Willemite, Zn2SiO4, the reddish mineral in this specimen, has a wide range of color. It's popular with collectors for its strong fluorescence. (more below)
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Willemite occurs with white calcite and black franklinite (a Zn and Mn-rich version of magnetite) in the classic locality of Franklin, New Jersey. In ultraviolet light the willemite glows bright green and the calcite shines red. But outside collectors' circles, willemite is a scarce secondary mineral that forms by oxidation of zinc vein deposits. Here it may take massive, fibrous or radiating crystal shapes. Its color ranges from white through yellow, bluish, green, red and brown to black. It may give off flashes of light when struck with steel, a property called triboluminescence. The Franklin specimens contain a small percentage of manganese to give the reddish color. In other places iron and copper may impart brown and blue colors, respectively.
Other Diagenetic Minerals