence [microform], USGS Microfiche I 19.76:86-85

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