Calcite is normally less than extraordinary. It pains me to say this since almost every fossil I’ve found is made of calcite. Limestone is primarily calcite, and the cement of many clastic sedimentary rocks is calcitic. In the metamorphic world, a large portion of marbles are composed of calcite. But something I’d never considered much was why calcite is so underrepresented in the igneous world. So I set off on an adventure deep in the Ouachita Mountains to Magnet Cove, Arkansas to investigate.
In the heart of Magnet Cove is something truly rare; a carbonatite. Even cooler than the name is the fact that it’s igneous calcite! There is something magical about thinking of the carbonate ooze, formed by microscopic organisms long ago, that compacts to form limestone that is melted by magma and recrystallized to form a dike of coarse, igneous calcite. However, when the carbonate rock was melted and incorporated into the intruding magma, why didn’t calcium in the calcium carbonate just mix with the silicate melt and form calcium-rich plagioclase? After all, plagioclase feldspar is one of the most common minerals on the planet, especially in igneous rocks.
While much more complex than what I’m about to explain, the carbonatite was formed as a result of liquid immiscibility and buoyancy. The carbonate melt, formed by partial melting of lime-rich crustal rock, is immiscible with the silicate magma, meaning it doesn’t like to mix. Another factor that separated the calcite from the typical igneous minerals was it’s relatively low density. A typical magma is somewhere in the ballpark of 2.5 g/cc. However, the carbonatite is estimated to have been between 2.2-2.3 g/cc. This means that the melt could have been separated simply due to buoyancy elevating the carbonate-rich melt above the silicate melt. This is supported by sampling of inclusions of primary “magma” in other minerals in Magnet Cove, as well as simply looking at the geometry of the igneous complex. The carbonatite is directly in the middle of the other ring dikes, suggesting that it was the last to fractionally crystallize.
As if you needed more reasons to love carbonatites, the Magnet Cove carbonatite also hosts a suite of exotic minerals containing rare earth elements, and even produced a new, Zr-rich garnet species called kimzeyite. Also, a perk to visiting Magnet Cove is being close to Hot Springs, AR, “America’s first resort.” Alongside the spas, attractions, and restaurants are large, folded rock faces along the strip.
10 out of 10, will rockhound again.
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