Simpsonite

Simpsonite has a general formula of Al₄(Ta,Nb)₃O₁₃(OH). It occurs as euhedral to subhedral tabular to short and prismatic crystals, commonly in subparallel groups. Under the petrographic microscope it has a very high relief.

Simpsonite
Locality: Alto do Giz pegmatite, Rio Grande do Norte, Brazil. Size: 1.6 x 1.5 x 1.8 cm.
General
Category	Oxide minerals
Formula (repeating unit)	Al4(Ta,Nb)3O13(OH)
Strunz classification	4.DC.10
Crystal system	Trigonal
Crystal class	Pyramidal (3) H-M symbol: (3)
Space group	P3
Unit cell	a = 7.37, c = 4.51 [Å]; Z = 1
Identification
Formula mass	813.65 g/mol
Color	White to cream, yellow to yellow-brown when altered
Crystal habit	Euhedral, prismatic, striated
Cleavage	None
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	7-7.5
Luster	Vitreous to adamantine
Streak	White
Diaphaneity	Semitransparent
Specific gravity	6.7
Optical properties	Uniaxial negative
Refractive index	nω = 2.045 nε = 2.025
Birefringence	δ = 0.020
Other characteristics	Blue-white cathodoluminescence and yellow fluorescence in SW UV
References	[1][2][3][4]

Discovered in 1938, it was named after Edward Sydney Simpson (1875–1939), government mineralogist and analyst of Western Australia.[3] It is an accessory mineral in some tantalum-rich granite pegmatites. It occurs in association with tantalite, manganotantalite, microlite, tapiolite, beryl, spodumene, montebrasite, pollucite, petalite, eucryptite, tourmaline, muscovite and quartz.[1] It is found in a few locations around the world, notably in the Onca and Paraiba mines of Rio Grande do Norte, Brazil and at Tabba Tabba, Western Australia.[1]