Hibonite

Hibonite is a mineral with the chemical formula (Ca,Ce)(Al,Ti,Mg)
12O
19, occurring in various colours, 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 found in some chondritic meteorites. Hibonite is closely related to hibonite-Fe (IMA 2009-027, (Fe,Mg)Al
12O
19)) an alteration mineral from the Allende meteorite.[3]

Hibonite
Hibonite, 1.6 cm sharp and lustrous crystal from Esiva eluvials, Maromby Commune, Amboasary District, Anosy (Fort Dauphin) Region, Tuléar (Toliara) Province, Madagascar
General
CategoryOxide minerals
Formula
(repeating unit)		(Ca,Ce)(Al,Ti,Mg)
12O
19
Strunz classification4.CC.45
Crystal systemHexagonal
Crystal classDihexagonal dipyramidal (6/mmm)
H-M symbol: (6/m 2/m 2/m)
Space groupP63/mmc
Unit cella = 5.56, c = 21.89 [Å]; Z = 2
Identification
ColorBrownish black to black; reddish brown in thin fragments; blue in meteorite occurrence
Crystal habitPrismatic platy to steep pyramidal crystals
Cleavage{0001} good, {1010} parting
FractureSubconchoidal
Mohs scale hardness​7 1⁄2-8
LusterVitreous
Streakreddish brown
DiaphaneitySemitransparent
Specific gravity3.84
Optical propertiesUniaxial (-)
Refractive indexnω = 1.807(2), nε = 1.79(1)
PleochroismO = brownish gray; E = gray
References[1][2]

A very rare gem, hibonite was discovered in 1953 in Madagascar by Paul Hibon, a French prospector.[4]

Colour

Hibonite can vary in colour, from a bright blue, to green, to orange, to a nearly black deep brown. The colour is related to the degree of oxidation; meteoritic hibonite tends to be blue.[5]

See also

References
  1. "Handbook of Mineralogy" (PDF).
  2. "Hibonite: Hibonite mineral information and data". www.mindat.org.
  3. "IMA Mineral List with Database of Mineral Properties". rruff.info.
  4. "Hibonite gemstone information". www.gemdat.org.
  5. Ihinger, Phillip D.; Stolper, Edward (May 1986). "The color of meteoritic hibonite: an indicator of oxygen fugacity". Earth and Planetary Science Letters. 78 (1): 67–79. doi:10.1016/0012-821X(86)90173-1.


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