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THE CRYSTAL STRUCTURE OF ILÍMAUSSITE-(Ce), (Ba,Na)₁₀K₃Na_4.5Ce₅(Nb,Ti)₆[Si₁₂O₃₆][Si₉O₁₈(O,OH)₂₄]O₆, AND THE "ILÍMAUSSITE" PROBLEM

Giovanni Ferraris^1,, Angela Gula¹, Natalia V. Zubkova², Dmitry Yu. Pushcharovsky², Elena R. Gobetchiya², Igor V. Pekov² and Knut Eldjarn³

¹ Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, and Istituto di Geoscienze e Georisorse, CNR, Via Valperga Caluso, 35, I-10125, Torino, Italy
² Faculty of Geology, Moscow State University, Vorobievy Gory, RU-119899 Moscow, Russia
³ SERO AS, Stasjonsveien 44, N-1375 Billingstad, Norway

E-mail address: giovanni.ferraris{at}unito.it

The crystal structure of ilímaussite-(Ce), (Ba,Na)₁₀K₃Na_4.5Ce₅(Nb,Ti)₆[Si₁₂O₃₆][Si₉O₁₈(O,OH)₂₄]O₆, from the type locality, the Ilímaussaq alkaline complex, South Greenland, has been solved and refined to R = 0.11 for 1663 observed reflections collected from a very poor crystal [a 10.770(3), c 61.05(2) Å; space group R32, Z = 3; MoK radiation; Nonius Kappa CCD X-ray diffractometer]. The structure consists of three pairs of (001) silicate sheets. These sheets sandwich: CeO₆ trigonal prisms and Na cations (A layer), NbO₆ octahedra, Ba, and K (O layer), and CeO₆ trigonal prisms (A’ layer). The layers are stacked according to the sequence AOA’O; the tetrahedral sites in A’ are only 50% occupied by Si, with consequent disorder of the basal oxygen atoms. Overall, the ratio of (Si + Al) to other cations is about 0.8. In the similar structure of a Ti-dominant analogue of ilímaussite-(Ce), to be compared with diversilite-(Ce), the same sites have been reported as being one-third occupied; in this case, the ratio of (Si + Al) to other cations is about 1.2. New chemical data are reported for diversilite-(Ce), in which the ratio of (Si + Al) to other cations is about 0.9. There is possibly a family of phases of "ilímaussite" type with compositions in the range 21 Si 24 apfu, with polytypism related to the disorder in the A’ layer.

Keywords: ilímaussite-(Ce), crystal structure, diversilite-(Ce), polytypism, Ilímaussaq complex, Greenland.