Hibonite

Hibonite
240px 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

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.

File:Hibonit-G-EmpireTheWorldOfGems.jpg

Faceted hibonite, 0.40 ct, Mogok, Myanmar

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

See also

• Glossary of meteoritics

References

• Webmineral