Carbonatite

In short

Carbonatite is a rare igneous rock made not of silicates but of carbonate minerals, mostly calcite. It crystallises from a carbonate-rich magma that comes from the mantle, and most occurrences lie in continental rifts together with alkaline rocks. At first sight it looks like marble, but it is not metamorphosed limestone as marble is. A large part of the world's niobium and rare earth elements comes from carbonatites. In Türkiye it occurs in the Kızılcaören complex in Eskişehir.

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How to recognise it

1. 1

   It is a coarse-grained rock of white, grey or beige calcite crystals that looks very much like marble or limestone.

2. 2

   It differs from marble in containing scattered igneous minerals such as black magnetite, green olivine, flakes of mica or apatite, and in often occurring with alkaline rocks such as syenite and phonolite.

3. 3

   The calcite-rich kinds fizz in dilute acid, like marble and limestone, so an acid test cannot tell them apart.

4. 4

   A firm identification needs chemical analysis and a study of the minerals.

Myths and misconceptions

• MythCarbonatite is just another name for marble or limestone.

  FactCarbonatite can resemble marble, but marble is metamorphosed limestone, while carbonatite is an igneous rock that crystallised from a carbonate magma coming from the mantle. Geochemical analysis is needed to tell them apart.

• MythLava is always a glowing red stream at over 1,000 °C.

  FactThe carbonatite lava of Ol Doinyo Lengai is only about 500-600 °C; in daylight it looks black or brown, and on contact with air it begins to turn white within hours.

Did you know?

Where is it found?

• Ol Doinyo Lengai, Arusha Region, Tanzania · Major depositThe world's only known active carbonatite volcano. Its natrocarbonatite lava erupts at about 500-600 °C, the coolest lava known; the black or brown lava begins to turn white within hours.
• Fen Complex, Telemark, Norway · Type localityThis roughly 3 km-wide circular area is the classic locality of the carbonatite types described by W. C. Brøgger in 1921; the carbonatites formed about 539 million years ago.
• Mountain Pass, California, USA · Major depositA carbonatite about 1.4 billion years old, intruded into gneiss; a major US rare earth mine works bastnäsite ore here.
• Bayan Obo, Inner Mongolia, China · Major depositOne of the world's largest rare earth, iron and niobium deposits. Its origin is still debated; it is often described as carbonatite-related, but there is no full agreement among scientists.
• Phalaborwa, Limpopo, South Africa · Major depositAn igneous complex with a carbonatite about 2.06 billion years old; copper, phosphate and magnetite are mined.
• Araxá, Minas Gerais, Brazil · Major depositThe Barreiro alkali-carbonatite complex, about 4.5 km across and intruded about 87 million years ago, is the world's largest niobium deposit; the niobium is in the mineral pyrochlore. Brazil supplies about 93% of world niobium production.
• Kızılcaören, Beylikova, Eskişehir, Türkiye · Known localityCarbonatite and carbonate-silicate dike-like bodies, which followed stocks of phonolite and trachyte, formed here about 25-24 million years ago. The fluorite-baryte-bastnäsite veins of the same complex form Türkiye's main rare earth deposit.

Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →