Prehnite from Tanzania
Contributed by: Michael Crawford
Date: Jul 12th, 2025
Locality: Merelani Hills, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania (See on Mindat)
Size: 4.5 x 7 cm
Description:
This is a specimen of yellow prehnite (Ca2Al(AlSi3O10)(OH)2) from Merelani Hills, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania. It has a bright blue fluorescence under longwave UV light. The fluorescence changes to blue and green under midwave light and changes to yellow and green under shortwave UV light. There is also a crystal of calcite on the specimen that fluoresces orange red under midwave and shortwave illumination.
The longwave emission spectrum is shown on the graph accompanying the fluorescent images. It is a broad peak with a maximum at 449 nm. There is no available information on the activation of this blue fluorescence. The fluorescence under midwave and shortwave is not bright enough to obtain a good spectral measurement of the prehnite emission.
The calcite is brightest under midwave illumination. The midwave emission of the calcite peaks at 618 nm. It is activated by manganese. The shortwave emission spectrum of this calcite is rather noisy, but it has enough definition (sharp peaks around 340 nm and 365 nm) in the ultraviolet region to show that cerium is present.
Summary of luminescence responses:
Prehnite (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Blue
- Fluorescence under Midwave (305nm LED) UV light: Green
- Fluorescence under Shortwave (255nm LED) UV light: Green
- Fluorescence under Midwave (305nm LED) UV light: Red