Multi-color Sphalerite from the Horn Silver Mine, Utah
Contributed by: Michael Crawford
Date: Oct 3rd, 2025
Locality: Horn Silver Mine, Frisco, San Francisco Mining District, Beaver County, Utah, USA (See on Mindat)
Size: 9.5 x 11 cm
Description:
A sphalerite (ZnS) specimen from the Horn Silver Mine, Beaver County, Utah. The sphalerite in this specimen has three colors of fluorescence, orange, green, and yellow-green, under longwavw UV illumination. The afterglow from exposure to longwave light also has three colors, orange, green, and yellow. There is very little fluorescence under midwave and shortwave light.
Green is an uncommon fluorescent color for sphalerite. Yellow to orange fluorescence is the most common fluorescent color of sphalerite. Sphalerite can also fluoresce blue, pink, red and near infrared. Different activators are responsible for the rainbow of colors of sphalerite fluorescence. Green sphalerite fluorescence is caused by copper and aluminum substituting for two zinc ions. Zinc ions have a 2+ charge, cuprous ions have a 1+ charge, aluminum ions have a 3+ charge. Two zinc ions (2 * 2+ = 4+) are replaced by a copper ion and an aluminum ion ((1+) + (3+) = 4+) in order to maintain charge balance. Details on how the copper-aluminum replacement activates the green sphalerite fluorescence are described by Waychunas in the 2020 FMS Journal.
The longwave emission spectrum of the green, fluorescent sphalerite has a peak at 503 nm. The green fluorescence of this Horn Silver Mine specimen is similar to other green, fluorescent sphalerites from Balmat, New York and Franklin, New Jersey. The longwave emission spectra of those sphalerites have peaks around 500 nm. The green fluorescence of willemite is slightly more yellow compared to green sphalerite fluorescence. The peak of willemite fluorescence is at 526 nm.
The spectrum of the yellow-green, fluorescent sphalerite is like the green sphalerite. Its maximum is at 505 nm; the width of the peak is slightly wider towards longer wavelengths.
The spectrum of orange, fluorescent sphalerite has two peaks at 465 nm and 584 nm. The peak at 584 nm is activated by manganese replacing zinc. Silver may be the activator for the blue, fluorescent peak. However, references cite that silver activated sphalerite fluorescence peaks around 450 nm.
Summary of luminescence responses:
Sphalerite (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Green
- Fluorescence under Longwave (365nm LED) UV light: Orange
- Fluorescence under Longwave (365nm LED) UV light: Yellow
- Afterglow after exposure to Longwave (365nm LED) UV light: Green
- Afterglow after exposure to Longwave (365nm LED) UV light: Orange
- Afterglow after exposure to Longwave (365nm LED) UV light: Yellow