Willemite from Berg Aukas, Namibia
Contributed by: Michael Crawford
Date: Nov 15th, 2025
Locality: Berg Aukas Mine, Grootfontein Constituency, Otjozondjupa Region, Namibia (See on Mindat)
Size: 7.5 x 11.5 cm
Description:
A willemite specimen from Berg Aukas Mine, Namibia. The willemite is a secondary mineral formed from sphalerite and other zinc minerals. The colorless, acicular willemite crystals precipitated in a vug of dolomite. The dolomite was deposited in the Late Precambrian and it is part of the Gauss Formation. The original mineralization occurred in the Late Precambrian to Cambrian on paleo-highs of karst breccias. The secondary willemite in this specimen formed more recently in the Quaternary period. The crystals are highly striated parallel to the c-axis and they have a flat termination. The crystals are up to 6 mm long.
The willemite has bluish white fluorescence under shortwave light, yellowish white under midwave light and blue fluorescence under longwave light. The willemite fluorescence from Berg Aukas is variable and a lot of it is non-fluorescent. This specimen also has long-lasting afterglow from exposure to both midwave and shortwave light. The afterglow is white to blue-white color.
Willemite fluorescence is often green due to manganese activation. The emission spectra of this specimen shows no evidence of manganese activation. The fluorescence may be activated by lead (Pb2+) replacing zinc or structural defects. The shortwave emission spectrum has two broad peaks at 349 nm and 437 nm. The midwave emission is shifted toward longer wavelengths. Its yellow fluorescence has a peak at 499 nm.
Arsenic is known to cause green afterglow in willemite with green fluorescence. Arsenic substitutes for silicon in the willemite structure. Arsenic is likely the cause of the afterglow in this willemite as well. Arsenic ions trap some of the electrons excited by the fluorescent activator and thermal energy slowly pushes the electrons back to the ground state with the emission of a photon of similar wavelengths of the fluorescence.
Summary of luminescence responses:
Willemite (Mindat) (RRUFF)
- Fluorescence under Midwave (305nm LED) UV light: Yellow
- Fluorescence under Shortwave (255nm LED) UV light: Blue
- Afterglow after exposure to Shortwave (255nm LED) UV light: Blue
- Afterglow after exposure to Midwave (305nm LED) UV light: Blue