Cathodoluminescence Characteristics and Mineral Chemistry of Tennessee Sphalerites (Tn-Usa)

Optical cathodoluminescence microscopy (CLM) can provide very useful information both on mineral zoning formed during crystal growth and on variations in chemical composition of sphalerites which is reflected by their visible colors. Gem quality sphalerite crystals associated with dolomite, calcite and fluorite having 3 different visible colors - green, honey and brown - were studied to compare their mineral chemistry and CL characteristics. Electron microprobe analyses indicated essentially no significant differences in terms of major components. Although the common CL-quencher ferrous iron contents of all three were lower than 0.25 wt %, none of the sphalerites cathodoluminescenced: two (brown and honey) were non-CL, the green sphalerite showed very week CL in faint brown color. Other trace elements that were analyzed include Fe, Cd, Mn, Ag, Hg, In, Bi, Cu, Ge, and Ga. Fe, Cd, Ga and Cu were relatively higher than the others. Most significant differences with respect to the common trace elements include honey colored sphalerites are essentially Fe- and Cd-poor but rich in Ga, green sphalerites are Fe- and Cd- rich but poor in all the others, and brown sphalerites are rich in Cd. No conclusive relation between non-CL behavior of all three sphalerites and their trace element chemistry was established, however; low quantities of common CL activators in sphalerite crystal structures such as Mn, Cu, and In were the main cause of the non-CL behavior.