Hibonite

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Hibonite
Hibonite - Esiva eluvials, Tulear Province, Madagascar.jpg
Hibonite, 1.6 cm sharp and lustrous crystal from Esiva eluvials, Maromby Commune, Amboasary District, Anosy (Fort Dauphin) Region, Tuléar (Toliara) Province, Madagascar
General
Category Oxide minerals
Formula
(repeating unit)
(Ca,Ce)(Al,Ti,Mg)12O19
Strunz classification 04.CC.45
Crystal symmetry Hexagonal dihexagonal dipyramidal
H-M symbol: (6/m 2/m 2/m)
Space group: P 63/mmc
Unit cell a = 5.56 Å, c = 21.89 Å; Z=2
Identification
Color Brownish black to black; reddish brown in thin fragments; blue in meteorite occurrence
Crystal habit Prismatic platy to steep pyramidal crystals
Crystal system Hexagonal
Cleavage {0001} good, {1010} parting
Fracture Subconchoidal
Mohs scale hardness 7½-8
Luster Vitreous
Streak reddish brown
Diaphaneity Semitransparent
Specific gravity 3.84
Optical properties Uniaxial (-)
Refractive index nω = 1.807(2), nε = 1.79(1)
Pleochroism O = brownish gray; E = gray
References [1][2]

Hibonite ((Ca,Ce)(Al,Ti,Mg)12O19) is a brownish black mineral with a hardness of 7.5-8.0 and a hexagonal crystal structure. It is rare, but is found in high-grade metamorphic rocks on Madagascar. Some presolar grains in primitive meteorites consist of hibonite. Hibonite also is a common mineral in the Ca-Al-rich inclusions (CAIs) found in some chondritic meteorites. Hibonite is closely related to hibonite-Fe (IMA 2009-027, ((Fe,Mg)Al12O19)) an alteration mineral from the Allende meteorite.[3]

A very rare gem, Hibonite was discovered in Madagascar by Paul Hibon, a French prospector.[4]

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