gems, spinel, Tanzania, Morogoro, inclusions, spinel inclusions, högbomite inclusions

Lamellar inclusions in gem spinels reportedly originating from the Morogoro area of Tanzania are identified as an iron-free, titanium-rich member of the högbomite series. The investigation concerns rough and faceted material obtained through several independent trade sources following the first receipt of samples in June 1987. The crystals include red, pink, orange, reddish purple, purplish blue and blue octahedra reaching approximately 1 cm. Most suppliers independently reported Morogoro as the source, and the occurrence of similar inclusions across the parcels supported a common locality; an earlier attribution to the Umba Valley was therefore considered probably incorrect. Conventional physical properties, trace-element compositions and absorption spectra did not distinguish these stones significantly from natural spinels from Sri Lanka and Myanmar. Their colour-related absorption was attributed to chromium, vanadium, iron in different valence states and cobalt, with cobalt detected by X-ray fluorescence despite concentrations below the electron microprobe detection limit. In contrast, approximately 70% of the examined Tanzanian samples contained characteristic lamellar inclusions. These occur in four orientations parallel to the octahedral {111} faces of the spinel host. Viewed along a ⟨110⟩ direction, two sets intersect at angles of 109.47° and 70.53°. Immersion microscopy with crossed polarizers established that the lamellae are doubly refractive. Their extreme thinness prevented successful identification by electron microprobe, but transmission electron microscopy of ion-beam-thinned petrographic sections resolved their structure and composition. Individual lamellae measured 0.01–0.1 µm thick. Electron diffraction indicated trigonal, rhombohedral symmetry, with unit-cell dimensions of approximately a = 5.7 Å and c = 55.8 Å. Quantitative energy-dispersive X-ray analyses using a 15 nm electron probe identified magnesium–aluminium–titanium oxides, supporting their assignment to iron-free, titanium-rich högbomite. The structural relationship between högbomite and spinel is discussed in terms of titanium substituting for magnesium, with related structures also occurring in taaffeite and musgravite. The authors propose these oriented, birefringent inclusions as a practical indication of natural Tanzanian spinel, potentially useful in distinguishing such material as flux-grown synthetic spinels become increasingly available.