Summary: The third terrestrial occurrence of hibonite is reported from granulite-facies rocks in the Furua Granulite Complex in southern Tanzania. The mineral forms yellowish-brown lath-shaped crystals in a grossular-anorthite rock containing subordinate sphene (clino)zoisite, hercynite, apatite, ilmenite, and corundumilmenite intergrowths.

Electron-microprobe analyses indicate a generalized formula (Ca_1−xRE_x)[(Al,Fe³⁺)_12−2a+x(Ti,Si)(Ti,Si)_a−x(Fe²⁺, Mg)_a]O₁₉, with RE = Ce + La + Nd, x = 0.2, and a = 0.8. Individual mineral analyses show a cation substitution of Ca + Ti + Fe³⁺ = RE + 2Al. Relatively high RE and Fe contents represent the main chemical differences with meteoritic hibonite. The hexagonal unit cell has a = 5.61 Å, c = 22.18, in good agreement with the other terrestrial hibonites.

Three compositional types of (clino)zoisite are distinguished: 1.8–3.1 wt% Fe₂O₃ (orthorhombic and monoclinic), 3.9–6.0 wt% Fe₂O₃ (monoclinic), and 5.8–7.9 wt% Fe₂O₃ with an average of 6.3 wt% RE₂O₃ (monoclinic).

Thermometric and barometric data for coexisting pyroxenes and garnet from adjacent rocks indicate granulite-facies equilibration conditions of 750 to 850 °C and 6 to 11 kb. During retrogression with increasing partial H₂O pressures, hibonite reacted with plagioclase and garnet to form spinel, sphene, and RE-bearing clinozoisite. Corundum-ilmenite inter-growths probably resulted from the breakdown of an Fe-högbomite.