English: Natrocarbonatite ash and lapilli (field of view ~5.2 cm across) from a mid-August to late October 1966 ash eruption of Ol Doinyo Lengai Volcano.

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Ol Doinyo Lengai in eastern Africa is only active volcano on Earth that erupts carbonatite lava and ash. All other volcanic materials on Earth are rich in silicate minerals. Ol Doinyo Lengai lava and ash have essentially zero silicate content. Instead, they are dominated by sodium carbonate minerals (plus some potassium carbonate and calcium carbonate). The high sodium content makes this volcanic ash natrocarbonatite. This is the rarest volcanic ash type on Earth. It occurs only at this volcano in northern Tanzania.

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. Both lava and ash alter 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. Nyerereite (Na0.8K0.2Ca0.5CO3; a.k.a. Na2Ca(CO3)2) forms small, grayish, rounded masses.

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.