Eskolaite . Pen for scale. Collected from Moscow, Russia, by Franklin S. Harris, 1929. Mineral collection of Bringham Young University Department of Geology, Provo, Utah. Photograph by Andrew Silver. BYU index 4-8046, Cr_2O_3. — Photo: Andrew Silver, USGS · Public domain
Eskolaite
Natural chromium(III) oxide — a rare green-black mineral named for Finnish geologist Pentti Eskola.
Intro adapted from Wikipedia (CC BY-SA).
Eskolaite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula Cr₂O₃.
It crystallizes in the hexagonal crystal system (space group R-3c). It is a member of the Corundum structural group.
Key properties: Mohs hardness 8; specific gravity 5.18; Pale green streak; Vitreous luster; optically uniaxial.
IMA symbol Esk, grandfathered status — known before the IMA’s founding, named in 1958.
Found in chromium-rich tremolite skarns, metaquartzites, and chlorite veins (Outokumpu, Finland); on greywacke pebbles in a glacial boulder clay deposit (Callowhill Upper, Ireland); as pebbles in streams (Merume River, Guyana).
It is found associated with Guyanaite, pyrrhotite, pentlandite, chalcopyrite, pyrite, chromian tremolite, uvarovite, chromium tourmaline, calcite, talc, quartz (Outokumpu, Finland).
The mineral was named to honor Professor Pentti Eelis Eskola (1883–1964), of the University of Helsinki, Helsinki, Finland.