Hour Glass Selenite from Canada
Contributed by: Michael Crawford
Date: Jun 13th, 2025
Locality: Willow Creek, Nanton, Alberta, Canada (See on Mindat)
Size: 7 cm
Description:
Image of gypsum var. selenite crystal showing “hour-glass” zoned fluorescence. The crystal is from Willow Creek, Nanton, Alberta, Canada. Crystal was illuminated with 365 nm longwave UV light. Shortwave and midwave UV give a similar response and the selenite crystal is phosphorescent in all UV wavelengths. Organic activators cause selenite fluorescence and afterglow.
The selenite grew by adding atoms to the surface of the crystal. The selenite crystalline structure forces the raw materials (calcium, sulfate and water) to be added in an organized fashion and they can only be added to certain surfaces and in order. Similarly, the organic activator may only fit on a single crystal face as the selenite grows. This explains why some selenite crystals show sector zoned fluorescence. The element activating fluorescence can only be added to one face and not the other face. Image 3 shows how the triangular zones form. The organic activators can attach only to the red edges of the growing crystal
Summary of luminescence responses:
Gypsum var. Selenite
- Fluorescence under Longwave (365nm LED) UV light: White
- Fluorescence under Midwave (305nm LED) UV light: White
- Fluorescence under Shortwave (255nm LED) UV light: White
- Afterglow after exposure to Longwave (365nm LED) UV light: White
- Afterglow after exposure to Midwave (305nm LED) UV light: White
- Afterglow after exposure to Shortwave (255nm LED) UV light: White