Multi-color Sphalerite from the Franklin Mine, New Jersey

Contributed by: Michael Crawford
Date: Nov 18th, 2025
Locality: Franklin, Sussex County, New Jersey, USA (See on Mindat)
Size: 8 x 14.5 cm

Description:
A multi-color sphalerite specimen from the Franklin Mine, Sussex County, New Jersey. The sphalerite fluoresces blue, violet, pink, and orange under longwave (365 nm LED) illumination. It also has strong afterglow from longwave exposure. The sphalerite loses the blue hues under midwave (310 nm LED) and shortwave (257 nm LED) illumination and fluoresces orange. There is no afterglow from midwave and shortwave exposure.

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. The emission spectrum of this blue fluorescence peaks around 450 nm. For more details on semiconducting minerals and band transitions see the article by Glenn Waychunas, in the 2020 FMS Journal.

Manganese replacing zinc in the 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 maxima around 610 nm to 620 nm. The 450 nm peak is still present in the emission of the orange, fluorescent sphalerite.

Fluorescent hydrozincite is also present in the specimen. It occurs as fracture coatings. It has a blue-greenish gray color under longwave light. It is blue under shortwave light. The emission peak moves from 447 nm under longwave light to 439 nm under shortwave light. The areas of hydrozincite coatings react to acid, indicating it is mixed with calcite or aragonite. The carbonate in the coating is responsible for its afterglow from all wavelengths of UV light.

Calcite in the specimen has weak red fluorescence under midwave light. It is much brighter red under shortwave light. The red fluorescence is activated by manganese.