Sphalerite and Willemite from the Sterling Hill Mine, New Jersey
Contributed by: Michael Crawford
Date: Dec 13th, 2025
Locality: Sterling Hill, Ogdensburg, Sussex County, New Jersey, USA (See on Mindat)
Size: 10 x 10.5 cm
Description:
A specimen of mahogany sphalerite in contact with a matrix of willemite and franklinite. The specimen comes from the Sterling Hill Mine, Sussex County, New Jersey. The sphalerite was emplaced after the willemite and franklinite formed. The emplacement of the sphalerite altered the matrix. At the contact, the matrix is all franklinite that is magnetic. The amount of willemite is reduced near the contact with the sphalerite and the willemite is no longer fluorescent. There is a dead zone of no fluorescence in the matrix at the contact with the sphalerite. The willemite in the matrix is fluorescent under all wavelengths of UV illumination and it is brightest under shortwave light. The emission spectra of this willemite is the same for all wavelengths of UV illumination and the emission has a maximum at 523 nm. Some of the willemite has brief afterglow from shortwave exposure.
There are two types of sphalerite in this specimen. One type of sphalerite fluoresces pink under longwave light. This sphalerite occurs closest to the contact with the matrix. The second type fluoresces orange under longwave light and it occurs as numerous veinlets. The longwave emission spectrum of the pink sphalerite (#2 in the annotated image) has a dominate peak at 450 nm and a very small broad peak around 611 nm. The longwave emission spectrum of the orange sphalerite (#3 in the annotated image) has two almost equal peaks. One peak at 446 nm like the pink sphalerite and the other peak is much broader with a maximum at 614 nm. Both sphalerites have long-lasting afterglow from exposure to longwave light. The afterglow is less intense from midwave exposure and no afterglow was observed from shortwave exposure.
An interesting feature of both sphalerite types is that the blue emission peak around 450 nm is reduced relative to orange emission peak around 611-614 nm under midwave and shortwave illumination. The sphalerite fluorescence appears orange under midwave and shortwave illumination.
Sphalerite is a semi-conductor. The band gap between the valance band and the conduction band is narrow enough that electrons can move to the conduction band when sphalerite is exposed to UV light. When the electrons return to the valance band, blue photons are emitted. It is not clear if the recombination of the electron from the conduction band to the valance band releases a photon of blue light or if the blue emission is activated by a silver ion replacing zinc. For more details on semiconducting minerals and band transitions see the article by Glenn Waychunas, in the 2020 FMS Journal.
Manganese replacing zinc activates the orange fluorescence. It takes less energy to create this manganese activated fluorescence, so the light emitted is at a longer wavelength. Emission spectra of this orange fluorescence is a broad peak with maximum at 614 nm. The 450 nm peak is still present in the emission of the orange, fluorescent sphalerite.
Summary of luminescence responses:
Sphalerite (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Pink
- Fluorescence under Longwave (365nm LED) UV light: Orange
- Fluorescence under Midwave (305nm LED) UV light: Orange
- Fluorescence under Shortwave (255nm LED) UV light: Orange
- Afterglow after exposure to Longwave (365nm LED) UV light: Pink
- Fluorescence under Longwave (365nm LED) UV light: Green
- Fluorescence under Midwave (305nm LED) UV light: Green
- Fluorescence under Shortwave (255nm LED) UV light: Green