Ankeritic carbonatite from the Cretaceous of Malawi, Africa. (~9.7 cm across at its widest)

Carbonatites are rare igneous rocks - they are composed of carbonate minerals, typically calcite (CaCO3 - calcium carbonate). Carbonatites are essentially igneous limestones.

The four categories of carbonatites are: 1) calciocarbonatite (calcite-dominated) 2) magnesiocarbonatite (dolomite-dominated) 3) ferrocarbonatite (siderite-dominated) 4) natrocarbonatite (sodium carbonate-dominated)

Calciocarbonatites are the most common type, whereas natrocarbonatites are extremely rare, which are only found at Ol Doinyo Lengai Volcano in eastern Africa.

The carbonatite shown above is from the Early Creatceous-aged Chilwa Alkaline Province, an igneous complex consisting of intrusive igneous rocks (alkali granites, syenites, alkaline syenites, nephilinites, and carbonatites) and extrusive igneous rocks (lavas - basanites, phonolites). The complex was intruded in phases throughout the Early Cretaceous, from about 111 to 138 million years ago.

The rock mildly effervesces in acid - it is dominated by the carbonate mineral ankerite, Ca(Fe,Mg,Mn)(CO3)2 - calcium iron magnesium manganese carbonate. There is a thin vein of calcite in this specimen - it readily bubbles in acid.

Geologic unit & age: Chilwa Alkaline Province, late Barremian Stage, mid-Early Creatceous, 126 Ma (= absolute date of Chilwa Island carbonatite)