A Palaeoproterozoic puzzle
Near Lake Onega in the Republic of Karelia, Russia, lies a massive deposit of a curious, carbon-rich rock known as shungite. These deposits are ancient, dating back approximately 2 billion years to the Paleoproterozoic Era. The total carbon reserve within the Shunga deposit is estimated to be over 250 gigatonnes, spread across an area of roughly 9,000 square kilometers. Shungite is not a mineral but a mineraloid, as it lacks a defined crystal structure. Its composition is primarily non-crystalline carbon, with some varieties containing up to 98% carbon.
The origin of this immense carbon deposit is a scientific debate. The prevailing theory asserts a biological origin, suggesting the carbon is the metamorphosed remains of vast blooms of ancient microorganisms, like algae or bacteria, from a brackish lagoon. These organisms would have been deposited in shallow water sediments and then transformed by heat and pressure over geological time. An alternative hypothesis suggests an abiogenic origin, where the carbon formed through non-biological processes deep within the Earth's crust or was even delivered by a meteorite. While the biological origin theory is now more widely accepted, the exact mechanisms that led to such a huge and concentrated carbon deposit remain a subject of study.
Buckyballs from the Precambrian
A feature of shungite is the presence of fullerenes—hollow spherical molecules made of carbon atoms, such as C₆₀ and C₇₀. These molecules are often called "buckyballs" because their structure resembles the geodesic domes designed by architect Buckminster Fuller. Fullerenes were first synthesized in a laboratory in 1985, and for a time, it was believed they could not occur naturally. Their discovery in shungite in 1992 by geochemists at Arizona State University was an event in the scientific community.
Shungite is classified into different types based on its carbon content. Type I, known as "elite" or "noble" shungite, has the highest carbon concentration (90-98%) and a metallic silvery luster. It is the rarest form, accounting for less than 1% of all shungite, and is found in small veins. The more common varieties contain lower percentages of carbon mixed with other minerals like quartz, mica, and pyrite. The main mining area is the Zazhoginsky deposit, which is the largest and most studied occurrence. This open-pit mine has estimated reserves of 35 million tons and produces around 200,000 tons of shungite annually.