Powellite and Scolecite from India
Contributed by: Michael Crawford
Date: Sep 16th, 2025
Locality: Nashik District, Nashik Division, Maharashtra, India (See on Mindat)
Size: 7 x 11 cm
Description:
A powellite (Ca(MoO4)) and scolecite (CaAl2Si3O10·3H2O) specimen from Nashik, India. These crystals formed within the gas-filled cavities of the Deccan Trap flood basalts. These basalts are one of largest volcanic provinces in the world that erupted approximately 66 million years ago. Hydrothermal fluids flowing through these cavities deposited the powellite and the zeolite, scolecite. Powellite is one of the rarest minerals from the Deccan Traps.
Powellite is one endmember of a solid solution series with the other endmember, scheelite (Ca(WO4)). Tungsten and molybdenum are readily interchangeable in the crystal structures of these endmembers. Both end members are intrinsically fluorescent. The molybdate ion activates powellite fluorescence and the tungstate ion activates scheelite fluorescence. Pure powellite fluoresces pale yellow and pure scheelite fluoresces bluish white under shortwave UV illumination. Research has shown that the shortwave scheelite fluorescence changes from bluish white when pure to white at 1.5 wt% molybdenum, and it becomes pale yellow at 10 wt% molybdenum. Adding more molybdenum keeps the fluorescent color the same as pure powellite (pale yellow). This powellite pale yellow fluorescence is equally bright under both shortwave and midwave UV illumination. Its fluorescence dims considerably under longwave light. The shortwave emission of powellite is a broad peak with a maximum at 505 nm.
The zeolite, scolecite, fluoresces bluish white under longwave UV light in this specimen. It is much dimmer under midwave and shortwave light. Most scolecite is non-fluorescent and reports about scolecite fluorescence describe its color as being yellow to brown. The longwave emission spectrum of scolecite peaks the visible blue region with two maxima at 436 nm and 437 nm. There is no reference material that provides information on the activator for this bluish white fluorescence.
Summary of luminescence responses:
Powellite (Mindat) (RRUFF)
- Fluorescence under Shortwave (255nm LED) UV light: Yellow
- Fluorescence under Midwave (305nm LED) UV light: Yellow
- Fluorescence under Longwave (365nm LED) UV light: White