Eskolaite

Eskolaite
Eskolaite from Russia
General
Category	Oxide mineral Hematite group Corundum structural group
Formula (repeating unit)	Cr2O3
Strunz classification	04.CB.05
Crystal symmetry	Trigonal hexagonal scalenohedral H-M symbol: (32/m) Space group: R3c
Unit cell	a = 4.95 Å, c = 13.58 Å; Z=6
Identification
Color	Black to dark green
Crystal habit	Hexagonal prisms and plates
Crystal system	Trigonal
Cleavage	None
Tenacity	Brittle
Mohs scale hardness	8
Luster	Vitreous or metallic
Streak	Pale green
Diaphaneity	Opaque, translucent in thin edges
Specific gravity	5.18
Optical properties	Uniaxial
Pleochroism	Noted; emerald-green to olive-green
References

Eskolaite is an uncommon chromium oxide mineral (chromium(III) oxide Cr₂O₃).

Discovery and occurrence

It was first described in 1958 for an occurrence in the Outokumpu ore deposit of eastern Finland. It occurs in chromium bearing tremolite skarns, metamorphosed quartzites and chlorite bearing veins in Finland; in glacial boulder clays in Ireland and in stream pebbles in the Merume River of Guyana. It has also been recognized as a rare component in chondrite meteorites.

The mineral is named after the Finnish geologist Pentti Eskola (1883–1964).

Structure and physical properties

File:Eskolaite-pV.svg

Molar volume vs. pressure at room temperature.

Eskolaite crystallizes with trigonal symmetry in the space group R3c and has the lattice parameters a = 4.95 Å and c = 13.58 Å at standard conditions. The unit cell contains six formula units. The lattice is analogous to that of corundum, with Cr³⁺ replacing Al³⁺.