Carbonatites are strange igneous rocks made up mostly of carbonates – common minerals like calcite, calcium carbonate. Igneous rocks that solidify from molten magma usually are high-temperature rocks containing lots of silicon which results in lots of quartz, feldspars, micas, and ferro-magnesian minerals in rocks like granite and basalt. Carbonatites crystallize from essentially molten calcite, and that’s rather unusual.
Most carbonatites are intrusive, meaning they solidified within the earth, and it wasn’t until 1960 that the first carbonatite volcano erupted in historic times, proving that they form from cooling magma. The eruption at Ol Doinyo Lengai in Tanzania occurred on a branch of the East African Rift System, and most carbonatites are associated with these breaks in continental crust where eventually a new ocean may form.
Eruptions at Lengai, whose name means “mountain of god” in the Maasai language, are the lowest-temperature magmas known because calcite melts at a much lower temperature than silica-rich compounds, around 510 degrees C versus 1000 degrees or more for most magmas. It isn’t even red-hot like most lava flows.
A simple and early interpretation of carbonatites was that they represented melting of limestone, but geochemical data indicate that they really do come from primary igneous material that probably originated in the mantle. Exactly how they form is debated, in part because they are so uncommon, but one idea is that they result from special cases of differentiation within more common magmas, or maybe an example of certain chemicals – the carbonates – separating out in an unusual way.
Another unusual aspect of carbonatites is the minerals associated with the dominant calcite. It’s common to get rare-earth compounds, tantalum, thorium, titanium, and many other minerals that are unusual in high concentrations in other settings. The Mountain Pass rare-earth deposit in California, once the largest producer of rare earths in the world, is in a Precambrian carbonatite. Rare earths are used in lots of modern technologies, including turbines for wind energy, batteries in electric car motors, cell phones, solar cells, and eyeglasses.
The largest rare-earth element deposit in the world, Bayan Obo in Inner Mongolia, China, has a complex history but certainly includes some carbonatite intrusion about 1,400 million years ago, possibly complicated by a later mineralization event about 440 million years ago (Fan and others, 2016, The giant Bayan Obo REE-Nb-Fe deposit, China: Controversy and ore genesis: Geoscience Frontiers, Volume 7, Issue 3, May 2016, p. 335-344).
The Bayan Obo image above is from NASA. Before you decry the mining devastation evident in the image, remember that you own rare-earth metals that were almost certainly mined and/or processed at the Bayan Obo complex. They are in your computer, smart phone, fluorescent lights and LEDs, speakers, and headphones. If you have an electric car, you have lots of metal from Bayan Obo, and if your electricity is generated by wind turbines, they rely on Bayan Obo metals for their motors. Assuming you use any gasoline, it was processed using lanthanum as a catalyst in oil refining, and assuming your car has a catalytic converter, there are probably some bits of Bayan Obo in there, too, and in the systems that power your car’s electric windows and starter motors. If you use a cigarette lighter for anything, its “flint” is an alloy including rare-earth elements.
More about Bayan Obo in this podcast episode from the History of the Earth Calendar.
Rare earths are also produced from the Mt. Weld carbonatite in Western Australia, but it’s more famous for its tantalum, an element that’s vital in capacitors for cell phones, video games, and computers. Australia has by far the greatest reserves of tantalum, but mining didn’t begin until 2011 and production is just now ramping up. In 2024 Australia remains a distant sixth in world tantalum production, after Congo (Kinshasa), Rwanda, Brazil, Niger, and China, barely ahead of tiny Burundi. The United States, which is 100% dependent on total imports for tantalum ores, scrap, and metal, imports most of it from Brazil, Rwanda, China, and Kazakhstan, although Australia is the primary source in the US for processed tantalum metal.
Magnetite is a common associated mineral in carbonatites, and at Magnet Cove, Arkansas, there’s enough to give the name to the place. It’s also rich in titanium, often in the form of the mineral rutile, titanium dioxide. I was there on a geology field trip in 1969, and I remember walking into the Kimzey Calcite Quarry. It was like walking into a huge calcite crystal, with gigantic cleavage faces the size of a person or bigger. We collected lots of cool rutile and pyrite crystals.
More common economic minerals can be associated with carbonatites as well. At one in South Africa the main products are copper and vermiculite.