Langite

Langite
Langite from Podlipa and Reinera Mines, Slovakia
General
Category	Copper minerals
Formula (repeating unit)	Cu4(SO4)(OH)6·2H2O
Strunz classification	7.DD.10
Dana classification	31.4.3.1
Crystal system	Monoclinic
Crystal class	Domatic (m) (same H-M symbol)
Space group	Pc
Unit cell	a = 7.118, b = 6.031 c = 11.209 [Å] β = 90.00−90.02°; Z = 2
Identification
Formula mass	488.32 g/mol
Color	Greenish blue, sky-blue to bluish green
Crystal habit	Crystals scaly, or as crusts; earthy
Twinning	Common on {110}, typically repeated
Cleavage	{001} perfect, {010} distinct
Fracture	Uneven
Mohs scale hardness	2.5 to 3
Luster	Vitreous, crusts silky
Streak	Blue green
Diaphaneity	Translucent
Specific gravity	3.28 3.48 to 3.5 3.5 2.28 to 3.34
Optical properties	Biaxial (-) r>v weak
Refractive index	nα = 1.708 nβ = 1.760 nγ = 1.798
Birefringence	δ = 0.090
Pleochroism	X = c = light yellowish green Y = b = blue-green Z = a = sky blue
Solubility	Insoluble in water, easily soluble in dilute acids or NH4OH
Other characteristics	May be altered to brochantite. Not radioactive
References

Langite is a rare hydrated copper sulfate mineral, with hydroxyl, found almost exclusively in druses of small crystals. It is formed from the oxidation of copper sulfides, and was first described in specimens from Cornwall, England. It is dimorphous with wroewolfeite. Langite was discovered in 1864 and named after the physicist and crystallographer Viktor von Lang (1838–1921), who was Professor of Physics at the University of Vienna, Austria.

Langite belongs to the monoclinic crystal class m, meaning that it has just one mirror plane, and no axes of rotational symmetry. The crystal is built up of identical unit cells stacked together, with no space in between. The unit cell for the monoclinic system has a base which is a rhombus, with sides a and c inclined at angle β. The third side b is perpendicular to both a and c. For langite the angle β is very close to 90°, so the unit cell is almost brick-shaped (as for the orthorhombic system). Different sources give slightly different values for the unit cell parameters, but they all fall within the ranges a = 7.118 Å to 7.137 Å, b = 6.031 Å to 6.034 Å, c = 11.209 Å to 11.217 Å, β = 90.00 to 90.02°, Z (the number of formula units per unit cell) = 2.

Langite usually occurs as druses of small greenish-blue crystals which may be scaly or earthy. It is translucent, with a vitreous to silky luster and a blue-green streak. It has perfect cleavage perpendicular to the c crystal axis, and distinct cleavage perpendicular to b. Twinning is common, and typically repeated to give snowflake or star shaped groupings. The mineral is soft, with hardness 2.5 to 3, a little less than that of calcite. Fracture is uneven, and specific gravity is in the range 3.28 to 3.50, a little less than that of diamond.