Unusual Sodalite from Greenland
Contributed by: Michael Crawford
Date: Jul 3rd, 2025
Locality: Ilímaussaq complex, Kujalleq, Greenland (See on Mindat)
Size: 2.5 x 3.5 cm
Description:
This is a sodalite var. hackmanite thumbnail from the Illimausaq Complex, Greenland. Hackmanite is not an official mineral of International Mineral Association (IMA) but the name is widely used by fluorescent mineral collectors to identify the tenebrescent or photochromatic variety of sodalite. The sodalite identification of this specimen has been confirmed with XRD analysis. This hackmanite is unusual because it is tenebrescent, but it has very weak red fluorescence in longwave UV illumination instead of the bright yellow-orange fluorescence typical of Greenland sodalite. The tenebresence is activated by all wavelengths of UV light and its color is violet. There is no afterglow from this sodalite specimen.
The crystal structure of sodalite is a framework of linked AlO4 and SiO4 tetrahedra in which Si and Al alternate on the tetrahedral sites. The linkage forms a cage that contains a centrally located anion (chlorine) surrounded by four cations (sodium) in tetrahedral coordination. The sodalite structure allows a wide range of anion and cation substitutions of potential fluorescent activators.
Tenebrescence requires two components. The first component is a cage with the central anion (chlorine) missing. The second component is an adjacent cage where the central anion (again chlorine) has been replaced with an anion that can act as an electron donor. In most hackmanites, the electron donor anion is disulfide which also activates the yellow-orange to orange longwave fluorescence of sodalite and hackmanite.
When UV light strikes sodalite, it frees an electron from disulfide that moves into the anion vacancy in the adjacent cage. The trapped electron creates a “F-center” (from the German word \"Farbzentrum\", where Farbe means color, and zentrum means center). The electron remains trapped in the F-center after the UV light is turned off. The F-center modifies the reflection of visible light from hackmanite. When visible light strikes the F-center, it absorbs the green wavelengths of visible light and some of the blue and red wavelengths. Visible light reflected from the tenebrescent specimen contains blue and red wavelengths to produce a purple to violet color. Exposure to light or thermal energy causes the trapped electrons in the F-centers to transfer back to the adjacent cages with the donor ions and the purple color fades away. Repetitive cycles of UV exposure and then bleaching the purple color does not diminish the intensity of the tenebrescent.
This sodalite is unusual because there is no disulfide activated fluorescence and therefore no disulfide to act as a donor of electrons to create tenebrescence. There must be another anion present to be an electron donor. The shortwave emission spectrum of this specimen has a broad peak at 684 nm that is activated by ferric iron replacing aluminum in the framework tetrahedra. There is another broad peak with a maximum at 430 nm. Whatever is activating this fluorescence may also be the electron donor.
Summary of luminescence responses:
Sodalite (Mindat) (RRUFF)
- Fluorescence under Midwave (305nm LED) UV light: Red
- Fluorescence under Shortwave (255nm LED) UV light: Red
- Tenebrescence after exposure to Shortwave (255nm LED) UV light: Violet