Hi Greg,
I extracted this from
Status Report On Gemstones From Afghanistan, by Gary W. Bowersox
on [
www.gems-afghan.com]
First published in GEMS & GEMOLOGY, Winter, 1985
Spodumene. The spodumene crystals from the Nuristan region are among the finest examples of this mineral ever found. Many details on the pegmatite deposits of spodumene are given in Rossovskiy et al. (1978) and Bariand and Poullen (1978). The transparent, gem-quality spodumene crystals from Nuristan come in a wide range of colors - purple and pink, as well as blue, green, and yellow. Some of these crystals are up to one meter in length. In general, they are well formed, with large, flat crystal faces, relatively sharp edges, a tabular shape, and are often twinned. As with tourmaline, the spodumene crystals are free of any attached minerals at the time they are sold to gem buyers. As is typical of spodumene, which is pleochroic, the crystals from this area display different hues when viewed in different orientations, with the strongest color for light passing parallel to the long direction (c-axis) of the crystal. Dunn (1974) describes some of the crystals as color zoned, but the crystals examined for this paper were more or less of uniform color.
From the study of a parcel of light pink spodumene crystal fragments and several additional faceted stones, refractive indices of 1.659 and 1.677 (±0.003) and specific-gravity values of approximately 3.20 (±0.02) were found. These fragments were pleochroic from brownish pink to pink. No features were visible in the hand spectroscope. When exposed to long-wave ultraviolet radiation, the fragments displayed a strong orangey pink fluorescence. When exposed to short-wave ultraviolet radiation, they exhibited a strong bluish pink fluorescence with a red phosphorescence that lasted for about one minute. When viewed with the microscope, the spodumene fragments revealed three-phase inclusions, growth tubes, and cleavages, and displayed twinning. In general, these properties are identical to those reported for Afghanistan spodumene by Dunn (1974) and Rossovskiy (l981).Most spodumene exhibits the property of tenebrescence, which involves a reversible darkening and lightening of its color with changes in conditions (Claffy, 1953). Pure spodumene is colorless; the various colors (pink, purple, green, yellow) are due to the presence of trace elements such as manganese and iron. Manganese substitutes for silicon, and iron for aluminum, in the spodumene crystal structure. According to Hassan and Labib (1978) and Nassau (1983), a darkening of the color of spodumene to pink or purple (kunzite) can be brought about by exposure to a source of high-energy radiation (gamma or X-rays) that removes an electron from the manganese and changes its oxidation state from 2+ to 3+. Further irradiation produces a coupled oxidation- reduction reaction involving both iron and manganese to turn the pink spodumene green.
Mn3+ + Fe3+ irradiation > Mn4+ + Fe2+
These radiation-induced color changes are thermally unstable, and the color-change sequence described above can be reversed by exposure to daylight, ultraviolet radiation, or moderate heat of a few hundred degrees Celsius. The exact color- alteration behavior of spodumene, and the relative persistence of radiation-induced colors, will vary depending on the nature of the trace elements and the color-treatment history of the stones in question. Because it is colored by chromium, which in spodumene is not susceptible to oxidation or reduction, hiddenite does not exhibit changes in coloration under similar conditions.
When mined, spodumene emerges from the ground with a blue-violet or green color. This suggests that the crystals have been exposed to some natural source of radiation that produced these colors by the mechanism described above. According to the miners, leaving the crystals in the sun for several days, often after having boiled them in water, is sufficient to turn the material to an attractive purple or pink color. Fade tests were conducted to document the thermal stability of the purple kunzite, and determined that heating crystal fragments to temperatures of 4000C for six hours was adequate to entirely bleach the pink color. Exposure of several pieces from a single pink crystal to direct sunlight produced fading to virtually colorless within several days (less than a week). As described above, the pink color can be restored by re-irradiation.
