Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Trigonal
    Chemistry : Zn₂SiO₄
    Rarity : Fairly common to uncommon

Willemite is an uncommon mineral in zinc deposits, mainly those hosted in limestone formations. It can be primary, as in Franklin (New Jersey), where it accompanies franklinite and zincite, but remains essentially secondary, formed by alteration of sphalerite in an arid climate. Its chemical composition is constant, only manganese can replace zinc in significant quantities (up to 7%): this is the troostite variety, brownish to pink in color. It was named in honor of William I, King of the Netherlands from 1815 to 1840. Willemite exists mainly in granular or fibrous masses with a greasy to vitreous luster, most often colorless or gray, sometimes colored yellow, brown or pink. Crystals of this species are rare and form short hexagonal prisms ending in a very flattened rhombohedron with curved faces. It was an ore of zinc in the rare deposits where it was abundant such as at Franklin (New Jersey). Gemmy crystals are sometimes cut for jewelry.

Main photo : 8 cm willemite from Franklin Mine, New Jersey, USA © Rock Currier

Willemite twins are rare, but known on {10-10}.

No fakes listed for this mineral species.