The rarest lava on Earth

Nearly all volcanoes on Earth erupt silicate lava--that is, lava which primarily forms minerals containing silicon and oxygen. Some rare exceptions to this can be seen in carbonatites, igneous rocks that consist primarily of carbonate minerals--usually dominated by calcite or dolomite, and often with silicate minerals mixed in. Only one volcano erupts carbonatite lava today: Ol Doinyo Lengai, a rift volcano in Tanzania (see crater in picture). But Ol Doinyo Lengai's lavas are unique even for carbonatites: while most carbonatites in the geologic record are dominated by calcite or dolomite, Ol Doinyo Lengai's lavas are extremely rich in sodium and potassium, and often contain less than 0.5% silica. As it cools below the surface, the magma mainly forms two rare sodium-calcium-potassium carbonate minerals, nyerereite and gregoryite. The lava is black and usually flows very easily, and may look like a mud flow. At the surface, however, the minerals are quickly weathered, turning the rock from black to almost white.

Geologists think that natrocarbonatite magma forms by separating from a body of silicate magma, similar to how a mixture of oil and water will separate. Ol Doinyo Lengai's rock record may suggest that this occurred fairly recently within the mountain: most of the mountain was built by eruptions of sodium- and potassium-rich silicate lava, with natrocarbonatite eruptions starting only recently in its history. Even today, eruptions alternate between nearly pure natrocarbonatite and relatively silica-rich episodes, and in a few eruptions, small spheroids of silicate material were carried along with the natrocarbonatite.


Image: Crater of Ol Doinyo Lengai, Tanzania, January 2011. Photo by Albert Backer, http://en.wikipedia.org/wiki/File:Crater_of_Ol_Doinyo_Lengai_(Jan_2011).jpg
References:
http://hvo.wr.usgs.gov/volcanowatch/archive/2003/03_04_17.html
http://blogs.stlawu.edu/lengai/what-is-carbonatite/
http://www.sciencedirect.com/science/article/pii/S0024493712002149