The crystal structure of ilímaussite-(Ce), (Ba,Na)10K3Na4.5Ce5(Nb,Ti)6[Si12O36][Si9O18(O,OH)24]O6, and the "ilímaussite" problem

Abstract

The crystal structure of ilimaussite-(Ce), (Ba,Na)(10)K3Na4.5Ce5(Nb,Ti)(6)[Si12O36][Si9O18(O,OH)(24)]O-6, from the type locality, the Ilimaussaq 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) Angstrom; space group R32, Z = 3; MoKalpha 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 ilimaussite-(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 "ilimaussite" type with compositions in the range 21 less than or equal to Si less than or equal to 24 apfu, with polytypism related to the disorder in the A' layer.