Hibonite

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Formula: Ca[Al₁₂]O₁₉
Multiple oxide, hibonite subgroup, magnetoplumbite group
Crystal System: Hexagonal
Specific gravity: 3.83 - 3.85 measured, 4.09 calculated
Hardness: 7½ to 8
Streak: Brown
Colour: Black to brownishk black , purple
Solubility: Disolves very slowly in a mixture of sulphuric and phosphoric acids
Common impurities: Fe,Si
Mildly RADIOACTIVE
Environments

Sedimentary environments
Metamorphic environments
Meteorites

Hibonite occurs in metamorphosed limestone, pyroxenite, gneiss and granulite of the amphibolite to granulite facies; it is also alluvial, and it is a common accessory in calcium- aluminium- rich inclusions in some carbonaceous chondrites (HOM).
Associated minerals include anorthite, calcite, corundum, diopside, marialite-meionite and spinel (Mindat).

Localities

At the Chyulu Hills, Makueni County, Kenya, hibonite has been found in bassanites in the volcanic field, in two xenoliths of mafic meta-igneous granulite dominated by clinopyroxene and plagioclase. It occurs as small grains forming intergrowths with spinel and sapphirine and showing reaction relationships with later mullite and sillimanite. Silica contents are high and exceed those in any other terrestrial and meteoritic hibonite (EJM 17.2.357–366).

At the type locality, the Esiva eluvials, Esiva, Maromby, Amboasary Sud, Anosy, Madagascar, hibonite occurs as large, black hexagonal prismatic crystals, up to 4 cm in size, in an alluvial deposit close to thorianite-bearing skarn. Associated minerals include plagioclase, corundum, spinel, thorianite and titanite (Mindat).

At Besakoa, Maromby, Amboasary Sud, Anosy, Madagascar and the Andakato thorianite deposit, Ambindandrakemba, Tranomaro, Amboasary Sud, Anosy, Madagascar, hibonite occurs in metamorphosed limestone rich in calcic plagioclase, with corundum, spinel and thorianite (Dana).

At the Shoriya Mountains, about 400 km southeast of Novosibirsk, Novosibirsk Oblast, Russia, hibonite is associated with vesuvianite, hercynite, corundum, andalusite, kyanite, diopside, rutile, titanite and magnetite (HOM).

At the Furura granulite complex, southwest of Mahenge, Morogoro Region, Tanzania, hibonite is associated with anorthite, grossular, titanite, zoisite and clinozoisite (HOM).

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