Willemite is an uncommon zinc silicate mineral (Zn2SiO4) that forms as a secondary mineral in zinc ore deposits. It was named after King William I of the Netherlands, where willemite was first discovered. Willemite is usually an aesthetically uninteresting mineral with a dull brown, green or white color, and it rarely forms crystals. What makes willemite a very popular mineral among collectors is that it often fluoresces a bright green when exposed to ultraviolet light.

Willemite is just one of many minerals to exhibit fluorescence. Perhaps the most famous location for fluorescing minerals is Franklin, New Jersey, where the old zinc mines now showcase their glowing rocks to visitors. The Franklin Marble is a very old calcite-based rock (about 1.295 billion years old, according to the New Jersey Geological and Water Survey). Hundreds of different minerals have been found in this marble, 90 of which fluoresce. This is an area unlike anywhere else in the world geologically, and the wide variety of minerals are the result of the rare deposits of zinc, iron and manganese.

For a mineral to fluoresce, certain ions called activators must be present in the right proportion.  Activators are usually trace elements, but some minerals like scheelite (CaWO4) and certain uranium minerals can fluoresce from the elements in their chemical formula. Willemite fluoresces because of Manganese 2+ activators substituting for zinc cations in the chemical structure. Manganese 2+ also causes the fluorescence in the other minerals in the Franklin Marble, such as calcite which glows a bright red-orange color, Hardystonite which glows blue, and esperite which glows yellow.

Come see examples of glowing willemite in the fluorescence display at the Dice Mineralogical Museum, open 12:30 to 4 p.m. on Mondays, Wednesdays and Fridays.