Eskolaite from Russia
Corundum structural group
|Crystal class||Hexagonal scalenohedral (3 m)
H-M symbol: (3 2/m)
|Unit cell||a = 4.95, c = 13.58 [Å]; Z = 6|
|Color||Black to dark green|
|Crystal habit||Hexagonal prisms and plates|
|Mohs scale hardness||8|
|Luster||Vitreous or metallic|
|Diaphaneity||Opaque, translucent in thin edges|
|Pleochroism||Noted; emerald-green to olive-green|
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
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+.
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