1911 Encyclopædia Britannica/Willemite
WILLEMITE, a mineral consisting of zinc orthosilicate, Zn2SiO4, crystallizing in the parallel-faced hemihedral class of the rhombohedral system. Crystals have the form of hexagonal prisms terminated by rhombohedral planes: there are distinct cleavages parallel to the prism-faces and to the base. Granular and cleavage masses are of more common occurrence. The colour varies considerably, being colourless, white, greenish yellow, apple-green, flesh-red, &c. The hardness is 5½, and the specific gravity 3.9-4.2. A variety containing much manganese replacing zinc is called “troostite.” Willemite occurs at Sterling Hill, Sussex county, and Franklin Furnace in New Jersey, where it is associated with other zinc ores (franklinite and zincite) in crystalline limestone. It has been found at only a few other localities, one of which is near Liége, and for this reason the mineral was named after William I. of the Netherlands. Under the influence of radium radiations, willemite fluoresces with a brilliant green colour. (L. J. S.)