Simpsonite: Gemstones Information
Simpsonite is Al4Ta3O13(OH) and forms euhedral to subhedral yellow toorange to brown tabular to short prismatic transparent crystals of the trigonal system with an adamantine lustre and hardness 7.5. The SG is 5.92–6.84. The RI for the ordinary and extraordinary rays is 2.035 and 1.977, uniaxial negative with birefringence 0.058. Crystals from western Australia show a bright blue-white under SWUV. Simpsonite occurs as an accessory mineral in some Ta-rich granite pegmatites. The most attractive simpsonite, of strong bright orange, is found at Tabba Tabba, western Australia; other material from the Alto do Giz pegmatite near Parelhas and the Orca mine in the same general district of Rio Grande do Norte, Brazil.