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Figure 1. The greenish yellow to yellow-green color of this clinohumite from Tanzania (cut stones 0.90–1.81 ct) is quite unusual. Courtesy of Intimate Gems; photo by Robert Weldon.
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During a buying trip to Tanzania in 2006, gem dealer Farooq Hashmi obtained some brownish yellow and yellow-green rough from a few small parcels that were represented as tourmaline from a new deposit at Sumbawanga in west-central Tanzania. All of the rough consisted of broken pieces (figures 1 and 2), so it was not possible to determine if they originated from primary or secondary deposits. Most of the pieces weighed <1 g, and there was an approximately equal percentage of the two colors available in each parcel. The appearance and properties of the material were not typical of tourmaline normally found in Tanzania, so Mr. Hashmi loaned several rough and cut examples of each color for examination at GIA.
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Figure 2. These brownish yellow gems from Tanzania (cut stones 0.64–0.80 ct) were identified as chondrodite. Courtesy of Intimate Gems; photo by Robert Weldon.
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The following properties were recorded on three greenish yellow cut stones (figure 1): color—greenish yellow to yellow-green, with no obvious pleochroism; RI—nα=1.620 and nγ=1.648–1.649; birefringence—0.028–0.029; hydrostatic SG—3.21; Chelsea filter reaction—none; fluorescence—inert to long-wave UV radiation and very weak to weak yellow to short-wave UV; and a weak absorption band at 490 nm visible with the desk-model spectroscope. These properties are similar to those reported for clinohumite by M. O’Donoghue (Gems, 6th ed., Butterworth-Heinemann, Oxford, UK, 2006, p. 400), except that the colors were reported as yellow to orange or dark brown and the RI values were higher (nα=1.623–1.702 and nγ=1.651–1.728). Microscopic examination revealed clouds of fine needles and flat, ribbon-like fluid inclusions. Electron backscatter diffraction analysis showed that these needles were parallel to the b axis (i.e., the 4.7 Å unit cell direction). All three samples were moderately abraded and contained numerous fractures.
The properties of three brownish yellow cut stones (figure 2) were: color—brownish yellow, with weak yellow to colorless pleochroism; RI—nα=1.589–1.593 and nγ=1.618–1.620; birefringence—0.027–0.029; hydrostatic SG—3.20–3.24; Chelsea filter reaction—none; fluorescence—inert to long-wave UV radiation and moderate yellow to short-wave UV; and no features visible with the desk-model spectroscope. These properties are consistent with those reported for chondrodite by O’Donoghue (2006, p. 399), except the colors reported in that publication were deep red and orange brown, and the RI values were slightly higher (nα=1.592–1.643 and nγ=1.621–1.636). Magnification revealed numerous abrasions, fractures, screw dislocations, and planar fluid inclusions.
Electron-microprobe analyses were performed on the three cut clinohumites (15 analyses total) and on one faceted chondrodite (5 analyses), and all of the data points showed a homogenous composition within each mineral; average analyses are shown in table 1. In addition to the elements expected from the chemical formula, (Mg,Fe2+)9(SiO4)4(F,OH)2, the clinohumites contained traces of Ti, Al, and K. Much lower Fe was present than in the data for clinohumite reported by W. A. Deer et al. (Rock-Forming Minerals—Orthosilicates, Vol. 1A, 2nd ed., Longman, London, 2001, pp. 381–417). The relatively small amount of Fe is responsible for the low RI values; although fluorine-rich clinohumite may also show low RI values, these stones contained intermediate amounts of F. The chondrodite, (Mg,Fe2+)5(SiO4)2(F,OH)2, contained traces of Al, Mn, Ca, Na, K, and Ti; the last element was present in considerably lower amounts than the range shown by Deer et al. (2001).
Vis-NIR spectroscopy of a polished piece of the rough clinohumite showed a broad peak centered at ~750 nm (figure 2)—quite unlike typical yellow-orange to orange clinohumite spectra, which are dominated by a peak at 435 nm related to Fe-Ti intervalence charge transfer (see K. Langer et al., "The crystal chemistry of the humite minerals: Fe2+-Ti4+ charge transfer and structural allocation of Ti4+ in chondrodite and clinohumite," European Journal of Mineralogy, Vol. 14, No. 6, 2002, pp. 1027–1032). A weaker feature near 1040 nm is seen in most clinohumite spectra, including the Tanzanian specimen, but dominantly only in one orientation; absorption in this region is due to Fe2+ (Langer et al., 2002). Further research is necessary to fully characterize the origin of the unusual yellow-green color in this clinohumite.
Clinohumite and chondrodite were recently documented from Tanzania by the SSEF laboratory in Basel, Switzerland ("Uncommon minerals as gemstones from Tanzania," SSEF Facette, No. 14, 2007, p. 6). Similar "golden" yellow chondrodite is shown in that article (along with yellow-brown to brown, red, and near-colorless material), whereas the greenish yellow clinohumite described here is quite different from the yellow-orange to orange stones shown in that report.
Eric A. Fritz and Christopher M. Breeding
GIA Laboratory, Carlsbad
George R. Rossman
California Institute of Technology
Pasadena, California
Brendan M. Laurs
William B. (Skip) Simmons and Alexander U. Falster
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