Two Color Fluorite from the Pena Blanca Mine, Colombia
Contributed by: Michael Crawford
Date: Jul 12th, 2025
Locality: Peña Blanca Mine, San Pablo de Borbur, Western Boyacá Province, Boyacá Department, Colombia (See on Mindat)
Size: 4 x 4 cm
Description:
This is a two-color fluorite (CaF2) specimen from the Pena Blanca Mine, Western Boyaca Province, Colombia. The specimen is mostly clear fluorite crystal with zones of green fluorite. The Pena Blanca Mine is the most northern of the emerald mines in the Western Emerald Bearing Belt in Colombia. The mineralogy of Peñas Blancas is very different from that found in the other emerald mines in Colombia, particularly because of the absence of pyrite and other sulfides and the presence of large fluorite crystals.
There are references that the green color in the fluorite is caused by chromium, like emeralds from the Pena Blanca Mine. There are also references that the green color is caused by yttrium. Chromium and yttrium are not the cause of the green color. The green color is caused by the rare earth elements, cerium (Ce3+) and samarium (Sm2+). These elements also contribute to the fluorescence of this fluorite.
The clear and green fluorite is fluorescent under all wavelengths of UV illumination. The blue fluorescence is activated by europium (Eu2+) replacing calcium. The green areas have a brighter fluorescence compared to the clear areas. The brighter fluorescence is caused by cerium and samarium acting as co-activators to europium increasing its ability to activate the blue fluorescence.
The emission spectra also show there is a small difference in the peak wavelength of the visible fluorescence in addition to the difference in brightness. The peak of the green fluorescence is at 418 nm. The peak of the clear fluorescence is at 420 nm. I measured several locations of green and clear fluorite on the crystal and this small difference was consistent for all measurements. The co-activators move the fluorescent peak to a slightly shorter wavelength.
The longwave and shortwave images of the fluorescence show the increased brightness for some of the green areas that can be seen in the white light image of the fluorite crystal. I also took images of the ultraviolet fluorescence under shortwave UV illumination. SW emission spectra clear and green fluorite show that the ultraviolet fluorescence is caused by cerium. The sharp peaks at 320 nm and 340 nm are caused by cerium. The ultraviolet fluorescence of the green fluorite is much brighter than the fluorescence of the clear fluorite. The image of ultraviolet fluorescence clearly distinguish the green zones in the crystal with the brighter fluorescence. The distinction between fluorite types is much clearer compared to the images of visible fluorescence.
Fluorite is transparent to ultraviolet light. This property allows the brighter areas within the crystal to be seen. The transparency allows the shortwave UV illumination to penetrate the crystal and the resulting UV fluorescence in the crystal’s interior to escape. The lens I used to take the ultraviolet fluorescent images contains lens elements made of quartz and synthetic fluorite because glass cannot transmit wavelengths shorter than 350 nm. The lens has uniform transmission across the ultraviolet 300 to 400 nm, visible 400 to 700 nm, near infrared 700 to 1100 nm.
Summary of luminescence responses:
Fluorite (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Blue
- Fluorescence under Midwave (305nm LED) UV light: Blue
- Fluorescence under Shortwave (255nm LED) UV light: Blue
- Fluorescence under Longwave (365nm LED) UV light: Blue