Gussiås suspected that the spinel sample was enriched in both cobalt and chromium due to its unusual color he has reported a similarly saturated purple stone, faceted and just under 2 ct from the same placer locality. They purchased it from miners on-site at the Khao Ka alluvial deposit, northeast of Nà Hà, An Phú, in the Luc Yen district (approx. The Vietnamese spinel (sample VN, figure 5) was acquired from Geir Atle Gussiås and Mai Tran of Balder Gems. Spinel sample AF from Badakhshan, Afghanistan. The second sample is a purple spinel with light to medium saturation, mined from a marble-hosted deposit in Badakhshan, Afghanistan (figure 4).įigure 6. One is a saturated purple sample from a placer deposit near Khao Ka in the Luc Yen district of Vietnam (figure 3). The current study compares the chemical composition and color of two purple spinel samples, from Vietnam and Afghanistan (figure 2), to each other and in the broader context of all gem spinel.
Purple spinel (figure 1) is generally colored by Fe or by a combination of Cr, V, and subordinate Fe (Andreozzi et al., 2019). Vivid spinels colored by cobalt (blue) and chromium (red) are particularly sought after in the gem trade. Gem-quality spinel occurs in a variety of colors as a result of a diverse array of possible chromophores and chromophore combinations (Schmetzer et al., 1989 Andreozzi et al., 2019): combinations of Fe 2+ and Fe 3+ (light blue/violet to green to black with increasing Fe concentration Hålenius et al., 2002) Co 2+ (blue Shigley and Stockton, 1984 D’Ippolito et al., 2015) Cr 3+ (red) and V 3+ (orange Andreozzi et al., 2019). Photo courtesy of Ikram Muhammad and ANAAR Gems, 2017.
The purple spinel deposit in Badakhshan, Afghanistan. Co/Fe, but this is not valid for spinel dominantly colored by V 3+ or containing significant concentrations of Fe.įigure 4. Cr 3+-, (Fe 2+,Fe 3+)-, and Co 2+-dominant (in terms of color) gem spinels are easily differentiated by chemical composition using a discrimination plot of Co/Cr vs. Sample VN represents an approximate intermediate between natural gem spinel populations colored dominantly by Cr 3+ vs. In contrast, the purple spinel from Afghanistan is relatively impoverished in all trace elements except Fe and Mn. This sample is enriched in Co, Ni, Cr, V, Zn, and Ga. In the Vietnamese purple spinel, the relatively low Fe concentration is likely crucial in creating the attractive saturated color. The dominant contributions of chromophores Cr 3+ and Co 2+ to the color of sample VN are responsible for its higher color saturation than that of sample AF, which is colored dominantly by Fe 2+ and Fe 3+ (3089 ppma total Fe) and contains negligible concentrations of other chromophores (<10 ppma V 3+, Cr 3+, and Co 2+). Sample VN is colored primarily by cobalt (17.9 ppma) and chromium (593 ppma), with minor color contribution from Fe (2437 ppma), and contains significantly more cobalt than typical Cr-enriched red, pink, and purple gem spinels (<6 ppma Co). UV-Vis-NIR spectra are compared along with trace element chemistry. Sample AF was from Badakhshan, Afghanistan, with light to medium saturation and containing phlogopite, amphibole, and probable fluid inclusions.
Sample VN, from a placer deposit in the Luc Yen district of Vietnam, exhibited strong saturation, orange-red fluorescence in UV, and magnesite and graphite inclusions. This study compares two purple gem spinel samples with regard to trace element chemistry and visible light absorption spectra, and presents the first reported example of a spinel with a saturated purple color caused predominantly by chromium and cobalt. The vivid colors seen in fine gem-quality spinel are derived from the mixing between multiple different chromophores in a range of ratios and concentrations, as in this fine 13.52 ct violetish purple spinel. Comprehensive CAD/CAM For Jewelry Certificateįigure 1.
0 kommentar(er)
