Durangite: Gemstones Information
Durangite is NaAl(AsO4)F and occurs characteristically as transparent to opaque euhedral monoclinic crystals which may show an attractive orange, orange-brown or pale yellow. The hardness is 5–5.5 and the SG about 3.9. Crystals are brittle. The RI is 1.597–1.634, 1.636–1.673,1.647–1.685, biaxial negative. Birefringence is near 0.050. Pleochroic colours are shades of orange-yellow with one direction near colourless. Durangite occurs in pegmatite dikes in granite or in tin placers derived from alkalic rhyolite. The main source of ornamental durangite is the Barranca tin mine, Durango, Mexico.