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carbonatite Unusual igneous rock, rich in calcite and other carbonate minerals (including dolomite and ankerite), which is considered to be mantle-derived. Carbonatites occur as intruded masses, dykes, and as cone sheets, and rarely as lavas and tephra; and are found in association with alkali-rich igneous rocks, notably those erupted by the volcanoes of the East African Rift. Rare elements, including the REEs (rare earth elements), barium, niobium, thorium, and phosphorus, are often enriched in comparison with many crustal and mantle-derived igneous rocks.