Carbonatite () is a type of intrusive or extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geochemical verification. Carbonatites usually occur as small plugs within zoned alkalic intrusive complexes, or as dikes, sills, breccias, and veins. They are almost exclusively associated with continental rift-related tectonic settings. It seems that there has been a steady increase in the carbonatitic igneous activity through the Earth's history, from the Archean eon to the present. Nearly all carbonatite occurrences are intrusives or subvolcanic intrusives. This is because carbonatite lava flows, being composed largely of soluble carbonates, are easily weathered and are therefore unlikely to be preserved in the geologic record. Carbonatite eruptions as lava may therefore not be as uncommon as thought, but they have been poorly preserved throughout the Earth's history. Carbonatite liquid compositions are significantly more alkaline than what is preserved in the fossil carbonatite rock record as composition of the melt inclusions shows. Only one carbonatite volcano is known to have erupted in historical time, the active Ol Doinyo Lengai volcano in Tanzania. It erupts with the coolest lava in the world, at . The lava is natrocarbonatite dominated by nyerereite and gregoryite. Genesis The magmatic origin of carbonatite was argued in detail by Swedish geologist Harry von Eckermann in 1948 based on his study of Alnö Complex. It was however the 1960 eruption of Ol Doinyo Lengai in Tanzania that led to geological investigations that finally confirmed the view that carbonatite is derived from magma. Carbonatites are rare, peculiar igneous rocks formed by unusual processes and from unusual source rocks. Three models of their formation exist: direct generation by very low-degree partial melts in the mantle and melt differentiation, liquid immiscibility between a carbonate melt and a silicate melt, peculiar, extreme crystal fractionation. Evidence for each process exists, but the key is that these are unusual phenomena. Historically, carbonatites were thought to form by melting of limestone or marble by intrusion of magma, but geochemical and mineralogical data discount this. For example, the carbon isotopic composition of carbonatites is mantle-like and not like sedimentary limestone. The age of carbonatites ranges from Archean to present: the oldest carbonatite, Tupertalik in Greenland, is 3007 Ma old, while Ol Doinyo Lengai volcano in Tanzania is presently active. Mineralogy Primary mineralogy is highly variable, but may include natrolite, sodalite,