New mineral data from the kamafugite-carbonatite association: the melilitolite from Pian di Celle, Italy

A detailed mineralogical investigation of a Plan di Celle sill rock (San Venanzo, Italy), classified as melilitolite and associated with venanzite and carbonatitic pyroclasts, revealed new and rare mineral parageneses, considered as characteristic of the kamafugite-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 (H2O, F CO2)-rich, Si-undersaturated 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.