New mineral data from the kamafugitecarbonatite association: The melilitolite from Pian di Celle, Italy

A detailed mineralogical investigation of a Pian di Celle sill rock (San Venanzo, Italy), classified asmelilitolite and associated withvenanzite and carbonatitic pyroclasts, revealed new and rare mineral parageneses, considered as characteristic of thekamafugite-carbonatite association. These are formed by several accessory minerals, including minerals of the cuspidine family, gotzenite, khibinskite, minerals of the rhodesite- delhayelite- macdonaldite family, pyrrhotite, bartonite and (Fe, Ni, Co) monoarsenide, mostly optically and chemically identified also in fluid inclusions. The chemical composition of these minerals and their probable crystallisation succession, deduced from textural relationships, demonstrates extensive atomic substitutions, notably for Ca, Ti, Mg and alkali, essentially reflecting high concentrations of REE, Sr, Ba, Nb and Zr, which significantly varied during crystallisation. Molecular alkali excess over Al and high Ca content in (H sub(2)O, F, CO sub(2))-rich, Siundersaturated liquid(s) are considered the dominant factors in controlling the stability of disilicate-type minerals. Separation of the carbonatite liquid from the silicate magma, constrained by textural and fluid inclusion data, was fundamental in moving the residuum onto a strongly peralkaline trend which stabilised the sulphides under changed redox conditions.Original Abstract: Eine eingehende mineralogische Untersuchung eines Lagerganges von Pian di Celle, der als Melilitolit klassifiziert and mit Venanzit and karbonatitischen Pyroklasten assoziiert ist, ergab neue and seltene Mineral-Paragenesen, die als charakteristisch fuer die Kamafugit-Karbonatit-Assoziation gelten. Diese bestehen aus verschiedenen akzessorischen Mineralien, darunter Perovskit, Cuspidin, Gotzenit, Khibinskit, Delhayelit, Macdonaldit, Bardonit and (Fe, Ni, Co) Monoarsenit; diese werden in Fluessigkeitseinschluessen mit optischen and chemischen Methoden identifiziert. Die chemische Zusammensetzung dieser Minerale and ihre wahrscheinliche Kristallisationsabfolge, aus texturellen Beziehungen abgeleitet, zeigt extensive Substitutionen, vor allem fuer Ca, Ti, Mg and Alkelien, die im wesentlichen hohe Gehalte an SEE, Sr, Ba, Mb and Zr andeuten, die waehrend der Kristallisation betraechtlichen Schwankungen unterlagen. Molekularer Alkali ueberschus ueber Al in (H sub(2)O, F, CO sub(2))-reichen Si-untersaettigten Fluiden werden als wichtigste Faktoren fuer die Stabilitaet von Mineralen des DisilikatTyps g