, gems

E.J. Gübelin examines four faceted specimens of gem-quality scorodite from Tsumeb, in present-day Namibia. Their dark blue color and pronounced pleochroism initially suggested tanzanite. Wet-chemical analysis of two small slabs showed that the material was close to the iron-rich end member of the scorodite–mansfieldite series, with minor aluminum and very little phosphorus. X-ray powder diffraction with a Gandolfi camera confirmed the orthorhombic scorodite structure. The measured refractive indices ranged from 1.785 to 1.816, with birefringence of 0.027–0.031, a positive optic sign, and an optic axial angle of about 75°. The average measured specific gravity was 3.29, close to the calculated density of 3.31. Gübelin compares these results with data from other localities and relates differences in optical properties and density to variations in composition. The Tsumeb stones showed pronounced trichroism, with violet, blue, and blue-green hues depending on viewing direction. Their visible spectrum showed a broad absorption band in the green, a cutoff near 450 nm toward shorter wavelengths, and substantial transmission at the red end. Microscopy revealed parallel arrays of hair-fine fibers, intersecting liquid feathers, and abundant dry cracks. The feathers contained densely concentrated droplets or fluid channels and resembled partly healed fractures in Sri Lankan sapphires. Small reddish to brownish-black aggregates showed a metallic, silvery luster where they reached a polished facet. Electron-microprobe analysis of one exposed aggregate indicated chalcocite, an iron-bearing phase interpreted as either hematite or goethite, and an aluminum oxyhydrate. Gübelin considers this assemblage consistent with the mineral association in Tsumeb’s cementation zone. He describes scorodite as a secondary hydrated ferric arsenate formed by oxidation of arsenic-bearing minerals. At Tsumeb, individual crystals up to 25 mm long and groups lining geode-like cavities were found in the second oxidation zone or at its contact with the cementation zone. With a Mohs hardness of only 3½–4 and a limited supply of facetable material, Gübelin regards scorodite chiefly as a collector’s gem.