Compositional variation in minerals of the chevkinite group

<jats:title>Abstract</jats:title><jats:p>The composition of chevkinite and perrierite, the most common members of the chevkinite group, is closely expressed by the formula<jats:italic>A</jats:italic><jats:sub>4</jats:sub><jats:italic>B</jats:italic><jats:italic>C</jats:italic><jats:sub>2</jats:sub><jats:italic>D</jats:italic><jats:sub>2</jats:sub>Si<jats:sub>4</jats:sub>O<jats:sub>22</jats:sub>, where<jats:italic>A</jats:italic>= (La,Ce,Ca,Sr,Th),<jats:italic>B</jats:italic>= Fe<jats:sup>2+</jats:sup>,<jats:italic>C</jats:italic>= (Fe<jats:sup>2+</jats:sup>,Fe<jats:sup>3+</jats:sup>,Ti,Al,Zr,Nb) and<jats:italic>D</jats:italic>= Ti. The<jats:italic>A</jats:italic>site is dominated by a strong negative correlation between (Ca+Sr) and the<jats:italic>REE</jats:italic>. Chondrite-normalized<jats:italic>REE</jats:italic>patterns are very variable, e.g. in<jats:italic>LREE/HREE</jats:italic>and Eu/Eu*. The<jats:italic>C</jats:italic>site is dominated by Ti, Al and Fe<jats:sup>2+</jats:sup>, 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</jats:p><jats:p><jats:disp-formula><jats:graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X0004531X_eqn1.png" /></jats:disp-formula></jats:p><jats:p>defines a linear array with r<jats:sup>2</jats:sup>= 0.91. Chevkinite and perrierite are shown to be compositionally distinct on the basis of CaO, FeO*, Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>and Ce<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>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.</jats:p>