Hardystonite crystals, Calcite and Willemite from Franklin, New Jersey
Contributed by: Michael Crawford
Date: Jun 26th, 2025
Locality: Franklin, Sussex County, New Jersey, USA (See on Mindat)
Size: 12 x 16 cm
Description:
This specimen contains hardystonite (Ca2Zn[Si2O7]) crystals in a matrix of willemite, calcite, zincite, and franklinite from the Franklin Mine, Sussex County, New Jersey. The first image shows the specimen illuminated by shortwave UV light (255nm LED). Calcite fluoresces red, willemite fluoresces green and the hardystonite fluoresces dark violet blue. All three minerals have dimmer fluorescence of the same colors under longwave and midwave illumination,
Shortwave emission spectra show that the calcite and hardystonite fluoresce in the ultraviolet region as well as the visible region. The ultraviolet fluorescence is considerable brighter than the visible fluorescence. The ultraviolet fluorescence in hardystonite and calcite is activated by lead. The ultraviolet fluorescence of calcite peaks at 313 nm. The hardystonite is a broader peak with a maximum at 350 nm. The dark violet-blue hardystonite fluorescence is the longer wave shoulder of this ultraviolet fluorescence. Hardystonite fluorescence is the brightest ultraviolet fluorescence that I have encountered.
The visible fluorescence of calcite and willemite is activated by manganese. The willemite emission spectrum peaks at 523 nm and calcite has a broader peak that has a maximum at 618 nm.
The second image is a false color image and shows the ultraviolet fluorescence. The narrow bandpass filters used to create this false color image are shown on the spectral plot image. The hardystonite tetragonal crystals are clearly defined in this image as white. The calcite appears blue in the ultraviolet image because of a fluorescent peak at 313 nm.
Summary of luminescence responses:
Hardystonite
- Fluorescence under Shortwave (255nm LED) UV light: Blue
- Fluorescence under Shortwave (255nm LED) UV light: Green
- Fluorescence under Shortwave (255nm LED) UV light: Red