Spatial speciation using cathodoluminescence

Cathodoluminescence (CL) is the term used to describe the generation of light when a material is bombarded by electrons. This signal can be collected in an electron probe microanalyser (EPMA) when fitted with an optical spectrometer.  The ability to quantify CL map data offers the potential to map minerals and materials with lower detection limits compared to standard trace element mapping by EPMA or micro proton-induced X-ray emission (μ-PIXE), as CL can have greater sensitivity to trace element levels, particularly on strongly active materials such as scheelite and calcite. In addition the CL signal is sensitive to the oxidation state of some trace elements.

CL is sensitive to both material composition and structure of the host lattice, as it originates from processes such as conduction to valence transitions and phonon modes, making CL sensitive to subtle effects such as trace-level dopants, valence of the activator, and concentrations of quenching ions. The most common activators are transition metal ions such as Cr3+,Mn2+,Mn4+,Sn2+,Pb2+,Fe3+  and rare earth elements (REE2+/3+). Due to the nature of the f orbital splitting, REE ions can occupy a number of energy levels giving rise to multiple spectral lines that can be used to differentiate the emitting ion.

A scheelite (CaWO4) sample from the giant Golden Mile deposit in Kalgoorlie (Western Australia) was examined using combined CL and x-ray analysis in an EPMA.  Scheelite is a minor mineral in many Au-bearing deposits and has highly variable REE content which can be used to decipher the processes leading to ore deposition in mineral deposits.

Fitted and calibrated CL maps extracted from the hyperspectral CL dataset, showing Dy3+ concentration in parts per million (left) and Er3+ concentration (right).

Fitted and calibrated CL maps extracted from the hyperspectral CL dataset, showing Dy3+ concentration in parts per million (left) and Er3+ concentration (right).

Fitting the peaks to the hyperspectral CL dataset allowed the extraction of a range of REE maps, showing REE zoning at concentrations down to 15 ppm with micron spatial resolution.

Publication

MacRae, C. M., Wilson, N. C., & Brugger, J. (2009). Quantitative cathodoluminescence mapping with application to a Kalgoorlie scheelite. Microsc Microanal, 15(3), 222-230. https://doi.org/10.1017/S1431927609090308