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Eskolaite from Russia
Category Oxide mineral
Hematite group
Corundum structural group
(repeating unit)
Strunz classification 4.CB.05
Crystal system Trigonal
Crystal class Hexagonal scalenohedral (3 m)
H-M symbol: (3 2/m)
Space group R3c
Unit cell a = 4.95, c = 13.58 [Å]; Z = 6
Color Black to dark green
Crystal habit Hexagonal prisms and plates
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 [1][2][3]

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

Discovery and occurrence[edit]

It was first described in 1958 for an occurrence in the Outokumpu ore deposit of eastern Finland.[2] 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.[1] It has also been recognized as a rare component in chondrite meteorites.[1]

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

Structure and physical properties[edit]

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+.