CL: Reference Block – Kurt Hollocher

What is this for?

The first thing you have to do when using the CL system is center the RPM mirror. Trying to do that on inhomogeneous materials, like zoned zircons, works poorly because the signal over the 2-d surface is irregular. You need a uniformly luminescent material. This block is a polished, 25.4 mm round mount with 20 different materials, all of which luminesce, plus copper as an orientation marker. Although only one luminescing material is really necessary, it wasn’t originally clear which ones would work best, so there are several. Also, these materials are handy for playing around with the system to learn how to collect spectra, images, and so on, without messing up your precious samples. Below are images of the block, brief explanations, and spectra.

This is a composite secondary electron image of the block, with labeled samples. Even though this is an SE image, I had the detector bias voltage set to bring out an atomic number effect. High atomic number materials are relatively bright, like copper, 1, 3, 4, and 20. Low atomic number materials are relatively dim, like 6 and 7-13. You can use the shapes to see where you are on the block.

This is a composite image made with the variable pressure detector. Under VP conditions the detector detects light emitted from gas molecules above the sample, excited by backscattered and secondary electrons. In this case, at high vacuum, it is detecting cathodoluminescence.

In the SE image above, samples 14-19 are all about the same brightness. Here, 14-16 are bright, but 17-19 are relatively dim. The detector is most sensitive to blue light (~400 nm), with a sharp cutoff above 300 nm and very low efficiency below about 650 nm.

One of the problems with CL imaging is streaking (blue arrow, above), caused by materials having long luminescence decay lifetimes. If you see streaking, the best way to reduce it is to slow down the scan speed. Alternatively, try using a color wheel filter to block the long-lifetime emission, letting through short-lifetime colors. For example, some orange-luminescing calcite streaks badly. However, streaking can be eliminated by using a blue color filter. It doesn’t let through a lot of light, but it does eliminate the streaks.

Compositions, spectra, and notes

The spectra below were all collected with the CL system CCD detector, using the 300 lines/mm grating, 7 mm slit (basically wide-open), a beam voltage of 10 keV, and a beam current of 5 nA. A narrower slit would resolve narrower REE lines. UV fluorescence refers to visible fluorescent color under UV illumination.

Sample	Material	Spectrum	Comments
01	Strontianite, SrCO₃	Spectrum	Bluish UV fluorescence.
02	Calcite, CaCO₃	Spectrum	Pink UV fluorescence.
03	Witherite, BaCO₃	Spectrum	Yellowish UV fluorescence.
04	Witherite, BaCO₃	Spectrum	Yellowish UV fluorescence.
05	Apatite, Ca₅(PO₄)₃(F,OH)	Spectrum	Not UV fluorescent, REE-activated.
06	Quartz, SiO₂	Spectrum	Not UV fluorescent, sandstone with K-feldspar grains.
07	Albite, NaAlSi₃O₈	Spectrum	Not UV fluorescent.
08	Albite, NaAlSi₃O₈	Spectrum	Not UV fluorescent.
09	Albite, NaAlSi₃O₈	Spectrum	Not UV fluorescent.
10	Labradorite, (Ca,Na)(Al,Si)₄O₈	Spectrum	Not UV fluorescent.
11	Perthite, (K,Na)AlSi₃O₈	Spectrum	Not UV fluorescent, coarse albite lamellae.
12	Perthite, (K,Na)AlSi₃O₈	Spectrum	Not UV fluorescent, coarse albite lamellae.
13	Perthite, (K,Na)AlSi₃O₈	Spectrum	Not UV fluorescent, microscopic albite lamellae.
14	Willemite, Zn₂SiO₄	Spectrum	Mn²⁺ activation.
15	Fluorite, CaF₂	Spectrum	Purple UV fluorescence.
16	Fluorite, CaF₂	Spectrum	Yellowish UV fluorescence.
17	Calcite, CaCO₃	Spectrum	Weak orange UV fluorescence, Mn²⁺ activation.
18	Calcite, CaCO₃	Spectrum	Weak orange UV fluorescence, Mn²⁺ activation.
19	Calcite, CaCO₃	Spectrum	Strong orange UV fluorescence, Mn²⁺ activation, 4% MnO.
20	Gallium arsenide, GaAs	Spectrum	Synthetic wafer, emission line at ~886 nm.

Here are combined spectra, for comparison purposes

Three calcite samples have Mn²⁺ peaks at about 620 nm, but sample 02 is quite different, with mostly blue REE-activated luminescence.

The witherites have two peaks at about 410 and 530 nm. The strontianite seems to have the same peaks, but shifted to shorter wavelengths by about 20 nm. The strontianite seems to have an additional peak at about 350 nm.

The all of these seem to have the same three broad peaks at about 400, 510, and 720 nm, but with very different peak heights. Albite 09 also seems to have a strong peak in the ultraviolet, absent in the others.

All three grains have albite exsolution lamellae in microcline hosts. They all seem to have the same three peaks in common, shifted slightly from one another. The albite lamellae are redder than the bluer microcline hosts.

The two fluorite grains have similar peak positions but different intensities. The apatite is activated by REE, including an infrared peak that is unusual in this sample set.

The quartz is of the brown variety, characteristic of graphite-bearing pelitic metamorphic source rocks. The willemite has a narrower peak at a different wavelength than most of the calcite samples, though it is activated by the same Mn²⁺. The gallium arsenide is a semiconductor with an emission peak in the near-infrared at ~886 nm, at which our detectors are not very sensitive.