Research on corundum and chrysoberyl from Tanzania completed

One year after the two students of earth science Katherine Waite and Susanne Pfenninger went to Tanzania to obtain gemstones for their research, they have now completed their Master's theses at the Mineralogical Institute of Basel University under the supervision of Prof. H.A. Hänni.

The first thesis from Katherine Waite is titled "Mineralogical and Gemmological Analysis of Corundum from Tunduru and Songea, southern Tanzania". The study characterises 50 samples, which were bought in Tunduru and include stones of various colours, size and quality from both the Songea and Tunduru deposits. The geology of these deposits is described, and gemmological properties, the chemical composition, UV-Vis and FTIR spectroscopic data are discussed. Inclusions were observed microscopically and analysed using Raman spectrometry. The conclusion from these data is that although the gemmological data, UV spectra and trace element contents of the sapphires from Tunduru and Songea are similar, the difference in appearance and internal features suggest two different host rocks. The samples from Tunduru are very similar in appearance to corundum from Sri Lanka and Madagascar and similar internal features have been reported from these countries. Hence, the Tunduru deposit is likely to be part of the Pan-African mineral belt, which was formed during the collision of East and West Gondwana in the late Proterozoic.

The second thesis from Susanne Pfenninger about the mineralogical and gemmological properties of chrysoberyl from Tunduru, is written in German. It describes 35 rough chrysoberyls of various colours and alexandrites from this secondary deposit. After a geological description of the area, the study concentrates on chemical composition and UV-Vis spectrometry of the various colours. Here, it is shown that yellow, green, and brown varieties owe their colour to Fe3+, the unique bluegreen variety, which has, so far, only been described from Tunduru and Madagascar, to V3+, and the colour-changing alexandrite to Cr3+ and Fe3+. Overall, the stones are relatively inclusion-free. Observed inclusions are healing fissures and hollow channels which, when abundant, form a cat's eye. Analysed inclusions were apatite, uraninite, quartz, and graphite.