Compositional variation in minerals of the chevkinite group

The composition of chevkinite and perrierite, the most common members of the chevkinite group, is closely expressed by the formulaA4BC2D2Si4O22, whereA= (La,Ce,Ca,Sr,Th),B= Fe2+,C= (Fe2+,Fe3+,Ti,Al,Zr,Nb) andD= Ti. TheAsite is dominated by a strong negative correlation between (Ca+Sr) and theREE. Chondrite-normalizedREEpatterns are very variable, e.g. inLREE/HREEand Eu/Eu*. TheCsite is dominated by Ti, Al and Fe2+, in very variable proportions. Most chevkinites and perrierites are close to stoichiometric, with cation sums between 12.9 and 13.5, compared to the theoretical 13. There is no single, generally applicable charge balancing substitution scheme in the group; however, the general relationship defines a linear array with r2= 0.91. Chevkinite and perrierite are shown to be compositionally distinct on the basis of CaO, FeO*, Al2O3and Ce2O3abundances. Chevkinite forms mainly in chemically evolved parageneses, such as syenites, rhyolites and fenites associated with carbonatite complexes. Perrierite is more commonly recorded from igneous rocks of mafic to intermediate composition. The compositional characteristics and possible structural formulae of other members of the chevkinite group are reviewed briefly.