WHAT IS FERROCARBONATITE - A REVISED CLASSIFICATION

The term ''ferrocarbonatite'' has been in use for about twenty years b ut is not adequately defined. The IUGS system of igneous rock nomencla ture defines it mineralogically as a carbonatite in which ''the main c arbonate mineral is iron rich'' and chemically as a carbonatite in whi ch (in weight percent) CaO:CaO + MgO + FeO + Fe2O3 + MnO < 0.8, and Mg O < FeO + Fe2O3 + MnO. However, most rocks that have been called ferro carbonatites do not have a high proportion of an Fe-rich carbonate min eral and it is clear from their chemical analyses that most are calcit e-hematite rocks or magnesian calcite-hematite rocks. Because the IUGS system treats FeO, Fe2O3 and MnO as a single component it is unable t o distinguish between calcite or dolomite carbonatites that contain he matite or magnetite, and carbonatites that are composed largely of Fe- rich ankerite, or of calcite and siderite. A modified chemical classif ication is proposed using molar rather than weight proportions in whic h the ferrocarbonatite field of the IUGS system is divided into two pa rts so as to recognise a group of rocks to be known as ferruginous cal ciocarbonatites, and to restrict the term ferrocarbonatite to much mor e Fe-rich rocks. The revised classification is as follows: calciocarbo natite: CCMF>0.75, magnesiocarbonatite: CCMF<0.75; MgO/FeO.>1.0, ferru ginous calciocarbonatite: 0.5<CCMF<0.75;MgO/FeO.<1.0, ferrocarbonatite : CCMF<0.5; MgO/FeO.<1.0 where CCMF is the molar ratio CaO/(CaO + MgO + FeO. + MnO) and FeO . refers to molar FeO if FeO and (FeO3)-O-2 are both determined and total Fe as FeO if not. It is proposed that the te rm ferrocarbonatite only be used in this modified chemical sense: carb onatites in which the main carbonatite is I-e rich are adequately desc ribed using the modal mineralogy. (C) 1997 Elsevier Science Limited.