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/