English: Igneous calcite from the Cretaceous of Arkansas, USA.

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 a partial large calcite crystal showing rhombohedral cleavage. It's from a calciocarbonatite body in central Arkansas' Magnet Cove Carbonatite, a mid-Cretaceous ring dike complex.

Geologic unit: Magnet Cove Carbonatite, Arkansas Alkaline Province, late Albian to Cenomanian, mid-Cretaceous, 96-102 Ma

Locality: Cove Creek, northern Hot Spring County, central Arkansas, USA