Carbonatite

Carbonatite is a rare type of igneous rock that is dominated by carbonate minerals, typically calcite or dolomite, with very low silica content. By convention, carbonatite rocks contain more than about 50 percent carbonate minerals in their modal mineralogy.

The mineral assemblage is varied but commonly includes calcite or dolomite as the principal phase, with lesser amounts of magnesite or siderite. Silicate minerals may be present as accessory or fenitizing phases, including nepheline, cancrinite, diopside, wollastonite, and phlogopite. Accessory minerals such as apatite, monazite, zircon, fluorite, magnetite, ilmenite, and pyrochlore are frequently important, the latter two carrying economically significant niobium, tantalum, and rare earth element contents.

Formation and genesis are linked to alkaline, low-silica magmatism. Carbonatites are thought to form from mantle-derived melts that either differentiate to high carbonate and volatile contents or directly melt carbonate-rich mantle sources. They are typically emplaced as intrusive stocks and dikes, though rare effusive natrocarbonatite lavas occur, notably at Ol Doinyo Lengai in Tanzania.

Occurrence and significance: carbonatites occur worldwide but are most common in continental intraplate and rift settings. They often host economically important deposits of niobium, tantalum, phosphate, and rare earth elements, in addition to various fluorine-bearing minerals. The study of carbonatites sheds light on mantle processes, carbon cycling, and the evolution of alkaline magmatism.