Multi-Wave Hardystonite - Franklin, New Jersey

Contributed by: Michael Crawford
Date: Jul 5th, 2026
Locality: Franklin Mine, Franklin, Sussex County, New Jersey, USA (See on Mindat)
Size: 13 cm

Description:
I started this study as a comparison of the fluorescence of hardystonite (Ca₂Zn[Si₂O₇]) under 340nm LED illumination. The specimens come from Franklin, New Jersey. The hardystonite shows violet fluorescence under 340 nm illumination, a UV wavelength that is not typically used to examine specimens for fluorescence. The violet fluorescence under 340 nm light is much brighter than the typical dark blue hardystonite fluorescence seen under SW (255 nm LED) illumination. Most of the hardystonite specimens in my collection fluoresce violet under 340 nm illumination. The specimen on the left shows this violet fluorescence. However, a couple of the hardystonite specimens have little or no reaction under 340 nm light. An example of this type of hardystonite is shown on the right.

The hardystonite specimen on the left is unusual because it fluoresces under all wavelengths of UV light. The spectra show the emission under SW (255nm), MW (305nm) and 340nm peak in the ultraviolet region. The maximum emission for each wavelength is different. It is at 350 nm with an inflection around 340 nm for SW. There are two peaks in the midwave spectrum (367 nm and 383 nm). Emission spectrum measured under 340 nm illumination has a peak at 394 nm just inside the ultraviolet region. The emission is very bright in the ultraviolet and its extension into the visible region produces a violet color. There is no definitive peak in the LW spectrum, but there is a rapid increase in emission in the blue violet region. There are also small peaks at 568 nm and 597 nm. These peaks may be fluorescence from hardystonite altered to clinohedrite.

Both specimens exhibit dark blue fluorescence under shortwave illumination. The shortwave emission spectra of the two specimens have a broad peak in the ultraviolet region with a maximum around 350 nm. The spectra also show an inflection or second peak around 340 nm. The broad ultraviolet peak extends into the visible violet blue producing the dark blue color we observe. The false color images of ultraviolet fluorescence indicate no differences between the two specimens.

The ultraviolet fluorescence of hardystonite is activated by lead. Not sure if lead is replacing calcium, zinc or both to activate the ultraviolet fluorescence. Trace amounts of lead are found in most hardystonite from Franklin. Hardystonite is a potential zinc ore, but the lead content made extraction of zinc by smelting very difficult, so hardystonite was sent to the waste pile. We can thank lead for leaving us an abundance of hardystonite specimens with interesting fluorescence.