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 × 1.5 × 1.8 cm.
General
Category	Oxide minerals
Formula	Al4(Ta,Nb)3O13(OH)
IMA symbol	Spn[1]
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	[2][3][4][5]

Discovered in 1938, it was named after Edward Sydney Simpson (1875–1939), government mineralogist and analyst of Western Australia.^[4] 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.^[2] It is found in a few locations around the world, notably in the Onca and Paraíba mines of Rio Grande do Norte, Brazil and at Tabba Tabba, Western Australia.^[2]