English: Natrocarbonatite (4.1 cm across) from a freshly flowing pahoehoe lava flow at Ol Doinyo Lengai Volcano back in 1963. This is from sample CML-9 of Peterson (1990) - Contributions to Mineralogy and Petrology 105: 143-155. Published chemical analyses on this very lava indicate that it has phenocrysts of gregoryite (77% sodium carbonate, 18% calcium carbonate, 5% potassium carbonate) and nyerereite (50% Ca-carbonate, 41% Na-carbonate, 9% K-carbonate), with a groundmass of 65% Na-carbonate, 20% Ca-carbonate, and 15% K-carbonate.

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Ol Doinyo Lengai in eastern Africa is only active volcano on Earth that erupts carbonatite lava. All other volcanic lavas on Earth are rich in silicate minerals. Ol Doinyo Lengai lava has essentially zero silicate content. Instead, it is dominated by sodium carbonate minerals (plus some potassium carbonate and calcium carbonate). The high sodium content makes this lava natrocarbonatite. Natrocarbonatite is the rarest lava type in the world.

Shown here is a sample freshly collected from an eruption back in the 1960s. Natrocarbonatite has the unfortunate tendency to alter upon exposure to the atmosphere & water. Molten natrocarbonatite has the appearance of very dark flowing mud, but it does glow red at night. After cooling, it alters relatively quickly to a whitish, crumbly material. This sample was collected before any significant chemical or physical alteration could occur.

Ol Doinyo Lengai natrocarbonatite is principally composed of the minerals gregoryite and nyerereite. Gregoryite (Na1.6K0.1Ca0.15CO3; a.k.a. (Na2K2Ca)CO3) forms dark-colored, platy-shaped, glassy-looking crystals in the lava. Nyerereite (Na0.8K0.2Ca0.5CO3; a.k.a. Na2Ca(CO3)2) forms small, grayish, rounded masses in the lava.

Ol Doinyo Lengai is one of many volcanoes in the East African Rift Valley, a long continental rift complex formed as the Afar Hotspot slowly rips Africa apart.

Locality: Ol Doinyo Lengai Volcano, ~9.5 miles south of Lake Natron, northern Tanzania, eastern Africa.