Gem News International Gems & Gemology, Spring 2020, Vol. 56, No. 1

Sunstone Plagioclase Feldspar from Ethiopia


Rough Ethiopian sunstones in a varies of colors and clarities.
Figure 1. Left: Forty-four Ethiopian rough sunstones exhibiting different colors and clarities. The largest stone weighs 54.41 ct. Right: Four Ethiopian rough sunstones ranging from 9.92 to 35.42 ct. All 48 rough stones were identified as labradorite by LA-ICP-MS, except for one spot as bytownite. Photos by Diego Sanchez (left) and Shunsuke Nagai (right).

Ethiopia, traditionally known for opal, has become an important source for emerald and sapphire. After these significant discoveries, a new type of Cu-bearing sunstone feldspar, first shown in 2015 to Tewodros Sintayehu (Orbit Ethiopia Plc.), was discovered in the Afar region (L. Kiefert et al., “Sunstone labradorite-bytownite from Ethiopia,” Journal of Gemmology, Vol. 36, No. 8, 2019, pp. 694–695). This material made its way to the jewelry market last year in Tucson.

To fully characterize this new production, GIA obtained 48 Ethiopian sunstones for scientific examination. Among them, 44 rough stones (figure 1, left) were borrowed from Stephen Challener (Angry Turtle Jewelry), who acquired them from an Ethiopian gem dealer in Tucson in February 2019. Another four rough stones (figure 1, right) were purchased by author YK from Amde Zewdalem (Ethiopian Opal and Minerals) and Benyam Mengistu, who facilitates mining and exporting samples from Ethiopia, at the Tokyo International Mineral Association show in June 2019. Prior to this discovery, the only verified occurrences of Cu-bearing feldspar were from Lake and Harney Counties in Oregon (e.g., the Dust Devil and Ponderosa mines). However, more than a decade ago there was a controversy about Cu-bearing feldspar on the market purportedly from Asia or Africa with an undetermined color origin, presumably Cu-diffused (G.R. Rossman, “The Chinese red feldspar controversy: Chronology of research through July 2009,” Spring 2011 G&G, pp. 16–30; A. Abduriyim et al., “Research on gem feldspar from the Shigatse region of Tibet,” Summer 2011 G&G, pp. 167–180). Gemological testing and advanced analytical methods helped distinguish this new Ethiopian material from the Oregon material and the controversial feldspar of questionable color origin mentioned above in order to ensure GIA’s accurate reporting of the natural origin of Cu-bearing feldspar.

Copper platelets (left), wispy dislocations (center), and yellowish crystals (right) in Ethiopian sunstone.
Figure 2. Left: Clouds of copper platelets were observed in numerous Ethiopian sunstones. Field of view 3.13 mm. Center: Dense networks of reddish wispy dislocations were seen in some Ethiopian sunstones, along with a slightly greenish blue bodycolor. Field of view 2.72 mm. Right: Yellowish crystals of what are likely fayalite were seen in one Ethiopian feldspar. These inclusions have also been documented in Oregon sunstone. Field of view 2.82 mm. Photomicrographs by Nathan Renfro.

Two polished rough Ethiopian samples gave RI readings of nα = 1.562 and nγ = 1.571 and birefringence of 0.009. Optic signs were biaxial positive. Hydrostatic SG measurements were 2.70 and 2.72. These RI and SG ranges overlapped with Oregon sunstone (RI—nα = 1.560–1.570, nγ = 1.568–1.579; birefringence—0.006–0.012; SG—2.70–2.72) but were significantly higher than other Cu-bearing feldspars with undetermined natural or treated color origin (nα = 1.551–1.559, nγ = 1.559–1.566; birefringence—0.004–0.010; SG—2.68–2.69). A total of 18 Ethiopian sunstones were tested under a desktop UV lamp. Eleven showed very weak orange fluorescence, while seven were inert under long-wave UV. All showed very weak red fluorescence under short-wave UV. Under magnification, many showed dense clouds of reflective copper platelets, much like those observed in material from Oregon (figure 2, left). Some stones also showed an interesting wispy network of reddish dislocation stringers with a greenish blue bodycolor in transmitted light (figure 2, center). Another example revealed several yellow crystals of what appeared to be fayalite, an inclusion also observed in Oregon sunstone (figure 2, right).

 
Chemical comparison of Oregon and Ethiopian sunstone and copper-bearing feldspar.
Figure 3. Chemical comparison of Oregon and Ethiopian sunstone and Cu-bearing feldspar with undetermined color origin. Mg (A and C) is the key trace element that separates Ethiopian from Oregon sunstone. The unknown Cu-bearing feldspar has a much higher Cu and Sr concentration than other natural untreated sunstones (A, D, E, and F). For major elements, Ethiopian sunstone overlaps with Oregon sunstone, with a slight lower Ca concentration (B and D). The unknown Cu-bearing feldspar contains more Na and less Ca than other natural untreated sunstones (B, D, and F).
Ternary diagram comparing natural Ethiopian sunstone, natural Oregon sunstone, and copper-bearing feldspar of unknown color origin.
Figure 4. Illustration of solid solution in the feldspars, modified after Deer et al. (Rock-Forming Minerals, Volume 4: Framework Silicates, 1963). The nomenclature of the plagioclase series and the alkali feldspars is also shown. All 48 Ethiopian rough stones (yellow dots) were classified as labradorite except one bytownite. Forty-five Oregon sunstones (blue and green dots) were classified as labradorite-bytownite. Twenty Cu-bearing feldspars with undetermined color origin (red dots) were classified as andesine-labradorite. Ethiopian sunstones are less calcic than Oregon sunstones. Variation of refractive indices ng and specific gravity with composition of feldspars were plotted in the ternary plot as pink and blue straight lines, respectively. Note: nγ = gamma, the highest RI of a biaxial crystal, light vibrating parallel to the Z optical direction.

Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) was used to measure the chemistry of all 48 Ethiopian sunstones, 26 Dust Devil sunstones, 19 Ponderosa sunstones, and 20 Cu-bearing feldspars with undetermined color origin. NIST 610 and USGS GSD-1G and GSE-1G glasses were used as external standards. 29Si was used as an internal standard. Ponderosa and Dust Devil sunstones yielded an end member composition of Ab28–32An67–72 Or0.3–0.4 and Ab28–36An64–72Or0.3–0.8, respectively (table 1). They are classified as labradorite-bytownite using an albite-anorthite-orthoclase (Ab-An-Or) ternary diagram (figures 3B and 4, blue and green dots). Ethiopian sunstones yielded an end member composition of Ab30–39An60–70 Or0.4–1.4, which is generally similar but less calcic than Oregon sunstones (table 1). All analyses of Ethiopian material indicated classification as labradorite except one spot that gave bytownite (figure 3B and figure 4, yellow dots). The Cu-bearing feldspar with undetermined color origin yielded an end member composition of Ab46–51An46–51Or2–3 (table 1). They were classified as andesine-labradorite, distinct from the Oregon and Ethiopian material (figure 3B and figure 4, red dots). In addition to the differences with major elements Na, Ca, and K, the analyses revealed that the trace elements Mg, Cu, Ga, and Sr were the four best discriminators providing clear separations among Oregon and Ethiopian sunstone and Cu-bearing feldspar with undetermined color origin. All Ethiopian sunstone had a lower Mg concentration (261–686 ppmw, table 1) than Oregon sunstone (>810 ppmw, table 1) (figure 3A). The Cu-bearing feldspar with undetermined color origin had a higher Cu (>405 ppmw) and Sr (>1120 ppmw) concentration than Oregon and Ethiopian sunstone (figure 3, A, D, E, and F). Interestingly, Ponderosa samples (figure 3, C and E) had a lower Ga concentration (<14.0 ppmw) than the Ethiopian sunstone and Cu-bearing feldspar with undetermined color origin. A group of Dust Devil stones with higher Mg, Ga, and Sr concentrations were separated from all other sources in figures 3A and 3C, further differentiating them from Ponderosa stones.

Copper-bearing sunstones from different sources are visually indistinguishable from one another. Gemological properties are usually sufficient to separate Ethiopian and Oregon sunstones from these Cu-bearing feldspars with undetermined color origin. However, accurate major and trace element chemical analysis obtained by methods such as LA-ICP-MS, XRF (Ga and Sr were first identified as reliable discriminators for separating Ethiopian from Oregon sunstones using XRF by author GRR before this work), or electron microprobe is critical to separating Ethiopian, Oregon, and Cu-bearing feldspar with undetermined color origin.

Ziyin Sun is research associate, Nathan D. Renfro is manager of colored stones identification, Aaron C. Palke is senior manager of research, Heidi Breitzmann is senior staff gemologist, Jonathan Muyal is former staff gemologist, Dylan Hand is senior analytics technician, Maxwell Hain is staff gemologist, and Shane F. McClure is senior director of identification, at GIA in Carlsbad, California. Yusuke Katsurada is a senior staff gemologist and Makoto Miura a staff gemologist (colored stones) at GIA in Tokyo. George Rossman is a professor of mineralogy at the California Institute of Technology.