OL DOINYO LENGAI
Ol Doinyo Lengai in eastern Africa (see pics) 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 below are samples freshly collected from eruptions 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. The samples below were 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. See thin section photo of these two minerals together.
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.
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.
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. This is the rarest volcanic ash type on Earth. It occurs only at this volcano in northern Tanzania.