Zinc spar

Smithsonite
A specimen of smithsonite from Kelly mine, Socorro County, New Mexico, United States.
General
Category	Carbonate mineral
Formula (repeating unit)	ZnCO3
Strunz classification	5.AB.05
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	R3c
Unit cell	a = 4.6526(7) c = 15.0257(22) [Å]; Z = 6
Identification
Color	White, grey, yellow, green to apple-green, blue, pink, purple, bluish grey, and brown
Crystal habit	Uncommon as crystals, typically botryoidal, reniform, spherulitic; stalactitic, earthy, compact massive
Twinning	None observed
Cleavage	Perfect on [1011]
Fracture	Uneven, sub-conchoidal
Tenacity	Brittle
Mohs scale hardness	4.5
Luster	Vitreous, may be pearly
Streak	White
Diaphaneity	Translucent
Specific gravity	4.4 - 4.5
Optical properties	Uniaxial (-)
Refractive index	nω = 1.842 - 1.850 nε = 1.619 - 1.623
Birefringence	δ = 0.223 - 0.227
Ultraviolet fluorescence	May fluoresce pale green or pale blue under UV
References

Smithsonite, or zinc spar, is zinc carbonate (ZnCO₃), a mineral ore of zinc. Historically, smithsonite was identified with hemimorphite before it was realised that they were two distinct minerals. The two minerals are very similar in appearance and the term calamine has been used for both, leading to some confusion. The distinct mineral smithsonite was named in 1832 by François Sulpice Beudant in honor of English chemist and mineralogist James Smithson (c.1765–1829), whose bequest established the Smithsonian Institution and who first identified the mineral in 1802.

Smithsonite is a variably colored trigonal mineral which only rarely is found in well formed crystals. The typical habit is as earthy botryoidal masses. It has a Mohs hardness of 4.5 and a specific gravity of 4.4 to 4.5.

Smithsonite occurs as a secondary mineral in the weathering or oxidation zone of zinc-bearing ore deposits. It sometimes occurs as replacement bodies in carbonate rocks and as such may constitute zinc ore. It commonly occurs in association with hemimorphite, willemite, hydrozincite, cerussite, malachite, azurite, aurichalcite and anglesite. It forms two limited solid solution series, with substitution of manganese leading to rhodochrosite, and with iron, leading to siderite.