Locality: Alto do Giz pegmatite, Rio Grande do Norte, Brazil. Size: 1.6 x 1.5 x 1.8 cm.
|Crystal class||Pyramidal (3)
H-M symbol: (3)
|Unit cell||a = 7.37, c = 4.51 [Å]; Z = 1|
|Formula mass||813.65 g/mol|
|Color||White to cream, yellow to yellow-brown when altered|
|Crystal habit||Euhedral, prismatic, striated|
|Mohs scale hardness||7-7.5|
|Luster||Vitreous to adamantine|
|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|
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.
Discovered in 1938, it was named after Edward Sydney Simpson (1875–1939), government mineralogist and analyst of Western Australia. 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. 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.
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