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.