From structure topology to chemical composition. XII. Titanium silicates: the crystal chemistry of rinkite, Na2Ca4REETi(Si2O7)2OF3

Abstract

AbstractRinkite, ideallyNa2Ca4REETi(Si2O7)2OF3, is a common mineral in alkaline and peralkaline rocks. The crystal structures of five rinkite crystals from three alkaline massifs: Ilimaussaq, Greenland; Khibiny, Kola Peninsula, Russia and Mont Saint-Hilaire, Canada, have been refined as two components related by the TWIN matrix (–1 0 0, 0 –1 0, 1 0 1)(Mo-αaradiation). The crystals,a =7.4132-7.4414,b =5.6595-5.6816,c =18.8181-18.9431 Å, β = 101.353-101.424(2)°,V=776.1-786.7 Å3, space groupP21lc, Z =2,Dcalc= 3.376—3.502 g cm–3, were analysed using an electron microprobe subsequent to collection of the X-ray intensity data. Transmission electron microscopy confirmed the presence of pseudomerohedral twinning in rinkite crystals. The crystal structure of rinkite is a framework of TS (titanium silicate) blocks. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block in rinkite exhibits linkage and stereochemistry typical for Group I (Ti = 1 a.p.f.u.) of Ti disilicate minerals: two H sheets connect to the O sheet such that two (Si2O7) groups link to thetransedges of a Na polyhedron of the O sheet. The crystal chemistry of rinkite and nacareniobsite-(Ce) is discussed.