Shungite

Shungite is named after the village of Shunga on Lake Onega in the Republic of Karelia, Russia. It occurs in a sedimentary-volcanic succession about 2.0 billion years old (Palaeoproterozoic) that covers some 9,000 km² north of Lake Onega, with an estimated carbon reserve of more than 25 × 10¹⁰ tonnes. Geologists interpret the deposits as a combination of a petrified oil field, petrified organosiliceous diapirs and oil spills; the Maksovo, Zazhogino and Nigozero deposits have economic significance.

The name covers materials of very different carbon content. Lustrous shungite (originally defined at about 98 wt% C) is black, sub-metallic and opaque. Shungite-bearing rocks range from metamorphosed oil shales with less than 50 wt% C to rocks with 50–75 wt% C, and diapiric shungite rocks contain 35–75 wt% SiO₂ and only 20–55 wt% C. A piece sold as "shungite" may therefore be anything from nearly pure carbon to a silica-rich rock.

Shungite has puzzled geologists since the 18th century. The name was introduced by Inostranzev in 1879 for a black, lustrous substance with about 98 wt% carbon occurring in veins and layers near Shunga; later classifications applied it more broadly, which has caused confusion. Some samples contain fullerenes, but according to Mindat only in trace amounts. Industrially, shungite and shungite rocks are used in metallurgy, water purification and other processes.