IGNEOUS ROCKS/Carbonatites 221 Table Some minerals found in carbonatites Carbonates Phosphates Calcite Dolomite Ankerite Siderite Bastnasite (Ce,La)FCO3 Burbankite Parisite Nyerereite (Na,K)2Ca(CO3)2 Gregoryite (Na,K)2CO3 Apatite Monazite Silicates Amphibole Aegerine augite Diopside Olivine Phlogopite Halides Fluorite Halite Sylvite Oxides Baddeleyite Hematite Ilmenite Magnetite Perovskite Pyrochlore and calcium carbonates, a variety known as natrocarbonatite Other minerals that are commonly found in carbonatites include apatite, amphibole, magnetite, mica, olivine, and pyroxene As an alternative to using a classification scheme based on mineralogy, carbonatites can also be classified on the basis of their chemical compositions (see Rocks and Their Classification) Figure shows one such classification scheme that divides carbonatites into calciocarbonatites, magnesiocarbonatites, and ferrocarbonatites The term silicocarbonatite is sometimes used for those rare carbonatites that contain greater than 20 wt% SiO2 Economic Deposits Sulphides Pyrite Pyrrhotite Galena Sphalerite Figure Classification scheme for carbonatites suggested by the IUGS based on chemical composition Oxides in wt percent Carbonatites are a major source of Nb, phosphates (see Sedimentary Rocks: Phosphates), F, and the REEs Minerals occur in magmatic, hydrothermal, and supergene deposits A list of some of the more important deposits is given in Table Carbonatites still constitute the world’s greatest source of Nb Pyrochlore, the most important Nb-bearing mineral, is principally mined in Brazil (e.g., Araxa´ , Tapira) and some is mined in Canada (St Honore´ ) However, not all carbonatites contain pyrochlore Because carbonate minerals are easily dissolved during chemical weathering, the more resistant minerals, such as pyrochlore and monazite, can be concentrated and easily mined as residual material Rare earth elements in carbonatites are common and can be found as magmatic minerals or in minerals directly precipitated from hydrothermal solutions Among such minerals are included monazite, bastnaesite, britholite, burbankite, parisite, and synchesite Although most of the world’s phosphate production comes from marine phosphorites, the presence of primary apatite in all carbonatites makes them a potential source for fertilisers More money is generated from this commodity than any other Table Some examples of mineral deposits associated with carbonatites Complex Commodity Details Araxa´, Brazil St Honore´, Canada Jacupiranga, Brazil Sukulu, Uganda Mountain Pass, USA Gakara Karonge, Burundi Tapira, Brazil Powderhorn, USA Phalaborwa, South Africa Ipanema, Brazil Amba Dongar, India Okurusu, Namibia Nb Nb Phosphate Phosphate REE REE Ti Ti Cu ỵ Vermiculite Vermiculite Fluorite Fluorite Pyrochlore in laterite Pyrochlore in carbonatite Primary apatite in carbonatite Residual apatite in weathered carbonatite Bastnaesite in carbonatite Hydrothermal veins of bastnaesite Anatase in laterite Perovskite in pyroxenite Vermiculite in pyroxene pegmatoid Vermiculite in glimmerite Hydrothermal overprint on fenite Hydrothermal overprint on fenite