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Eskolaite from Russia
CategoryOxide mineral
Hematite group
Corundum structural group
(repeating unit)
Strunz classification4.CB.05
Crystal systemTrigonal
Crystal classHexagonal scalenohedral (3 m)
H-M symbol: (3 2/m)
Space groupR3c, No. 167
Unit cella = 4.95, c = 13.58 [Å]; Z = 6
ColorBlack to dark green
Crystal habitHexagonal prisms and plates
Mohs scale hardness8
LusterVitreous or metallic
StreakPale green
DiaphaneityOpaque, translucent in thin edges
Specific gravity5.18
Optical propertiesUniaxial
PleochroismNoted; emerald-green to olive-green

Eskolaite is a rare 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+.