Elpidite from Mt St Hilaire
Contributed by: Douglas Bank
minID: 6EX-21Q
Date: May 1st, 2026
Locality: Poudrette quarry (De-Mix quarry; Demix quarry; Uni-Mix quarry; Carrière Mont Saint-Hilaire; MSH), Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Québec, Canada (See on Mindat)
Size: 2 x 1 x 2.3 cm
Weight: 13.11 g
Description:
This specimen consists of well-developed sprays of elpidite crystals from the Poudrette Quarry, exhibiting a radiating habit with clean terminations and a dense, intergrown arrangement across the matrix. In white light, the crystals are colorless to pale cream with a vitreous luster, forming attractive, fan-like aggregates typical of the species from this locality.
Under ultraviolet illumination, the specimen fluoresces under both longwave and shortwave UV, with a stronger and more vivid response under shortwave. The fluorescence is bright green and highlights individual crystal groups and internal structure that are less apparent in daylight. This behavior is consistent with fluorescence attributed to trace uranyl ions within the crystal structure, a feature occasionally observed in elpidite from Mont Saint-Hilaire.
Radiation measurements using a Mazur PRM-9000 indicate levels exceeding 4.5 µSv/hr, substantially above typical background. The measured radiation decreases when the specimen is enclosed, but remains elevated, indicating that the emission is not limited to alpha radiation. The combination of well-formed crystal sprays, strong UV response, and measurable radioactivity makes this a representative and scientifically interesting example from this classic alkaline complex locality.
Summary of luminescence responses:
Elpidite (Mindat) (RRUFF)
- Fluorescence under Shortwave (254nm Lamp/Mercury) UV light: Green
- Fluorescence under Longwave (365nm LED) UV light: Green
- Fluorescence under Midwave (305nm LED) UV light: Green
- Fluorescence under 222nm excimer UV light: Green