Genthelvite from Sterling Hill

Contributed by: Michael Crawford
Date: Jun 15th, 2025
Locality: Passaic pit (Marshall Mine; Passaic Mine), Sterling Hill, Ogdensburg, Sussex County, New Jersey, USA (See on Mindat)
Size: 6 x 6.5

Description:
This specimen of genthelvite (Be3Zn4(SiO4)3S) was sourced from the \"genthelvite trench\" within the Passaic Pit at Sterling Hill Mine, Ogdensburg, Sussex County, New Jersey. The genthelvite exhibits bright fluorescence under longwave UV illumination, with diminished fluorescence under shortwave UV illumination, and no fluorescence under midwave UV illumination.

The emission spectrum of genthelvite under longwave UV light shows a peak at 508 nm, consistent with the spectra of genthelvite samples from Mt. St. Hiliare and Argentina. This peak occurs at a shorter wavelength compared to the longwave fluorescent peak (525 nm) of gemmy green willemite from Franklin, New Jersey. The green fluorescence of genthelvite is triggered by manganese substitution for zinc, akin to the activation mechanism in willemite fluorescence.

Under shortwave UV illumination, genthelvite also peaks at 508 nm. The specimen includes a mineral that fluoresces red in shortwave UV light and a small patch of willemite. The shortwave fluorescence of willemite is significantly brighter than that of genthelvite.

The emission spectrum of the red-fluorescent mineral under shortwave UV light reveals a broad peak with a maximum near infrared at 720 nm. Comparison with spectra of other common red fluorescent minerals suggests it is albite, which exhibits red fluorescence due to ferric iron (Fe3+) replacing aluminum (Al3+). The last two images illustrate the shortwave near infrared fluorescence of the specimen. The monochrome image was captured using a 730 nm bandpass filter, highlighting areas of albite mineralization. The second image is a color infrared composite, with the green and red visible bands assigned to blue and green respectively, and near infrared assigned to red.

The shortwave fluorescence peak of willemite occurs at 520 nm, slightly shorter than the typical 525 nm peak observed in Franklin/Sterling Hill willemite specimens. The cause of this spectral shift remains unidentified.