Spinel and Pseudomorph Ruby after Spinel from Mahenge, Tanzania
Contributed by: Michael Crawford
Date: Aug 13th, 2025
Locality: Mahenge, Morogoro Region, Tanzania (See on Mindat)
Size: 4 x 4.5 cm
Description:
This is a specimen of an octahedral spinel (MgAl2O4) crystal that has small areas that have been partially replaced by corundum var. ruby (Al2O3). Several examples of ruby pseudomorphs after spinel are shown in mindat.org (https://www.mindat.org/photo-1219478.html). The specimen comes from Mahenge, Morogoro Region, Tanzania. The specimen also contains a small amount of pargasite (NaCa2(Mg4Al)(Si6Al2)O22(OH)2) that fluoresces greenish white under shortwave UV illumination and an unknown mineral that fluoresces yellow under shortwave UV illumination. The unknown mineral might be either clinohumite (Mg9(SiO4)4F2) or chondrodite (Mg5(SiO4)2F2). These minerals are known to occur at Mahenge and they can have shortwave yellow fluorescence.
The red color of spinel and ruby is caused by traces of chromium. Chromium also 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. The red fluorescence of spinel is brightest under longwave UV light.
The ruby rich areas of the spinel crystal have a pinkish hue in daylight, and they have a slightly brighter red fluorescence. Spinel is darker red. The difference in brightness of the fluorescence is greater under shortwave light. 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 4 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 emission spectrum of ruby rich areas in this specimen has five peaks like pure spinel, but it has a prominent central peak at 693 nm that is much brighter than the other four peaks. This central peak does not match the peaks of either ruby or spinel. The central peak of ruby is at 692 nm and the central peak of spinel is at 696 nm. The other four peaks of the ruby rich areas are at the same wavelengths of pure spinel (673 nm, 684 nm, 705 nm and 715 nm).
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 Shortwave (255nm LED) UV light: Green