Carbonatites are strange igneous rocks made up mostly ofcarbonates – common minerals like calcite, calcium carbonate. Igneous rocksthat solidify from molten magma usually are high-temperature rocks containinglots of silicon which results in lots of quartz, feldspars, micas, andferro-magnesian minerals in rocks like granite and basalt. Carbonatitescrystallize from essentially molten calcite, and that’s really unusual.

Most carbonatites are intrusive, meaning they solidifiedwithin the earth, and it wasn’t until 1960 that the first carbonatite volcanoerupted in historic times, proving that they form from cooling magma. Theeruption at Ol Doinyo Lengai in Tanzania occurred on a branch of the EastAfrican Rift System, and most carbonatites are associated with these breaks incontinental crust where eventually a new ocean may form.

Mt Lengai, Tanzania, photo by Clem23
(Creative Commons License - source)Eruptions at Lengai, whose name means “mountain of god” inthe Maasai language, are the lowest-temperature magmas known because calcitemelts at a much lower temperature than silica-rich compounds, around 510degrees C versus 1000 degrees or more for most magmas. It isn’t even red-hotlike most lava flows.

A simple and early interpretation of carbonatites was thatthey represented melting of limestone, but geochemical data indicate that theyreally do come from primary igneous material that probably originated in themantle. Exactly how they form is debated, in part because they are so rare, butone idea is that they result from special cases of differentiation within morecommon magmas, or maybe an example of certain chemicals – the carbonates – separatingout in an unusual way.

Another unusual aspect of carbonatites is the mineralsassociated with the dominant calcite. It’s common to get rare-earth compounds,tantalum, thorium, titanium, and many other minerals that are unusual in highconcentrations in other settings. The Mountain Pass rare-earth deposit inCalifornia, once the largest producer of rare earths in the world, is in aPrecambrian carbonatite. Rare earths are used in lots of modern technologies,including turbines for wind energy, batteries in electric car motors, cellphones, solar cells, and eyeglasses.

Rare earths are also produced from the Mt. Weld carbonatitein Western Australia, but it’s more famous for its tantalum, an element that’svital in capacitors for cell phones, video games, and computers. Australia hasby far the greatest reserves of tantalum, but mining didn’t begin until 2011and production is just now ramping up. The United States, which is 100%dependent on imports for tantalum, imports most of it from Brazil, Rwanda,China, and Kazakhstan.

Magnetite is a common associated mineral in carbonatites,and at Magnet Cove, Arkansas, there’s enough to give the name to the place. It’salso rich in titanium, often in the form of the mineral rutile, titanium dioxide.When I was there on a geology field trip in 1969, I remember walking into theKimzey Calcite Quarry. It was like walking into a giant calcite crystal, withgigantic cleavage faces the size of a person or bigger. We collected lots of coolrutile and pyrite crystals.

More common economic minerals can be associated withcarbonatites as well. At one in South Africa the main products are copper andvermiculite.

While I said earlier that carbonatites are really rare,there are still a few dozen known. It’s possible that their rarity is areflection of the fact that calcite is much more easily eroded and dissolvedthan the typical basaltic rocks that derive from most volcanoes, so they maysimply be poorly preserved.

—Richard I. Gibson