English: Carbonatites are essentially “igneous limestones”. That's pretty odd when you think about it. Carbonatites are rare igneous rocks - some are intrusive (formed deep underground by relatively slow cooling of magma), but some are demonstrably extrusive (formed at the Earth's surface by relatively quick cooling of lava). They are principally composed of carbonate minerals (hence the term "carbonatite"), including calcite (CaCO3 - calcium carbonate), dolomite (CaMg(CO3)2 - calcium magnesium carbonate), siderite (FeCO3 - iron carbonate), and sodium & potassium carbonate minerals.

Carbonatites composed of calcite are called calciocarbonatites. Those with sodium-rich carbonate minerals are natrocarbonatites. Examples with a significant dolomite component are magnesiocarbonatites. If a significant siderite component is present, they're called ferrocarbonatites. Calciocarbonatites are the most common variety - they tend to be very coarsely crystalline. An alternate rock name for calciocarbonatite is sövite.

This sample is from Colorado's Gem Park Complex, which is dominated by pyroxenites and gabbros. Carbonatites in the complex are reported to be magnesiocarbonatites and ankeritic magnesiocarbonatites (Parker & Sharp, 1970). However, the rock readily bubbles in acid, so a significant calcite component is present. The color suggests it is ferruginous. A more precise assessment would require thin section analysis.

Geologic unit: Gem Park Complex, Neoproterozoic or Cambrian, 551 ± 55 Ma

Locality: unrecorded site in Gem Park, far-southern Fremont County, south-central Colorado, USA

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Reference cited:

Parker & Sharp (1970) - Mafic-ultramafic igneous rocks and associated carbonatites of the Gem Park Complex, Custer and Fremont Counties, Colorado. United States Geological Survey Professional Paper 649. 24 pp. (pubs.usgs.gov/pp/0649/report.pdf)