Hibonite

Hibonite is a mineral with the chemical formula (Ca,Ce)(Al,Ti,Mg)12O19, 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)Al12O19)) an alteration mineral from the Allende meteorite. Hibonites were among the first minerals to form as the disk of gas and dust swirling around the young sun cooled.

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

Hibonite

General

Category Oxide minerals

Formula (Ca,Ce)(Al,Ti,Mg)12O19

IMA symbol Hbn

Strunz classification 4.CC.45

Crystal system Hexagonal

Crystal class

Dihexagonal dipyramidal (6/mmm)

H-M symbol: (6/m 2/m 2/m)

Space group P63/mmc

Unit cell a = 5.56, c = 21.89; 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

Cleavage {0001} good, {1010} parting

Fracture Subconchoidal

Mohs scale hardness 7+1⁄2–8

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

Etymology

Hibonite was first discovered in 1956 in the eluvial deposit of Esiva near Maromby, in the Amboasary district, Anosy region (Fort Dauphin) in the Toliara province of Madagascar, and described by Curien, Guillemin, Orcel, and Sternberg. It was named after its discoverer, Paul Hibon.

Classification

In the outdated 8th edition of the mineral classification according to Strunz, hibonite belonged to the mineral class of “oxides and hydroxides” and there to the section “M 2 O 3 and related compounds”, where it is listed together with the hogbomite series and magnetoplumbite and in the appendix with plumboferrite in the “β-corundum-magnetoplumbite group” with the system number IV/C.05.

In the Lapis classification by Stefan Weiß, last revised in 2018 and formally based on the 8th edition of Karl Hugo Strunz 's old classification, the mineral was given the system and mineral number IV/C.08-030. This corresponds to the class of "Oxides and Hydroxides" and there to the section "Oxides with a metal:oxygen ratio of 2:3 (M2O3 and related compounds)", where hibonite, together with barioferrite, bartelkeit, batiferrite, chihuahuaite, diaoyudaoite, haggertyite, hawthorneite, lindqvistite, magnetoplumbite, nežilovite, otjisumeite, plumboferrite, yimengite and zenzénite, forms the "magnetoplumbite group" with the system number IV/C.08. 

The 9th edition of Strunz's mineral classification, last updated in 2009 by the International Mineralogical Association (IMA), classifies hibonite in the class of "Oxides (Hydroxides, V -Vanadates, Arsenites, Antimonites, Bismuthites, Sulfites, Selenites, Tellurites, Iodates)" and there in the division "Metal: Oxygen = 2: 3, 3: 5 and comparable". Here the mineral can be found in the subdivision "With large and medium-sized cations", where it forms the "Magnetoplumbite group" with the system number 4.CC.45 together with batiferrite, barioferrite, diaoyudaoite, haggertyite, hawthorneite, lindqvistite, magnetoplumbite, nežilovite, plumboferrite and yimengite.

In the Dana classification of minerals, which is primarily used in English-speaking countries, hibonite has the classification and mineral number 07.04.01.01. This corresponds to the class "Oxides and Hydroxides" and the division "Multiple Oxides." Here, it is found within the subdivision "Multiple Oxides with O 19 Groups" in the group "(Hexagonal, P63/mmc)," which also includes yimengite and hawthorneite.

Crystal structure

Hibonite crystallizes hexagonally in the space group P 6 3 / mmc (space group no. 194) with the lattice parameters a = 5.613 Å and c = 22.285 Å and two formula units per unit cell.

Characteristics

Like many rare earth minerals, hibonite contains traces of uranium and thorium. For this reason, it is classified as weakly radioactive. It has a specific activity of approximately 88.2 Bq /g (for comparison, natural potassium has 31.2 Bq/g).

Chemical characteristics

It is an oxide of aluminium, calcium and cerium, but the former is so frequently substituted totally or partially by titanium or magnesium that the International Mineralogical Association considers these five metals as constituents of its formula in its latest classifications. The magnetoplumbite group in which it is classified are all complex oxides of various metals, which are crystallographically related to corundum or spinel.

In addition to the elements in its formula, it usually contains impurities such as iron and silicon.

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.

Formation and deposits

At its type locality it was described in alluvial deposits near skarns containing thorianite. It has also been described in limestones, pyroxenites and gneisses of the amphibolite to granulite facies. 

 It has been described on every continent of the world, including Antarctica, where it has been described in a meteorite. It is a very common accessory mineral in environments rich in calcium and aluminium and in some carbonate chondrites.

 It is usually found associated with other minerals such as calcic plagioclase, corundum, spinel, thorianite, titanite, anorthite, grossularia, zoisite, clinozoisite, vesuvianite, hercynite, andalusite, kyanite, diopside, rutile or magnetite. 

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