Sphalerite – Balmat, New York

Contributed by: Michael Crawford
Date: Jul 15th, 2026
Locality: Balmat, Fowler, St. Lawrence County, New York, USA (See on Mindat)
Size: 8 x 10 cm

Description:
Sphalerite specimen from Balmat, St. Lawrence County, New York. The sphalerite fluoresces bright green, violet-blue, and dark orange under longwave UV light. The green, fluorescent sphalerite has modest green afterglow from exposure to longwave light. The violet-blue sphalerite has violet-blue afterglow that is dimmer than the green afterglow. Under midwave UV light, most of the sphalerite fluoresces orange. The fluorescence under shortwave light is similar to the midwave response.

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. Impurities such as copper, manganese, and silver can alter the band gap energy difference and activate a wide range of colors. 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. The longwave emission spectrum shows the green fluorescence peak at 499 nm in the Balmat specimen.

The blue fluorescence of the sphalerite has an emission spectrum with a peak at 450 nm. This blue fluorescence may also be caused by copper (Cu⁺) and aluminum impurities replacing zinc, but the replacement copper aggregates with interstitial copper in defect sites to activate blue fluorescence. Silver is also known to activate blue fluorescence, but the reported fluorescent peak is at 450 nm and the presence of silver is not reported at Balmat.

Longwave and midwave emission spectra of the orange, fluorescent sphalerite has a broad peak in the blue region with a maximum at 459 nm. There are smaller peaks in the spectra around 569 nm and 601 nm in the yellow and orange regions.