Willemite and Calcite from the Republic of the Congo
Contributed by: Michael Crawford
Date: Oct 9th, 2025
Locality: Mindouli District, Pool Department, Republic of the Congo (See on Mindat)
Size: 6 x 7.5 cm
Description:
Pictured is a specimen of willemite and calcite. The specimen also contains crystals of non-fluorescent green dioptase. It is from Gotala, Mindouli, Mindouli District, Pool Department, Republic of the Congo. The willemite fluoresces yellow under longwave and midwave UV illumination, and greenish white in SW illumination. The willemite has yellow afterglow after exposure to all UV wavelengths.
The calcite is weakly fluorescent under midwave light and fluoresces bright orange red under shortwave light. Emission spectra of the minerals are shown in the last image. Calcite has a strong, sharp peak at 311 nm. This ultraviolet fluorescence is activated by lead. The orange-red fluorescence of calcite peaks around 620 nm. This visible light peak is considerably less intense than the ultraviolet peak. The orange-red fluorescence is activated by manganese. The false color image of ultraviolet fluorescence shows the calcite as light blue from the lead activated peak at 311 nm.
The shortwave willemite fluorescence of this Congo specimen has a broad peak with a maximum around 483 nm. This peak is much broader compared to the fluorescence of willemite from Franklin, New Jersey and its maximum is shifted to shorter wavelengths around 483 nm. The green color of Franklin willemite is shown in the sharp peak with a maximum at 525 nm. The broader width of the Congo willemite fluorescence corresponds to the whitish color that contains more visible wavelengths. The green fluorescence is activated by manganese like the orange-red fluorescence of calcite. The presence of more fluorescent wavelengths in the Congo willemite indicates that manganese and additional activators are present. Additional activators in the Congo willemite are also indicated by the yellow fluorescence in LW and MW illumination. Yellow willemite fluorescence also occurs in specimens from Puttapa, Australia and Balmat, New York. When Franklin willemite fluoresces under longwave and midwave light, it is green like the shortwave fluorescence. Arsenic activates afterglow in willemite, and it may activate the yellow longwave and midwave fluorescence. The additional shortwave fluorescence may be activated by arsenic, lead or a rare earth. For a full discussion of the willemite afterglow, see Bostwick/Waychunas/Green in UV Waves, supplement to v48 n6, Nov-Dec 2018.
Summary of luminescence responses:
Willemite (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Yellow
- Fluorescence under Midwave (305nm LED) UV light: Yellow
- Fluorescence under Shortwave (255nm LED) UV light: White
- Afterglow after exposure to Longwave (365nm LED) UV light: Yellow
- Afterglow after exposure to Midwave (305nm LED) UV light: Yellow
- Afterglow after exposure to Shortwave (255nm LED) UV light: Yellow
- Fluorescence under Shortwave (255nm LED) UV light: Red