WILLEMITE, a mineral consisting of zinc orthosilicate, 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. It varies considerably in colour, being colourless, white, greenish-yellow, apple-green, flesh-red, etc. The hardness is 51-, and the specific gravity 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 lime stone. It has been found at only a few other localities, one of which is near Liege, 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.