Witherite

Witherite
Witherite from Cave-in-Rock (size: 4.9 x 3.7 x 3.2 cm)
General
Category	Carbonate mineral
Formula (repeating unit)	BaCO3
Strunz classification	5.AB.15
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pmcn
Unit cell	a = 5.31 Å, b = 8.9 Å c = 6.43 Å; Z = 4
Identification
Color	Colorless, white, pale gray, with possible tints of pale-yellow, pale-brown, or pale-green
Crystal habit	Striated short prismatic crystals, also botryoidal to spherical, columnar fibrous, granular, massive.
Twinning	On {110}, universal
Cleavage	Distinct on {010} poor on {110}, {012}
Fracture	Subconchoidal
Mohs scale hardness	3.0 - 3.5
Luster	Vitreous, resinous on fractures
Streak	White
Diaphaneity	Subtransparent to translucent
Specific gravity	4.3
Optical properties	Biaxial (-)
Refractive index	nα = 1.529 nβ = 1.676 nγ = 1.677
Birefringence	δ = 0.148
2V angle	Measured: 16°, calculated: 8°
Dispersion	Weak
Ultraviolet fluorescence	Fluorescent and phosphorescent, short UV=bluish white, long UV=bluish white
References

Striated short prismatic crystals, also botryoidal to spherical,

Witherite is a barium carbonate mineral, BaCO₃, in the aragonite group. Witherite crystallizes in the orthorhombic system and virtually always is twinned. The mineral is colorless, milky-white, grey, pale-yellow, green, to pale-brown. The specific gravity is 4.3, which is high for a translucent mineral. It fluoresces light blue under both long- and short-wave UV light, and is phosphorescent under short-wave UV light.

Witherite forms in low-temperature hydrothermal environments. It is commonly associated with fluorite, celestine, galena, barite, calcite, and aragonite. Witherite occurrences include: Cave-in-Rock, Illinois, US; Pigeon Roost Mine, Glenwood, Arkansas, US; Settlingstones Mine Northumberland; Alston Moor, Cumbria; Anglezarke, Lancashire and Burnhope,County Durham, England; Thunder Bay area, Ontario, Canada, Germany, and Poland (Tarnowskie Góry and Tajno at Suwałki Region).