Willemite and Calcite from the Sterling Hill Mine, New Jersey
Contributed by: Michael Crawford
Date: Jul 17th, 2025
Locality: Sterling Hill, Ogdensburg, Sussex County, New Jersey, USA (See on Mindat)
Size: 7 x 8 cm
Description:
This is a specimen I collected in 2024 off the conveyor belt at the Sterling Hill Mine, New Jersey. It is some of the last ore extracted before the mine closed in 1986 and it had been sitting on the conveyor belt for 38 years. It had a very thick coating of mud and willemite dust that had to be scrubbed off. The occasional tours of the headframe and conveyor system at Sterling Hill allow visitors to collect one sample from the conveyor and they give you a certificate to verify its origin. After 38 years, it is amazing that there were still rocks available and I got a nice specimen for my collection.
The specimen has a matrix of willemite var. troostite, calcite, and franklinite. Troostite is a manganese bearing variety of willemite with MnO content of 3 to 12 mass percent. The manganese gives troostite a brown daylight color. The brown troostite and black franklinite are equigranular grains around 2 mm in size. These grains are in coarse grained calcite matrix. There is a vein of willemite on the side of the specimen. The vein has slickensides on it indicating that the vein was emplaced along a small fault.
The all types of willemite fluoresce green in all wavelengths of UV illumination. None of the willemite has afterglow. The calcite fluoresces red and the franklinite is non-fluorescent. The shortwave emission spectra of the willemite vein and troostite grains are slightly different. Their peaks are at 521 nm for the vein and 523 nm for the troostite grains. Spectra of calcite taken at various locations on the specimen have a broad peak with a maximum at 623 nm typical of calcite, but there is a secondary peak at 521 nm. Apparently, some very fine willemite is mixed in with the calcite. The fluorescence of both the calcite and willemite is activated by manganese.
Summary of luminescence responses:
Willemite (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Green
- Fluorescence under Midwave (305nm LED) UV light: Green
- Fluorescence under Shortwave (255nm LED) UV light: Green
- Fluorescence under Longwave (365nm LED) UV light: Red
- Fluorescence under Midwave (305nm LED) UV light: Red
- Fluorescence under Shortwave (255nm LED) UV light: Red