A silicate mineral Zn4[Si2O7](OH)2.H2O which accompanies zinc, iron, and lead sulphides and is associated with smithsonite ZnCO3, a principal ore of zinc; sp. gr. 3.4–3.5; hardness 4–5; orthorhombic; when massive, white, grey, yellow, brown, green, or light blue, sometimes colourless; vitreous lustre; crystals can be small, tabular grains, but normally form aggregates of radial or earthy masses; cleavage perfect prismatic {110}; occurs in the oxidation zone of lead-sulphide/zinc-sulphide deposits, in hydrothermal veins near the surface, and as a primary mineral in hydrothermal veins, associated with sulphides and fluorite.