Langite
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, United Kingdom. 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. Unit cell. 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. Physical properties. 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. Optical properties. Langite is biaxial (-). Since it is monoclinic, it has three different refractive indices, corresponding to the three crystallographic directions. All the refractive indices are in the range 1.64 to 1.80, which is comparatively large, almost as high as garnet. Different sources give these values: The mineral is pleochroic, with X light yellowish green, Y blue-green and Z sky blue. Occurrence. Langite is an uncommon but widespread secondary mineral in the oxidised zone of copper sulfide deposits , which may be of post-mine formation. It is associated with wroewolfeite, posnjakite, serpierite, devilline, chalcophyllite, connellite, brochantite, malachite and gypsum. There are two type localities for langite, Fowey Consols, Tywardreath, Par Area, St Austell District, and St Just, St Just District, both in Cornwall, England. The type material is conserved at the Natural History Museum, Vienna, Austria, reference A.a.4353. Other reported occurrences include: