Formula: Cu4(SO4)(OH)6 · 2H2O

Colour: Blue, greenish-blue

Lustre: Vitreous, Silky

Hardness: 2½ – 3

Specific Gravity: 3.48 – 3.5

Crystal System: Monoclinic

Name: Named by Nevil Story-Maskelyne in 1864 in honor of Victor von Lang (1838-1921), physicist and crystallographer, Professor of Physics, University of Vienna, Vienna, Austria. At the time of the mineral’s discovery von Lang was employed at the British Museum (1862-1864). He became a pioneer of crystal physics.

Co-Type Localities: St Just, Cornwall, England, UK;Fowey Consols (incl.Wheal Treasure; Wheal Fortune; Wheal Chance), Tywardreath and Par, Cornwall, England, UK.

Dimorph of: Wroewolfeite

A secondary copper mineral formed from the oxidation of copper sulphides. Often forms pseudohexagonal twins. Some habits are easily confused with posnjakite and wroewolfeite. May be altered to brochantite.