A strongly thermoluminescent spodumene
Author: Gagan Choudhary
(This article was first appeared in Gems & Gemology, Vol. 46, No. 4, pp 322 - 323)
Thermoluminescence is a property of some minerals whereby they display a bright glow when heated to a certain temperature. Minerals known to display this property include chlorophane (a variety of fluorite), apatite, calcite, lepidolite, and spodumene (see www.galleries.com/minerals/property/pleochro.htm#thermo). I, in April 2010, at the Gem Testing Laboratory of Jaipur, examined a spodumene that displayed a striking example of thermoluminescence.
Figure 1: This 16.17 ct green spodumene was notable for its strong thermoluminescence at low temperature.
The pear-shaped green stone (figure 1) weighed 16.17 ct and measured 19.94 x 11.55 x 10.82 mm. Its colour was reminiscent of green beryl or emerald from Nigeria, but the bright lustre and liveliness ruled out the possibility of beryl. The following gemmological properties were recorded: RI—1.660–1.675; birefringence—0.15; hydrostatic SG—3.17; fluorescence—strong orange to long-wave UV radiation and strong pink to short-wave UV (figure 2); and a weak absorption band visible in the blue region at around 440 nm in the desk-model spectroscope (no chromium lines were detected). These properties are consistent with those reported for spodumene (R. Webster, Gems, 5th ed., rev. by P. G. Read, Butterworth-Heinemann, Oxford, UK, 1994, pp. 186–189). With magnification, a few liquid “fingerprints” were observed under the table and crown facets. Cleavage planes, a common feature in spodumene, were not evident.
Figure 2: The spodumene in figure 1 fluoresced strong orange when exposed to long-wave UV radiation (left) and strong pink under short-wave UV (right).
Microscopic examination was conducted with the aid of a fibre-optic lamp. Curiously, when the examination was completed, the green spodumene appeared bright orange (figure 3). Within a few minutes, however, the original green colour returned. The orange glow was caused by the heat of the fibre-optic lamp exciting the spodumene’s activator elements to produce thermoluminescence. The effect was similar to the stone’s fluorescence reaction to long-wave UV radiation (again, see figure 2, left). The stone was reheated with the fibre-optic lamp and glowed orange again after three minutes of exposure, before returning to its original colour within two to three minutes after removal of the lamp. These steps were repeated several times with consistent results.
Figure 3: The heat generated by a fibre-optic lamp caused the spodumene to thermoluminesce bright orange. Within three minutes after the stone was removed from the lamp, the original green colour returned.
EDXRF analysis revealed the presence of Al, Si, and Fe. Mn, a common constituent in spodumene that is also responsible for its strong fluorescence (see M. Robbins, Fluorescence: Gems and Minerals Under Ultraviolet Light, Geoscience Press, Arizona, 1994, pp. 265–266), was not detected in this specimen. Therefore, the cause of the fluorescence and thermoluminescence is unknown.
This was the first time I had seen the heat of a fibre-optic lamp causing thermoluminescence in spodumene, although fluorescence had been observed several times in diamonds, synthetic spinels or even fluorite. Webster (1994) mentioned this effect occurring at temperatures around 200oC. This spodumene’s thermoluminescence at such a low temperature makes it quite an unusual specimen.
All photographs by Gagan Choudhary