Spinel and Ruby from Mahenge, Tanzania
Contributed by: Michael Crawford
Date: Aug 15th, 2025
Locality: Mahenge, Morogoro Region, Tanzania (See on Mindat)
Size: 6.5 x 9.5 cm
Description:
This specimen from Mahenge, Tanzania is composed of large pink crystals of spinel (MgAl2O4) that has been partially replaced and coated with corundum var. ruby (Al2O3). Both minerals fluoresce bright red under longwave UV illumination and somewhat dimmer under shortwave light. Chromium activates the red fluorescence of spinel and ruby. There is no reference available if the chromium replaces aluminum or magnesium or both in spinel. Most likely that chromium (Cr3+) replaces aluminum since it has the same charge (Al3+). Chromium replaces aluminum in ruby.
Longwave emission spectra clearly show the location of unaltered spinel and areas coated with ruby. There are also areas where the spinel has been partially replaced by ruby. The emission spectra of pure spinel and ruby are very different. The ruby longwave emission spectrum has a very bright peak at 692 nm. This peak is nearly four to five times brighter than peaks (659 nm, 669 nm, 706 nm and 713 nm) on either side of this prominent peak. No peaks in the spinel longwave emission dominate its signature. The spinel spectrum has five peaks (673 nm, 684 nm, 696 nm, 706 nm and 715 nm) that makes it distinct from the ruby spectrum.
The spectrum of the ruby coating differs slightly from the spectrum of a reference specimen of ruby. The ruby coating has a prominent central peak at 692 nm like the reference spectrum. However, three of the smaller peaks (673 nm, 684 nm, and 705 nm) match the peaks of spinel. The spectra of mixed spinel and ruby show how the peaks change from end member spectra. The central peak changes from 696 nm for spinel to 692 nm as the amount of ruby increases. The relative intensity of the central peak compared to the peaks on either side increases as the relative amount of ruby increases.
Summary of luminescence responses:
Spinel (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Red
- Fluorescence under Shortwave (255nm LED) UV light: Red
- Fluorescence under Longwave (365nm LED) UV light: Red
- Fluorescence under Shortwave (255nm LED) UV light: Red