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

• Published in: Mineralogical Magazine
• Main Authors: CAMARA ARTIGAS, Fernando, Sokolova E, Hawthorne FC
• Other Authors: Cámara F
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2011
• Subjects: rinkite; crystal structure; EMP analysi; transmission electron microscopy; chemical formula; Ti disilicate mineral; Group I; Greenland; kola peninsula
• Online Access: http://hdl.handle.net/2318/92729; https://doi.org/10.1180/minmag.2011.075.6.2755; http://www.minersoc.org/pages/e_journals/minmag.html

Rinkite, ideally Na2Ca4REETi(Si2O7)2OF3, is a common mineral in alkaline andperalkaline rocks. The crystal structures of five rinkite crystals from three alkaline massifs: Ilimaussaq, Greenland; Khibiny, Kola Peninsula, Russia andMont Saint-Hilaire, Canada, have been refinedas two components related by the TWIN matrix (-1 0 0, 0 -1 0, 1 0 1) (Mo-Ka radiation). The crystals, a = 7.4132 - 7.4414, b = 5.6595 - 5.6816, c = 18.8181- 18.9431 Å, b = 101.353 - 101.424(2)º, V = 776.1786.7 Å3, space group P21/c, Z = 2, Dcalc = 3.376 - 3.502 g cm3, were analysedusing 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 the trans edges of a Na polyhedron of the O sheet. The crystal chemistry of rinkite and nacareniobsite-(Ce) is discussed.