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(after King Willem I of the Netherlands), a mineral, Zn2SiO4 (73.0 percent ZnO, 27.0 percent SiO2). Admixtures of Mn2+ are common. Crystallizes in a trigonal system. The structure is complex, composed of tetrahedrons of SiO4 and ZnO4 linked by common oxygens. Prismatic or acicular crystals are rare; they are usually granular or radial-fibrous aggregates, incrustations, and so on.
Willemite is colorless, greenish, or yellowish brown, and it phosphoresces brightly in ultraviolet light or cathode rays. Hardness on the mineralogical scale, 5-6; density, 3,890-4,180 kg/m3. It formed both in oxidation zones of lead-zinc deposits and in certain contact-metasomatic deposits (at Franklin in the USA). It is used as a zinc ore. It is also used to make fluorescent screens in cathode-ray tubes and other instruments.