Willemite

Willemite is a zinc silicate named after Dutch King William I. It crystallizes in the trigonal system (space group R3̄) and forms typically in oxidation environments of zinc deposits, often associated with franklinite, zincite, and other metal oxides. Its hardness of 5.5 on the Mohs scale makes it relatively brittle, but its most fascinating characteristic is the brilliant green fluorescence under long-wave ultraviolet radiation (365 nm), caused by the presence of manganese traces as an activator. This phenomenon has made it a highly sought-after collector's gem, although it is rarely transparent in gem quality. The most beautiful and transparent crystals come from the famous Franklin mine in New Jersey, United States, where it was discovered in 1829 and where it remains today the most representative specimen.

Willemite (Zn₂SiO₄, space group R3̄, a = 13.98 Å, c = 9.30 Å) is an anhydrous silicate with a modified olivine-type structure. It exhibits a relative density of 4.0–4.2 g/cm³ and hardness of 5.5 Mohs. The refractive index ranges from nω = 1.691 to nε = 1.719 (negative birefringence), with birefringence Δn ≈ 0.028. Fluorescence is exceptional: under UV radiation at 254 nm (short-wave) it emits brilliant green, while at 365 nm (long-wave) the fluorescence is even more intense; the activator is Mn²⁺ substituting for Zn²⁺. Raman spectroscopy shows characteristic Si–O stretching bands around 900–950 cm⁻¹. Willemite forms in supergene oxidation zones of zinc deposits, often in association with franklinite (ZnFe₂O₄), zincite (ZnO), and other secondary minerals. Gem-quality specimens are extremely rare; the finest come from Franklin (New Jersey, USA), where willemite was the signature mineral of the mine. Composition may contain traces of Mn, Fe, and Cd. Under ordinary visible light it appears colorless, pale yellow-green or green, sometimes with orange tones due to franklinite inclusions.