Carbonatites

Carbonatites are composed of carbonate minerals that appear to have formed from carbonate liquids. They typically contain many minerals of unusual composition that are seldom found elsewhere. Carbonatites have been mined for a variety of elements, including the rare-earth elements niobium and thorium. The rare-earth elements are used as phosphors in television picture tubes and in high-quality magnets in stereo systems; niobium is used in electronics and in steel to make it resist high temperatures.

Composition of Carbonatites

Carbonatites are unusual igneous rocks because they are not primarily composed of silicate minerals, as are most other rocks formed from molten rock material. Instead, carbonatites are composed mostly of carbonate minerals and of minor amounts of other minerals that are rare in other rocks. The carbonate minerals composing carbonatites are usually calcite (calcium carbonate) or dolomite (calcium magnesium carbonate). The other minerals found in carbonatites often contain large concentrations of elements that rarely become concentrated enough to form these minerals in other rocks.

Carbonatites are often associated with rare silicate rocks injected at about the same time as the carbonatites. These silicate rocks are unusual, as they normally lack feldspars (calcium, sodium, and potassium aluminum silicate minerals), which are abundant in most igneous rocks formed within the earth. Instead of feldspar, many of these silicate rocks contain varied amounts of the minerals nepheline (sodium aluminum silicate) and clinopyroxene (calcium, magnesium, iron silicate) and minor amounts of minerals not commonly found in other igneous rocks. The silicate rock consisting of more or less equal amounts of nepheline and clinopyroxene is called ijolite.

Occurrence of Carbonatites

Carbonatites often occur as small bodies of varying size and shape that cut across the surrounding rocks. Often, they occur as dikes that may be only a few feet to tens of feet wide, but they may be greater in length. An example of carbonatites occurring as dikes is found at Gem Park near Westcliffe, Colorado. Carbonatites sometimes occur as somewhat equi-dimensional bodies that are larger than those forming dikes. The Sulfide Queen carbonatite at Mountain Pass, California, for example, is about 800 meters long and 230 meters wide. Often, carbonatites contain foreign rock fragments. The cross-cutting relations and foreign rocks found within the carbonatites suggest that they form by the injection or intrusion of molten carbonate into the surrounding solid rock. The foreign rock fragments within the carbonatite could be solid rocks ripped off the walls by the moving molten carbonate material as it was injected through solid rock.

The occurrence of molten carbonate at the active volcano at Ol Doinyo Lengai in northern Tanzania, Africa, confirms that such material exists. The abundance of recently formed volcanic material composed of carbonatite at Ol Doinyo Lengai and other locations may indicate that some carbonatite can form near the surface. The carbonatite liquids at the surface may flow as lava out of the volcano or be ejected explosively into the air, similar to that of other volcanoes formed mostly from silicate liquids. Carbonatites of volcanic origin are especially abundant in Africa along a portion of the continent called the East African rift zone. In this location, Africa is slowly being ripped apart by forces within the earth. Some carbonatites that intruded below the surface as dikes may also at one time have fed volcanoes at the surface. Erosion of the extinct volcano and associated silicate rocks may have exposed dikes composed of solidified carbonatite.

Complexes

The carbonatites, ijolites, and other associated igneous rocks that formed at the same place and were injected at about the same time are called complexes. These complexes have small areas of exposure at the surface; most are exposed over a surface area of only about 1 to 35 square kilometers. The Magnet Cove complex in Arkansas, for example, has about a 16-square-kilometer exposure. The Gem Park complex in Colorado is only about 6 square kilometers in area. The associated silicate rocks usually compose most of the area of the exposed complex; only a small portion is carbonatite. The overall shape of these complexes is often circular, elliptical, or oval, but departures from these shapes are common. Often, the different rock types within a complex are built of concentric zones much like the layers of an onion.

Examples of these complexes include Seabrook Lake, Canada, where the complex is roughly circular and is about 0.8 kilometer across. Its “tail,” however, extends from the main circular body to about 1.2 kilometers to the south. The central core of carbonatite, composed mostly of calcite, is about 0.3 kilometer across. Smaller carbonatite dikes can be found within other rocks. The largest carbonatite body is surrounded by a dark rock containing angular blocks composed mostly of carbonate, clinopyroxene, or biotite (a dark, shiny potassium, iron, magnesium, and aluminum silicate). This latter body is surrounded by a mixture of ijolite and pyroxenite (a rock containing only pyroxene). The ijolite and pyroxenite also compose much of the tail to the south.

Another complex is located at Gem Park in Colorado. Gem Park is oval in shape, roughly 2 by 3.3 kilometers. There is no central carbonatite there. Instead, scores of small, dolomite-rich carbonatites have intruded as dikes across the silicate rocks of the complex. The main silicate rocks of the complex are pyroxenite and a feldspar-clinopyroxene rock called gabbro. The pyroxenite and gabbro form concentric rings with one another. The large amount of gabbro makes this complex unusual, as gabbros are seldom present with carbonatite in the same complex.

The complexes at Gem Park and Seabrook Lake are rather simple, as they contain very few rock types. A wide variety of minerals can be found in some complexes, resulting in a large number of rock types. The Magnet Cove complex in Arkansas, for example, has twenty-eight major rock types listed on the geologic map. (The reading accompanying the map extends the total rock types to an even greater number.) A large proportion of the rock names in geology are generated by the wide variation of minerals found in these complexes that compose merely a tiny portion of the earth’s surface.

Hypotheses of Carbonatite Formation

Up until the 1950s, geologists believed that carbonatites were limestones that melted and were intruded as molten rock material, or that circulating waters formed carbonatites by replacing silicate minerals with carbonate minerals. Some geologists thought that carbonatites could have formed from limestones that were remobilized by a solid, plastic flow—much like the flow of toothpaste squeezed out of a tube.

A major problem with the suggestion that carbonate material could melt, however, was the apparently high melting point of pure calcite or dolomite. Few geologists could believe that the temperature within the earth was high enough to melt calcite or dolomite. Another problem concerning the belief that limestones were the source of carbonatite was that in some areas, no limestones could be found anywhere near the occurrences of carbonatites. Also, the concept of the intrusion of limestones by the plastic flow of a solid was difficult to reconcile with many observations of carbonatites, including the occurrence of foreign igneous rock fragments composed of silicate minerals within them. The absence of fossils in the “limestone” also was noted as unusual, as most limestones have abundant fossils. The lack of fossils could be explained, however, by the melting hypothesis: the fossil evidence would have been destroyed during the melting.

Experiments in furnaces at temperatures and pressures similar to those expected deep within the earth have done much to support the melting hypothesis for the igneous formation of carbonatites. Several experiments in the late 1950s showed that carbonate minerals could melt at reasonably low temperatures (about 600 degrees Celsius) if abundant carbon dioxide and water vapor coexisted with the carbonate minerals. This dispelled the notion that molten carbonate could not exist within the earth. Also, the discovery in 1960 that the Ol Doinyo Lengai volcano was extruding carbonate lavas confirmed that carbonate liquids could exist within the earth. Similar experiments at high temperature and pressure on the composition of carbon dioxide or water vapor suggested that their composition could not produce carbonatites by the replacement of silicate minerals with carbonate minerals.

These experiments, combined with field observations (including the way carbonatites cross-cut surrounding rocks and the presence of foreign rock fragments), confirmed that most carbonatites formed by the intrusion of carbonate liquids. Even so, the experiments fell short of dispelling the notion that carbonate liquids could have been derived from melted limestone. The melted limestone hypothesis met objections, because isotopic and element concentrations in carbonatites were much different from those observed in limestone. For example, the elements lanthanum and niobium are hundreds of times more concentrated in carbonatites than they are in limestone. There is no known process that can produce this magnitude of element enrichment, either by melting or leaching the carbonate liquid from the solid rock through which it moved. Such observations have caused the limestone origin of carbonatites to be rejected by most geologists.

Continuing Study of Carbonatite Formation

Scientists continue to try to understand how carbonatites form. Experiments in furnaces suggest that some carbonate liquids may separate from some silicate liquids similar to those occurring with carbonatites. This process would be like the separation of oil and water as immiscible liquids. Other experiments in furnaces suggest that rocks more than 80 kilometers deep within the earth may melt in small amounts and produce the carbonate liquids and associated silicate liquids similar in composition to those observed in the natural rocks. Although these experiments fail to prove that carbonate liquids form in these ways, several other lines of evidence have convinced geologists that either of these possibilities could produce carbonatites in nature. For example, some possible source rocks that could melt and produce carbonatites or associated silicate rocks are sometimes carried up with lava from deep within the earth. The strontium-87 to strontium-86 ratios and the element contents of these possible source rocks have been measured and are similar to those expected for rocks that would melt and produce carbonate and silicate liquids. Strontium-87 forms from the decay of rubidium-87, and the ratio of strontium-87 to strontium-86 is a measurement of the age of the possible source rocks, and can help to identify the rocks that gave rise to the carbonatite magma.

Geologists want to understand how carbonatites form partly because of their economic importance. With this knowledge, they can design better strategies to find carbonatites or the associated silicate rocks that are as yet undiscovered. For example, carbonatites are very small targets to find on the surface of the earth, but if the silicate rocks associated with carbonatites contain abundant magnetic minerals, an aerial survey of the area can detect magnetic fields due to the rocks by using a magnetometer device. Once geologists find areas with magnetic anomalies, they can collect soil or stream samples in the area to see if any unusual minerals associated with carbonatites or the associated silicate rocks are present. They can also drill the area to see if any carbonatites are below the surface.

Economic Value of Carbonatites

The unusually high concentrations of some elements in certain minerals make carbonatites potential ores for these elements. Carbonatites have high concentrations of niobium, thorium, and the rare-earth elements of lower atomic number (such as lanthanum and cerium). Iron, titanium, copper, and manganese also have been mined from carbonatites.

Niobium has been economically extracted from the mineral pyrochlore at Fen, Norway, and at Kaiserstuhl, Germany. Niobium is used as an alloy in steel to resist high temperature; this steel is then used in gas turbines, rockets, and atomic power plants. Rare-earth elements have been mined from the carbonatite at Mountain Pass, California. The reserves of rare-earths are enormous at Mountain Pass (averaging about 7 percent) compared to other carbonatites. The mine closed in the late 1990s and briefly reopened from 2012 to 2015 before it went bankrupt and was forced to shutter its operations. The mining company MP Materials took control of the mine in 2017 and resumed operations in 2018. Four years later, in 2022, the Mountain Pass Mine produced 42,499 metric tons of rare earth minerals, about 14 percent of the worldwide total.

Other carbonatites enriched with rare-earths occur in Malawi in Africa. The rare-earth elements are concentrated in many minerals, including perovskite, monazite, xenotime, and a variety of rare-earth carbonate minerals. The rare-earths are used as color phosphors in television picture tubes and as components in high-quality magnets used in stereo speakers and headphones. Thorium, often enriched along with the rare-earths in many carbonatites, tends to concentrate in the same minerals and deposits as do the rare-earth elements. Thorium is radioactive and has been used as a source of atomic energy. It has also been used for the manufacture of mantles for incandescent gas lights. The Oka Complex west of Montreal, Canada, has also been mined for rare-earth elements. Rising demand for rare-earths for electronics, plus dependency on China as the leading refiner of rare-earths, has spurred additional exploration for new resources.

Principal Terms

calcite: a mineral composed of calcium carbonate

dike: a tabular igneous rock formed by the injection of molten rock material through another solid rock

dolomite: a mineral composed of calcium magnesium carbonate

igneous rocks: rocks formed from liquid or molten rock material

ijolite: a dark-colored silicate rock containing the minerals nepheline (sodium aluminum silicate) and pyroxene (calcium, magnesium, and iron silicate)

isotopes: different atoms of the same element that have different numbers of neutrons (neutral particles) in their nuclei

limestone: a sedimentary rock composed mostly of calcium carbonate formed by organisms or by calcite precipitation in warm, shallow seas

mineral: a naturally occurring element or compound with a more or less definite chemical composition

rock: a naturally occurring consolidated material that usually consists of two or more minerals; sometimes, as in carbonatites, rocks may consist mainly of one mineral

silicate mineral: a mineral composed of silicon, oxygen, and other metals, such as iron, magnesium, potassium, and sodium

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