Willemite and Mimetite from the M\'fouati Mine, Republic of the Congo
Contributed by: Michael Crawford
Date: Aug 31st, 2025
Locality: Mfouati Mine, Mfouati, Mfouati District, Bouenza Department, Republic of the Congo (See on Mindat)
Size: 3 x 3 cm
Description:
This is a thumbnail specimen of mimetite (Pb5(AsO4)3Cl) and willemite (Zn2SiO4) from the M`fouati Mine, M`fouati, Bouenza Department, Republic of the Congo. The mimetite fluoresces orange and is brightest under midwave UV illumination. The brightness of mimetite fluorescence is modest even under midwave light. It takes a strong UV light to see the fluorescence. The willemite fluoresces bright yellow under longwave UV light, it is a dimmer yellow under midwave, and it fluoresces greenish white under shortwave illumination. The willemite has long-lasting afterglow from exposure to all wavelengths of UV illumination.
The last picture is an enlargement that shows zoning in the willemite crystals revealed in the image of shortwave fluorescence. The outer rims of the crystals have brighter fluorescence.
Willemite from New Jersey is famous for its bright green fluorescence that is activated by manganese replacing zinc. The shortwave emission spectrum for this green fluorescence has a much narrower bandwidth than the shortwave willemite spectrum of this specimen and its maximum is at 521 nm compared to the maximum at 471 nm for this specimen. Manganese likely contributes to the fluorescence of this specimen but there are additional activators making the spectrum wider and shifting the peak to shorter wavelengths. This willemite fluoresces yellow under longwave and midwave light. The longwave emission spectrum peaks at 566 nm. This yellow fluorescence may be activated by arsenic. Arsenic activates the willemite afterglow.
The emission spectrum of the mimetite in this specimen is very noisy. There are three peaks in the spectrum around 570 nm, 600 nm, and 680 nm that may be true signal of the emission. There is no available information of what activates mimetite’s orange fluorescence.
Summary of luminescence responses:
Mimetite (Mindat) (RRUFF)
- Fluorescence under Midwave (305nm LED) UV light: Orange
- Fluorescence under Shortwave (255nm LED) UV light: Orange
- Fluorescence under Longwave (365nm LED) UV light: Yellow
- Fluorescence under Midwave (305nm LED) UV light: Yellow
- Fluorescence under Shortwave (255nm LED) UV light: White