Carbonatites are predominantly composed of carbonate minerals. They are rare, peculiar igneous rocks formed by unusual processes and from unusual source rocks. The origin of carbonatite magma is obscure. Most carbonatites occur close to intrusions of alkaline rocks (those rich in potassium or sodium relative to their silica contents) or to the ultramafic igneous rocks. These associations suggest a common derivation, but details of the way that carbonatite magmas might concentrate geochemically scarce metals remain conjectural. There are about 330 known occurrences of carbonatites worldwide. They often occur as minor members of larger intrusions of silicate igneous rocks, commonly referred to as ‘carbonatite complexes’. Most are calcite carbonatites.
Carbonatites are, almost exclusively, associated with continental rift-related tectonic settings. In historical time only one volcano actively erupting carbonatite to the surface is known to us, Oldoinyo Lengai in Tanzania. Image: Extrusive calciocarbonatite (calciocarbonatitic lapillistone; welded calciocarbonatite, 7.4 cm across), erupted during the Miocene from Germany’s extinct Kaiserstuhl Volcano. This carbonatite is special: it was the first ancient carbonatite ever that demonstrated to have originally been erupted onto the surface. (© James St. John, via Flickr)
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