Hodgkinsonite, Willemite, from Franklin, New Jersey

Contributed by: Michael Crawford
Date: Mar 7th, 2026
Locality: Franklin Mine, Franklin, Sussex County, New Jersey, USA (See on Mindat)
Size: 4 x 6 cm

Description:
This is a specimen of pink hodgkinsonite (Mn²⁺Zn₂(SiO₄)(OH) ₂), willemite (Zn₂SiO₄) and franklinite (Zn²⁺Fe³⁺₂O₄) from the Franklin Mine in New Jersey. The hodgkinsonite fluoresces dull red under longwave and midwave UV illumination. It has no shortwave fluorescence. The willemite fluoresces green under all wavelengths of UV light. The willemite fluorescence is so bright that it masks the nearby hodgkinsonite fluorescence especially under longwave illumination. Hodgkinsonite appears brighter compared to willemite under midwave light.

The longwave emission spectrum of hodgkinsonite has a broad peak with a maximum at 661 nm. The fluorescence is likely activated by manganese. Manganese is part of hodgkinsonite’s chemistry which suggests the fluorescence is intrinsic. However, not all hodgkinsonite fluoresces, so there is some other activator or quencher than influences the fluorescence.

There are two types of willemite in the specimen. The longwave emission spectra of willemite associated with hodgkinsonite has a peak at 518 nm. The willemite associated with franklinite has an emission peak at 524 nm. The cause of this difference might be the concentration of the manganese activator replacing zinc in the willemite.

This writeup and new pictures update the post originally on Nature's Rainbows dated April 3, 2017.