IngentaConnect Compositional variation in minerals of the chevkinite group

Authors: R. MacDonald; H.E. Belkin

Source: Mineralogical Magazine, Volume 66, Number 6, 1 December 2002 , pp. 1075-1098(24)

Abstract:

The composition of chevkinite and perrierite, the most common members of the chevkinite group, is closely expressed by the formula A₄BC₂D₂Si₄O₂₂, where A = (La,Ce,Ca,Sr,Th), B = Fe²⁺, C = (Fe²⁺,Fe³⁺,Ti,Al,Zr,Nb) and D = Ti. The A site is dominated by a strong negative correlation between (Ca+Sr) and the REE. Chondrite-normalized REE patterns are very variable, e.g. in LREE/HREE and Eu/Eu*. The C site 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

(Ca+Sr)^A + Ti_C = REE_A + M_C^3+,2+

defines a linear array with r² = 0.91. Chevkinite and perrierite are shown to be compositionally distinct on the basis of CaO, FeO*, Al²O³ and Ce₂O₃ abundances. 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.