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THE AUSTRALIAN GEMMOLOGIST | New Finds of Cobalt-Bearing Spinel Near Mahenge, Tazania

New Finds of Cobalt-Bearing Spinel Near Mahenge, Tazania

Dr Tom Stephan, MSc (Geosciences), FGG, EG, Vice-Managing Director of the German Gemmological Training Centre. German Gemmological Association, Prof.-Schlossmacher-Str. 1, 55743 Idar-Oberstein, Germany
t.stephan@dgemg.com
Dr Ulrich Henn, Dip Min, Managing Director of the German Gemmological Association. German Gemmological Association, Prof.-Schlossmacher-Str. 1, 55743 Idar-Oberstein, Germany
ulihenn@dgemg.com
Stefan Müller, MSc (Geosciences), Research Gemmologist. DSEF German Gem Lab, Prof.-Schlossmacher-Str. 1, 55743 Idar-Oberstein, Germany

Acknowledgements

The authors thank the companies Paul Wild, Kirschweiler, Germany, as well as Mahenge Gems, Singapore, for the kind loan and part donation of the samples described in this article.

Abstract

Since November 2021, bright blue spinels have been recovered from the Mahenge region of Tanzania. These stones are cobalt-bearing and are called cobalt spinels in the international trade. The samples investigated for this article have been identified as relatively pure magnesium-spinels, with iron content (FeO) of 1.232-2.150 wt % and cobalt content (CoO) of 0.014-0.022 wt %.

Despite the low concentrations, the influence of cobalt on the colour of these spinels can be proved by their absorption spectrum. A characteristic inclusion of the spinels of the new finds in Tanzania often show birefringent lamellae, most likely högbomite, as known also for spinels in other colours from this region.

Figure 1. Cobalt-spinels from new finds in Mahenge, Tanzania; 1.03-2.05ct. Photo courtesy of T. Stephan, collection of Paul Wild, Kirschweiler.

Figure 1. Cobalt-spinels from new finds in Mahenge, Tanzania; 1.03-2.05ct. Photo courtesy of T. Stephan, collection of Paul Wild, Kirschweiler.

Introduction

The East African country, Tanzania, has several occurrences of gem-quality spinel that supply the international gemstone trade with pink to red as well as blue to violet material. Various mines are located in the Umba valley in the northeastern part of the country (Bank et al., 1989) as well as near Handeni in the Tanga region. In the south different coloured spinels are found on placers of the Tunduru-Songea area (Henn and Milisenda, 1997). However, the most important occurrences are located in the Morogoro region near Matombo and in the area of Mahenge (see, for example, Pardieu et al., 2008; Pardieu and Vertriest, 2016; Kukharuk and Manna, 2019), known for its ruby occurrences.

In 2007, the Tanzanian spinel occurrence came into focus when a cluster weighing 52kg with up to 20cm large red spinel crystals was discovered south of Mahenge near Ipanko. This find made Tanzania one of the most important suppliers of high quality pink to red spinel. The Mahenge area additionally produces orange, reddish-purple to purple (mauve) as well as blue to blue-violet spinel.

In November 2021, bright blue cobalt-bearing spinel crystals from an occurrence 20km south of Mahenge appeared in the trade which expanded the supply of cobalt blue material beyond previous producers like Vietnam and Sri Lanka. The place of discovery is located in the Mahenge valley at the opposite side of the occurrence of the pink and red material.

Several short notes on the new finds were published online as press releases and blog entries by different authors and institutions, and the titles and URLs for these notes are included at the end of the reference list.

Figure 2. Lamellae, most-likely composed of högbomite in the blue spinels from Mahenge, Tanzania: The lamellae are oriented along octahedral planes, show strong reflection in reflective light (A) and often show strong interference colours (B); under diffuse lighting the reflections are weaker (C); under crossed polarisation filters, the parallel orientation of the lamellae is obvious (D). Magnification 40-50x. Photos courtesy of T. Stephan.

Figure 2. Lamellae, most-likely composed of högbomite in the blue spinels from Mahenge, Tanzania: The lamellae are oriented along octahedral planes, show strong reflection in reflective light (A) and often show strong interference colours (B); under diffuse lighting the reflections are weaker (C); under crossed polarisation filters, the parallel orientation of the lamellae is obvious (D). Magnification 40-50x. Photos courtesy of T. Stephan.

Until spring 2022, large quantities were mined. Since then, the finds have been declining and production has now come to a standstill; it is even possible that it was just one pocket (Wez Barber, Mahenge Gems Ltd., pers. comm., October 2022). Cut stones (Figure 1) of under one carat up to more than forty carats are on offer at relatively high prices. Some stones show a colour change from blue to violet or a colour shift from violet-blue to bluish-violet.

Cobalt-bearing blue spinel was first mentioned in 1977 by Mitchell. Up to this point of time the presence of cobalt was used as a diagnostic feature for synthetic spinel in gemmological testing, especially the proof of Co2+-bands in the absorption spectrum routinely determined with a hand spectroscope.

Since the 1980s, several occurrences of gem quality cobalt-bearing blue spinel were described especially in Sri Lanka (Shigley and Stockton, 1984; Harder, 1986); Tanzania´s Umba valley (Bank et al., 1989) and Tunduru (D’Ippolito et al., 2015); Vietnam (Chauviré et al., 2015); and Pakistan (Schollenbruch et al., 2021). In addition to spinel, cobalt-bearing gahno-spinel and gahnite are also known (Stephan et al., 2021 and 2022). Fundamental research into the role of cobalt and iron in spinel, gahnospinel and gahnite, as well as their influence on the colour, has been published elsewhere (for example Schmetzer et al. ,1989; Taran et al., 2005, 2009; D’Ippolito et al., 2013, 2015; Fregola et al., 2014; Palke and Sun, 2018; and Belley and Palke, 2021).

This article provides a detailed description of the physical and chemical properties, as well as inclusion characteristics, of the new blue cobalt-bearing spinel from Tanzania. Particularly, the authors have considered the description of the colour mechanism, especially the role of the cobalt-iron ratio as an indicator of the brightness of the blue colour.

Materials and Methods

For this article, five cut and fifteen rough cobalt-bearing spinels reportedly from the new finds near Mahenge, Tanzania, were investigated. Refractive indices were measured with a Schneider refractometer, using Anderson liquid (n = 1.79) as contact liquid. Photomicrographs were taken with an immersion microscope equipped with a Zeiss Stemi2000 optic. Paraffin oil (n = approx. 1.49) was used as immersion liquid.

Spectroscopic data were collected using a Flame-T-XR1-ES spectrometer from Ocean Insight. The device covers the spectral range between 200-1000nm. UV/Vis/NIR-absorption spectra were recorded using a halogen bulb (380-1000nm) as light source. To measure photoluminescence (PL) spectra the same device was used, equipped with an optical fibre and a long wave (LW) UV-LED (365nm). Chemical compositions were measured with an energy dispersive X-ray fluorescence (XRF) using a Thermo Scientific ARL Quant’X EDXRF. Inclusions were identified with Raman micro-spectroscopy using a Renishaw inVia unit equipped with a 514nm and a 785nm laser.

Physical Characteristics

The refractive indices of the cobalt-bearing spinels from Tanzania were determined as n = 1.718-1.720 and the density as 3.60-3.65 g/cm3, as expected for relatively pure magnesium spinel.

Microscope Characteristics

As a typical inclusion of the spinels from the Morogoro region of Tanzania, Schmetzer and Berger (1990, 1992) described groups of fine lamellae which are oriented parallel to the octahedral faces. These are wafer-thin lamellae composed of iron-free, titanium-rich högbomite, which, due to its birefringence, shows strong interference colours under crossed polarisation filters (Figure 2). Strong interference colours are also observed using strong reflective light, due to interference at the thin högbomite layers. These thin layers were also mentioned in the short notes found (see “online references” of SSEF and Gemworld International), but were just mentioned as oriented lamellae with interference colours, without mentioning the publications by Schmetzer and Berger (1990, 1992).

In the samples investigated for this report thin lamellae were frequently observed, providing the first hints regarding their origin. Although not confirmed by this present study, these lamellae are most likely högbomite.

Furthermore, zircon, apatite and dolomite inclusions were observed, as well as fine particles, either arranged like dust or pearl chain-like, or as fine lines along the octahedral faces (Figure 3).

Figure 3. Dust-like exsolutions are often observed as fine lines along the corners of the octahedron (A), partly they also form star-like arrangements on octahedral planes (B). Photos courtesy of T. Stephan.

Figure 3. Dust-like exsolutions are often observed as fine lines along the corners of the octahedron (A), partly they also form star-like arrangements on octahedral planes (B).
Photos courtesy of T. Stephan.

Table 1. Chemical composition of selected cobalt-bearing blue spinels from the new finds in Mahenge, Tanzania (in wt-%). bdl = below detection limit.

Table 1. Chemical composition of selected cobalt-bearing blue spinels from the new finds in Mahenge, Tanzania (in wt-%). bdl = below detection limit.

Table 2: Iron- and cobalt-content (wt %) of the blue cobalt-bearing spinels investigated spectroscopically for Figure. 6.

Table 2: Iron- and cobalt-content (wt %) of the blue cobalt-bearing spinels investigated spectroscopically for Figure. 6.

Chemical Composition

The chemical composition of the five faceted cobalt-bearing spinels from Tanzania is shown in Table 1. For spectroscope comparison, samples from Vietnam and Sri Lanka were measured and their iron- and cobalt-contents are shown in Table 2.

Fluorescence

Under LWUV, a strong green fluorescence was observed (Figure 4). Under shortwave (SW) UV the fluorescence was much weaker and most of the samples were inert. Using the Chelsea filter, a weak reddish reaction was visible.

The moderate to strong green fluorescence is caused by an emission band in the green spectral region visible in PL-spectra, with a maximum at 511nm (Figure 5).

According to Kammerling and Fritsch (1991), this 511nm band is caused by tetrahedrally coordinated bivalent manganese. For samples with comparable iron-content, we observed stronger green fluorescence for those with higher manganese content, suggesting the assignment by Kammerling and Fritsch is correct. Additionally, for most of the samples the typical emission bands for chromium-bearing spinels in the red spectral region around 690-700nm were observed (see for example Malickova et al., 2021; Liu et al., 2022).

Figure 4. Cobalt-bearing spinels from Mahenge, Tanzania under daylight (A) and under LWUV (365nm) (B). Note: The samples at the bottom left in picture (B) were directly illuminated with LWUV, the samples in the background indirectly; the larger the distance, the weaker the fluorescence is seen in this picture. Photos courtesy of T. Stephan.

Figure 4. Cobalt-bearing spinels from Mahenge, Tanzania under daylight (A) and under LWUV (365nm) (B).
Note: The samples at the bottom left in picture (B) were directly illuminated with LWUV, the samples in the background indirectly; the larger the distance, the weaker the fluorescence is seen in this picture. Photos courtesy of T. Stephan.

Figure 5. Photoluminescence spectrum of one of the blue cobalt-bearing spinels from Mahenge, Tanzania (Sp1.536 in Table 1).

Colour Causes and UV/Vis/NIR Spectroscopy

The colour of natural and synthetic blue spinel is well known and described in the literature (Schmetzer et al., 1989; Taran et al., 2005, 2009; D’Ippolito et al., 2013, 2015; Fregola et al., 2014; Palke and Sun, 2018; Belley and Palke, 2021): Natural blue spinel is coloured by iron, which is incorporated into the crystal lattice in either bivalent or trivalent (Fe2+, Fe3+) state. On the contrary, synthetic blue spinel is usually doped with bivalent cobalt (Co2+) to produce bright blue colours, so that the detection of cobalt, routinely with the hand-held spectroscope, has long been considered to be diagnostic evidence of synthetic material. In 1977, Mitchell detected Co2+ as a colouring element also in natural spinels.

The difficulty with the detection of cobalt is that the absorption bands of Fe2+ and Fe3+ as well as Co2+ overlap almost completely, especially in the visible part of the spectrum. A visual differentiation therefore is challenging. For a clear separation of the influence of all chromophores, more complex mathematical data interpretation is necessary (see Stephan et al., 2021, 2022). The most important band positions of the chromophores are listed in Table 3. Spectroscopically, the influence of Co2+ can usually be estimated, as it has already been described in the literature (Schmetzer et al., 1989; Taran et al., 2005, 2009; D’Ippolito et al., 2013, 2015; Fregola et al., 2014; Palke and Sun, 2018).

Table 3. Maxima and assignment of the most important absorption bands in the visible region for cobalt- and iron-bearing blue spinels. Assignment has been carried out with Schmetzer et al., 1989; Taran et al., 2005, 2009; D’Ippolito et al., 2013, 2015; Fregola et al., 2014; Palke and Sun, 2018. IV = tetrahedral coordination VI = octahedral coordination IVCT = Intervalence Charge Transfer

Table 3. Maxima and assignment of the most important absorption bands in the visible region for cobalt- and iron-bearing blue spinels. Assignment has been carried out with Schmetzer et al., 1989; Taran et al., 2005, 2009; D’Ippolito et al., 2013, 2015; Fregola et al., 2014; Palke and Sun, 2018.
IV = tetrahedral coordination
VI = octahedral coordination
IVCT = Intervalence Charge Transfer

Figure 6. UV/Vis/NIR-spectra of selected blue cobalt-bearing spinels from Vietnam (A) and Sri Lanka (C and D) in comparison to the spectrum of one of the blue spinels from Mahenge, Tanzania (B; Sp1.249 in Table 1). The iron- and cobalt-content of these samples are shown in Table 2.

Figure 6. UV/Vis/NIR-spectra of selected blue cobalt-bearing spinels from Vietnam (A) and Sri Lanka (C and D) in comparison to the spectrum of one of the blue spinels from Mahenge, Tanzania (B; Sp1.249 in Table 1). The iron- and cobalt-content of these samples are shown in Table 2.

To explain the bright blue colour of the new cobalt-bearing spinels from Mahenge, Tanzania, and especially their brightness, the absorption spectrum of one of the Tanzanian samples was compared to other iron- and cobalt-bearing blue spinels (Figure 6; see Table 2 for the chemical compositions). Spectrum (A) is recorded from a bright blue spinel from Vietnam; this stone with relatively low iron- and relatively high cobalt-content. In the visible region the spectrum is therefore dominated by the three Co2+-bands between 500-700nm, and the strongest peak is the Co2+-band at 580nm. In addition, no iron bands are observed between 450-480nm.

Spectrum (B) shows the absorption spectrum of one of the blue spinels from the new finds near Mahenge, Tanzania. For this stone, the Co2+-band at 580nm is dominant as well, but weaker Fe-bands are observed between 450-480nm. On the contrary, spectrum (C), measured from a blue cobalt-bearing spinel from Sri Lanka, the Co2+-band at 580nm is less dominant and the Fe-bands between 450-480nm are stronger. Spectrum (D) shows strong Fe-bands between 450-480nm, and the Co2+-band at 580nm is not the dominant peak, but there is the Fe2+-band at 556nm. In this area, the absorption bands of iron and cobalt overlap strongly, but the dominance of the 556nm band is caused by the influence of iron. This spectrum was also measured on a blue spinel from Sri Lanka, with low cobalt but high iron content.

The bright blue colour of the new cobalt-bearing spinels from Mahenge, Tanzania, can be explained by the ratio between the iron and cobalt bands. Compared to other sources of comparable material, the Tanzanian samples showed weak to moderate influences of iron with stronger influence of the cobalt bands. Therefore, their colour is brighter and less greyish as observed for many blue spinels.

Of course, it should be noted that other chromophores such as chromium and manganese can also influence the colour of a blue spinel.

Conclusion

After Sri Lanka, Vietnam and Pakistan, Tanzania is another source of attractive bright blue cobalt-bearing spinels. The results of this study show that the cobalt-concentrations of the new cobalt-bearing spinels from Mahenge, Tanzania are usually relatively low, and iron is usually present as an additional chromophore. The blue colour is therefore produced by a combination of cobalt and iron and thus depends on the Co/Fe-ratio (see for example Schmetzer et al. 1989), as known also for blue cobalt-bearing spinels from other sources. Compared to reference stones from other sources, however, the iron-content of Tanzanian spinel is quite low, explaining their bright blue colour. Brighter blue colours are typically known for cobalt-bearing spinels from Vietnam, which possess very low iron content.

For cobalt-bearing spinels, a dominance of the Co2+-absorption bands between 490-680nm with a simultaneous weak to moderate influence of the Fe-bands between 450-480nm increases the brightness of the blue colour, which is honoured internationally with the trade name “cobalt spinel” or “cobalt blue spinel”.

References

Bank, H., Henn, U. and Petsch, E., 1989. Spinelle aus dem Umba-Tal, Tansania. Z. Dt. Gemmol. Ges., 38(4), pp.166-168.

Belley, P. M. and Palke, A. C., 2021. Purple gem spinel from Vietnam and Afghanistan: comparison of trace element chemistry, cause of colour, and inclusions. Gems & Gemology, 57(3), pp.228-238.

Chauviré, B., Rondeau, B., Fritsch, E., Ressigeac, P. and Devidal, J.-L., 2015. Blue spinel from the Luc Yen District of Vietnam. Gems & Gemology, 51(1), pp.2-17.

D’Ippolito, V., Andreozzi, G. B., Bosi, F., Halenius, U., Mantovani, L., Bersani, D. and Fregola, R. A., 2013. Crystallographic and spectroscopic characterization of a natural Zn-rich spinel approaching the endmember gahnite (ZnAl2O4) composition. Mineralogical Magazine, 77(7), pp.2941-2953.

D’Ippolito, V., Andreozzi, G. B., Halenius, U., Skogby, H., Hametner, K. and Günther, D., 2015. Color mechanisms in spinel: Cobalt and iron interplay for the blue colour. Physics and Chemistry of Minerals, 42, pp.431-439.

Fregola, R. A., Skogby, H., Bosl, F., D’Ippolito, V., Andreozzi, G. B. and Halenius, U., 2014. Optical absorption spectroscopy study of the cause for color variations in natural Fe-bearing gahnite: Insights from iron valency and site distribution data. American Mineralogist, 99, pp.2187-2195.

Harder, H., 1986. Natürliche kobaltblaue Spinelle von Ratnapura, Sri Lanka. Neues Jahrbuch für Mineralogie, 3, pp.97-100.

Henn, U. and Milisenda, C. C., 1997. Neue Edelsteinvorkommen in Tansania: Die Region Tunduru-Songea. Z. Dt. Gemmol. Ges., 46(1), pp.29-43.

Kammerling, R. C. and Fritsch, E., 1991. Gem Trade Lab Notes: Spinel, with unusual green fluorescence. Gems & Gemology, 27(2), pp.112-113.

Kukharuk, M. and Manna, C., 2019. The spinels of Mahenge, Tanzania. InColor, 43, pp.54-58.

Liu, Y., Qi, L, Schwarz, D. and Zhou, Z., 2022. Color mechanism and spectroscopic thermal variation of pink spinel reportedly from Kuh-i-Lal, Tajikistan. Gems & Gemology, 58(3), pp.338-353.

Malickova, I., Bacik, P., Fridrichova, J., Hanus, R., Illasova, L., Stubna, J., Furka, D., Furka, S. and Skoda, R., 2021. Optical and luminescence spectroscopy of varicolored gem spinel from Mogok, Myanmar and Luc Yen, Vietnam. Minerals, 1192), pp.1-13.

Mitchell, R. K., 1977. African grossular garnets; blue topaz; cobalt spinel; and grandidierite. Journal of Gemmology, 15(7), pp.354-358.

Palke, A. C. and Sun, Z., 2018. What is cobalt spinel? Unraveling the causes of colour in blue spinels. Gems & Gemology, 54(3), pp.262.

Pardieu, V., Hughes, R. W. and Boehm, E., 2008. Spinel Resuraction of a classic. InColor, 8, pp.10-18.

Pardieu, V. and Vertriest, W., 2016. Update on coloured stone mining in Tanzania. Gems & Gemology, 52(3), pp.318-321.

Schmetzer, K. and Berger, A., 1990. Lamellar iron-free högbomite-24R from Tanzania. N. Jb. Miner. Monatshefte, 9, pp.401-412.

Schmetzer, K. and Berger, A., 1992. Lamellar inclusions in spinels from Morogoro area, Tanzania. Journal of Gemmology, 23(2), pp.93-94.

Schmetzer, K., Haxel, C. and Amthauer, G., 1989. Colour of natural spinels, gahnospinels and gahnites. Neues Jahrbuch für Mineralogie, 160(2), pp.159-180.

Schollenbruch, K., Blauwet, D., Malsy, A.-K. and Bosshard, V., 2021. Cobalt-blue spinel from Northern Pakistan. Journal of Gemmology, 37(7), pp.726-737.

Shigley, J. E. and Stockton, C. M., 1984. ”Cobalt-Blue“ Gem Spinel. Gems & Gemology, 20(1), pp.34-41.

Stephan, T., Henn, U. and Müller, S., 2021. Blauer Gahnit aus Nigeria. Z. Dt. Gemmol. Ges., 70(3/4), pp.3-18.

Stephan, T., Henn, U. and Müller, S., 2022. On the colour mechanism of blue gahnite from Nigeria. Journal of Gemmology, 38(2), pp.183-193.

Taran, M. N., Koch-Müller, M. and Langer, K., 2005). Electronic absorption spectroscopy of natural (Fe2+,Fe3+)-bearing spinels of spinel s.s.-hercynite and gahnite-hercynite solid solutions at different temperatures and high-pressures. Physics and Chemistry of Minerals, 32(3), pp.175-188.

Taran, M. N., Koch-Müller, M. and Langer, K., 2009. Optical spectroscopic study of tetrahedrally coordinated Co2+ in natural spinel and staurolite at different temperatures and pressures. American Mineralogist, 94(11-12), pp.1647-1652.

Online-References
(sorted after publishing date)

SSEF conducts analysis of cobalt-blue spinel from a newly reported source in Tanzania – SSEF (https://www.ssef.ch/ssef-conducts-analysis-of-cobalt-blue-spinel-from-a-newly-reported-source-in-tanzania/)

A New Source in Tanzania Is Producing Cobalt-Blue Spinel | B. Branstrator, National Jeweler (https://nationaljeweler.com/articles/10908-a-new-source-in-tanzania-is-producing-cobalt-blue-spinel)

Out of the Blue | Blue Cobalt Spinel from Mahenge, Tanzania – Mahenge Gems (https://mahenge.com/out-of-the-blue-blue-cobalt-spinel-from-mahenge-tanzania/)

New Cobalt Blue Spinel from Tanzania – A. Butler, IGI GemBlog (https://www.igi.org/gemblog/new-cobalt-blue-spinel-from-tanzania/)

Cobalt Spinel – C. Lüle, Gemworld International
(https://www.gemguide.com/cobalt-spinel-2/)

Neue Funde von Kobalt-Spinellen in Tansania – U. Henn,
T. Stephan und S. Müller, DGemG-Blog (https://blog.dgemg.com/gemmologie/104-co-spinell-tansania.html

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