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 (Cal_ xRE~) [(Al,Fe3+)12_ 2~+ x(Ti,Si) Three compositional types of (clino)zoisite are distinguished: 1.8-3.I wt 700 Fe203 (orthorhombic and monoclinic), 3.9-6.0 wt % FezO3 (monoclinic), and 5.8-7.9 wt 9/o Fe203 with an average of 6. 3 wt % RE203 (monoclinic).Thermometric and barometric data for coexisting pyroxenes and garnet from adjacent rocks indicate granulite-facies equilibration conditions of 75o to 85o ~ and 6 to I I kb. During retrogression with increasing partial H20 pressures, hibonite reacted with plagioclase and garnet to form spinel, sphene, and RE-bearing clinozoisite. Corundum-ilmenite intergrowths probably resulted from the breakdown of an Fe-h6gbomite.THE rare mineral hibonite, with the theoretical composition of CaO'6AI203, is well known from Ca-Al-rich inclusions in carbonaceous chondritic meteorites (Keil and Fuchs, I971; Blander and Fuchs, I975; Macdougall, i979; Smith, i979). In the Allende and Leoville meteorites, for instance, hibonite occurs in association with anorthite, perovskite, gehlenite, spinel, wollastonite, andradite, and fassaite. Textural, thermodynamic, and isotopic anomaly data suggest that hibonite in meteorites may be regarded, at least in several cases, as an early high-temperature condensate from a primitive solar nebula (Shimizu et al., I978; Smith, x979; Lorin, pets. comm.).The presence of hibonite in terrestrial rocks,
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