Spinel Crystal from Tanzania
Contributed by: Michael Crawford
Date: Jul 24th, 2025
Locality: Mahenge, Morogoro Region, Tanzania (See on Mindat)
Size: 4.5 x 4.5 cm
Description:
An octahedral spinel (MgAl2O4) crystal from Mahenge, Morogoro Region, Tanzania. The red color of spinel is due to traces of chromium and vanadium. Spinels are red when the chromium content is greater than the vanadium content, while spinels appear orange when vanadium is greater than the chromium content. The chromium also activates the red fluorescence of this spinel. There is no reference available if the chromium replaces aluminum or magnesium or both. Most likely that chromium (Cr3+) replaces aluminum since it has the same charge (Al3+).
The red fluorescence of this spinel is very bright under longwave UV light. It is very dim red under midwave and shortwave UV light. The longwave emission spectrum has five sharp peaks in the red and near infrared. The peaks are 9 nm to 12 nm apart with the maximum at 696 nm. Longwave emission spectra can be used to distinguish spinel from ruby. Ruby has a single very large peak around 692 nm with several much smaller peaks, whereas the spinel emission spectrum has five prominent peaks.
Summary of luminescence responses:
Spinel (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Red