Bismutite

Bismutite or bismuthite is a bismuth carbonate mineral with formula Bi₂(CO₃)O₂ (bismuth subcarbonate). Bismutite occurs as an oxidation product of other bismuth minerals such as bismuthinite and native bismuth in hydrothermal veins and pegmatites.[2] It crystallizes in the orthorhombic system and typically occurs as earthy to fibrous masses.[1]

Bismutite
Bismutite from Schneeberg, Germany
General
Category	Carbonate mineral
Formula (repeating unit)	Bi2(CO3)O2
Strunz classification	5.BE.25
Dana classification	16a.03.05.01
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) (same H-M symbol)
Space group	Immm
Unit cell	a = 3.865 Å, b = 3.862 Å, c = 13.675 Å; Z = 2
Identification
Color	Yellow to brown, greenish, green-grey, grey or black
Crystal habit	Very rare as platy crystals; typically radially fibrous to spheroidal, in crusts and earthy to dense massive aggregates
Twinning	pseudo-merohedral twinning simulates tetragonal symmetry
Cleavage	Distinct/Good on {001} (microscopically observable)
Mohs scale hardness	2.5 - 3.5
Luster	Vitreous, waxy, may be dull to earthy
Streak	Grey
Diaphaneity	Opaque to transparent in small grains
Specific gravity	6.7 - 7.4 measured, 8.15 calculated
Optical properties	Biaxial (-) (appears uniaxial due to twinning)
Refractive index	a=2.12-2.15, b=2.12-2.15, g=2.28
Birefringence	0.1300-0.1600
2V angle	45
References	[1][2][3][4]

It was first described in 1841 for an occurrence in Saxony.[1]

The term bismuthite has been used in the past for bismuthinite.[5]

References

1. Webmineral
2. Mindat
3. Handbook of mineralogy
4. Grice, Joel D., A Solution to the Crystal Structures of Bismutite and Beyerite, The Canadian Mineralogist, Vol. 40, pp. 693-698 (2002)
5. Chisholm, Hugh, ed. (1911). "Bismuthite" . Encyclopædia Britannica. 4 (11th ed.). Cambridge University Press. p. 11.