Minerals that exhibit CL include diamond, quartz, corundum, rutile, cassiter-
ite, benitoite, willemite, halite, fluorite, spinel, calcite, dolomite, kaolinite,
apatite, barite, strontianite, sphalerite, zircon, feldspar, jadeite, diopside, wol-
lastonite, forsterite and fayalite.
Localised energy levels in the gap between valence and conduction bands,
arising from lattice defects, interstitial ions, or substitutional impurity atoms,
often play an important role in CL emission (see Fig. 2.14). Certain elements
behave as ‘activators’, small concentrations of which are sufficient to produce
CL. Others (notably divalent Fe) have the effect of ‘quenching’ CL emission.
The intensity of some forms of CL emission is strongly influenced by the
density of defects, which is dependent on factors such as temperature of
formation, cooling rate, deformation and irradiation. High defect densities,
however, may suppress CL by promoting alternative modes of de-excitation.
The colour of CL emission depends on the difference in energy between the
states concerned. Most commonly the energy is not narrowly defined and the
emission takes the form of a band (Fig. 2.15(a)). In a few cases line spectra
characteristic of the impurity element are produced (Fig. 2.15(b)).
Excitation of CL is not very sensitive to the beam accelerating voltage, but
sometimes it is advantageous to use a relatively high value (at least 20 kV)
because this enables the electrons to penetrate the non-luminescent damaged
surface layer. Prolonged electron bombardment tends to cause fading, whereby
the intensity declines, sometimes irreversibly. Cooling the specimen below room
temperature significantly increases CL intensity for certain types of sample.
Cathodoluminescence can be detected in SEMs and EMPs with appropriate
light-detection equipment (Section 3.12.2). Owing to the complexity of the
factors governing CL emission in natural materials, unambiguous elemental
analysis is problematic, but nevertheless information not easily obtained by
other means in a range of geological applications can be obtained from CL
images (see Section 4.8.4).
2.9 Specimen heating
The fraction of the energy in the incident electron beam which reappears in the
form of X-rays, light, etc. is small, most being converted into heat in the target.
The temperature rise T can be estimated from the following expression:
T ¼ 4:8E
0
i=ðkdÞ; (2:5)
where E
0
is the incident electron energy (keV), i the current (mA), k the thermal
conductivity (W cm
1
K
1
)andd the beam diameter (mm). For metals k is
2.9 Specim en heating 19