Carbonatite
Igneous rock with >50% carbonate minerals (calcite, dolomite)
A rare igneous rock made mostly of carbonate minerals, source of the world's most important rare-earth-element and niobium deposits.
- Mohs hardness
- 3-4
- Color
- White, grey, yellowish to brown
- Type
- igneous
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Overview
Carbonatite is an unusual igneous rock containing more than 50% carbonate minerals, chiefly calcite or dolomite, that crystallized from carbonate-rich magma rather than from sedimentary precipitation. Because it superficially resembles marble or limestone, it is one of the most surprising members of the igneous family.
Varieties are named for their carbonate: soevite (calcite-rich) and alvikite are common, while natrocarbonatite, erupted from Ol Doinyo Lengai in Tanzania, is the only carbonatite lava observed erupting in modern times.
Despite its rarity, carbonatite is economically vital because it concentrates rare earth elements, niobium, and phosphate.
Formation & geology
Carbonatite forms from carbonate-rich magmas generated in the mantle, typically in continental rift zones and stable cratonic settings. These magmas may arise by very low-degree partial melting of carbonated mantle, by liquid immiscibility separating a carbonate melt from a silicate melt, or by extreme fractional crystallization.
Carbonatites usually occur as plugs, dikes, and ring complexes associated with alkaline silicate rocks such as ijolite, nepheline syenite, and phonolite. The famous active example, Ol Doinyo Lengai, erupts cool, runny natrocarbonatite lava that weathers white within hours.
How to identify it
Carbonatite looks deceptively like marble or limestone: pale, fine-to-coarse grained, and soft enough to scratch with a knife (Mohs 3 for calcite). The decisive field test is that it fizzes vigorously in dilute hydrochloric acid, like any carbonate.
Distinguish it from limestone and marble by its geological setting (associated with alkaline igneous rocks and ring complexes), the presence of igneous accessory minerals such as apatite, magnetite, pyrochlore, and rare-earth minerals, and igneous textures. Radioactive thorium-bearing minerals also commonly accompany it.
Uses & significance
Carbonatites are the single most important source of rare earth elements (e.g., Mountain Pass, California; Bayan Obo, China) and a leading source of niobium (Brazil) and phosphate for fertilizer. They are prime exploration targets in the modern technology-metals economy.
The rock has no gemstone or metaphysical tradition, but its economic and scientific importance is immense, providing direct evidence of deep mantle carbon and unusual magma processes.
Frequently asked questions
How is carbonatite different from limestone?
Both are carbonate-rich and fizz in acid, but carbonatite is igneous, crystallized from magma and found with alkaline igneous rocks, while limestone is sedimentary.
Why is carbonatite economically important?
It hosts the world's largest deposits of rare earth elements and niobium, plus significant phosphate, fluorite, and copper.
Is there an active carbonatite volcano?
Yes. Ol Doinyo Lengai in Tanzania erupts cool, fluid natrocarbonatite lava, the only carbonatite volcano active today.
Is carbonatite radioactive?
Some carbonatites contain thorium- and uranium-bearing minerals and can be mildly radioactive, so samples should be handled accordingly.
Carbonatite guides
In-depth guides for identifying, valuing, and understanding Carbonatite.
