Ol Doinyo Lengai in Tanzania, approximately 20miles (30km) to the south of Lake Natron, is one of the most peculiar volcanoes.
Whereas all other volcanoes eject silicate based lava at a temperature of 1000°c plus, this gorgeous monster, at 2980metres (9780ft), ejects molten natrocarbonate at around 5-600°C. and is the only known active example of the type.
Natrocarbonate is a mixure of sodium and potassium carbonates along with a minor quantity of silicate minerals and this type of lava and ejecta so produced are called Carbonatites. The silicate minerals are silica-undersaturated suites, as feldspathoids and nepheline along with olivine and pyroxenes ie minerals with ultra-basic affinities.
When newly erupted, these rocks are dark grey to black in colour and quickly weather to a pale grey to white. Unlike silicate high temperature lavas that are incandescent in daylight, the relatively low carbonatite temperatures give a lava that is black, glowing dull red at night.
Ol Doinyo is one of the volcanoes along the line of the African Rift Valley system. This rift system extends from the Mozambique coast opposite Madagascar in the south, northwards through Africa into the Red Sea by the Horn of Africa and extends past Sinai into the Dead Sea area. This rift system is the present day activity associated with the contunued break-up of the Gondwanaland super-continent and continental drift.
An excellent suite of photos of Ol Doinyo volcano and landforms are found here:
Carbonatite intrusives are widespread, but very uncommon and include dykes, sills, pipes and veins.
Although rare, the presence of free carbon (rather than carbonates) is not unusual in igneous rocks, as evidenced by Diamonds in widespread, but rare, Kimberlite pipe intrusions, graphite in doleritic intrusions – eg the intrusion in Borrowdale (English Lake District, the original graphite source for the Cumberland Pencil Co. in Keswick) and in extra-terrestrial carbonaceous chondrite meteorites.
Picture taken from http://www.eoearth.org/article/Diamond