Radiating Willemite from Franklin, New Jersey
Contributed by: Michael Crawford
Date: Jun 12th, 2025
Locality: Franklin, Sussex County, New Jersey, USA (See on Mindat)
Size: 6 x 5 x 9 cm
Description:
This specimen is a nice example of radiating willemite from Franklin, Sussex County, New Jersey. The initial image displays the highly intense shortwave (SW) fluorescence. This variant of willemite is renowned for its prolonged afterglow, which can persist for hours. Although Afghan hackmanite may rival this longevity, it is believed that the afterglow of radiating willemite is superior in duration. The green fluorescence of willemite is activated by trace amounts of manganese replacing zinc.
Arsenic is responsible for activating the willemite\'s afterglow. Arsenic (As5+) replaces some of the silicon (Si4+) within the willemite structure. When UV illumination excites electrons in manganese ions within willemite to higher energy states, some of these electrons are trapped by proximate arsenic atoms. These electrons can remain trapped until sufficient thermal energy enables their return to the ground state of the Mn2+ atom, releasing photons perceived as afterglow. The afterglow emission spectrum corresponds to the fluorescence emission observed under UV illumination. The duration of the afterglow can be adjusted by altering the specimen’s temperature; cooling extends the afterglow while heating reduces it.
Willemite within the matrix also exhibits phosphorescence, albeit with a considerably shorter afterglow duration compared to radiating willemite. Shortwave emission spectra reveal that the peak emission of radiating willemite occurs at 519 nm, whereas the matrix willemite peaks at 524 nm. The cause of this discrepancy remains unidentified.
In this specimen, radiating willemite is located within a secondary vein intersecting the gneissic willemite and franklinite matrix. Radiating willemite forms due to hydrothermal alteration of primary zinc ores, liberating zinc into solution and subsequently precipitating as willemite within fractures. Hydrothermal fluids also alter the matrix adjacent to the vein, replacing willemite with dolomite and tan serpentine. Franklinite persists in the altered zone. The altered zone near the vein loses most of its fluorescence because the dolomite and serpentine replacements are non-fluorescent.
The specimen was fractured along the vein to expose the radiating willemite, which typically occurs in veins. Willemite crystals radiate outward from a central point, creating flower-like patterns. The intensity of the afterglow appears greatest at the center of the flower.
The area of the white radiating willemite is 6 cm by 5 cm.
Summary of luminescence responses:
Willemite
- Fluorescence under Longwave (365nm LED) UV light: Green
- Fluorescence under Midwave (305nm LED) UV light: Green
- Fluorescence under Shortwave (255nm LED) UV light: Green
- Afterglow after exposure to Midwave (305nm LED) UV light: Green
- Afterglow after exposure to Shortwave (255nm LED) UV light: Green