The term "shungite" was originally used in 1879 to describe a mineraloid with more than 98 percent carbon. More recently the term has also been used to describe shungite-bearing rocks, leading to some confusion. Shungite-bearing rocks have also been classified purely on their carbon content, with Shungite-1 having a carbon content in the range 98-100 weight percent and Shungite-2, -3, -4 and -5 having contents in the ranges 35-80 percent, 20-35 percent, 10-20 percent and less than 10 percent, respectively. In a further classification, shungite is subdivided into bright, semi-bright, semi-dull and dull on the basis of their luster (the terms lustrous and matte are also used for bright and dull).
Shungite has two main modes of occurrence, disseminated within the host rock and as apparently mobilised material. Migrated shungite, which is bright (lustrous) shungite, has been interpreted to represent migrated hydrocarbons and is found as either layer shungite, layers or lenses near conformable with the host rock layering, or vein shungite, which is found as cross-cutting veins. Shungite may also occur as clasts within younger sedimentary rocks.
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Shungite has to date mainly been found in Russia. The main deposit is in the Lake Onega area of Karelia, at Zazhoginskoye, near Shunga, with another occurrence at Vozhmozero. Two other much smaller occurrences have been reported in Russia, one in Kamchatka in volcanic rocks and the other formed by the burning of spoil from a coal mine at high temperature in Chelyabinsk. Other occurrences have been described from Austria, India, Democratic Republic of Congo and Kazakhstan.
Shungite has been regarded as an example of abiogenic petroleum formation, but its biological origin has now been confirmed. Non-migrated shungite is found directly stratigraphically above deposits that were formed in a shallow water carbonate shelf to non-marine evaporitic environment. The shungite bearing sequence is thought to have been deposited during active rifting, consistent with the alkaline volcanic rocks that are found within the sequence. The organic-rich sediments were probably deposited in a brackish lagoonal setting. The concentration of carbon indicates elevated biological productivity levels, possibly due to high levels of nutrients available from interbedded volcanic material.
The stratified shungite-bearing deposits that retain sedimentary structures are interpreted as metamorphosed oil source rocks. Some diapiric mushroom-shaped structures have been identified, which are interpreted as possible mud volcanoes. Layer and vein shungite varieties, and shungite filling vesicles and forming the matrix to breccias, are interpreted as migrated petroleum, now in the form of metamorphosed bitumen.