A preliminary report on the rhombohedral structure of the ordered yttrofluorite mineral, tveitite (possible formulae: Ca~a+sY6_~F44_ ~, Ca~4YsF4a or Ca~aY 6-F420), is given. In this structure discrete M6Xa7 groups occur, which consist of an octahedral arrangement of cation-centred square antiprisms sharing corners around the threefold axis to enclose a filled cuboctahedron of anions. The same group is found in other known fluorite-related superlattices, which suggests that it is a basic structural unit. Its formal derivation from the analogous element of fluorite is described.Most naturally-occurring yttrofluorite is a disordered, fluorite-type solid solution containing <20% (Y,RE)F 3. Recently, however, Bergstal, Jensen & Neumann (1977) reported an ordered yttrofluorite from Norway (tveitite) with about 30% (Y,RE)F 3, and an analogous mineral has been found independently by Crook (1978), occurring as small inclusions in normal yttrofluorite from the Barringer Hill district of Texas, USA.A small sample of the Norwegian tveitite was kindly made available to us, and further studies by powder X-ray and single-crystal electron diffraction have revealed its unit-cell geometry and place as the member n = 19 of the homologous series of fluorite-related compounds, MnX2~+ 5, in the system CaF2-YF 3 (Gettmann & Greis, 1978). A structure determination on tveitite has now been carried out. The full details will be reported subsequently, but we present here the result and, emerging from it, a new structural principle which explains the existence of the series * To whom correspondence should be addressed.0567-7394/80/060889-03 $01.00The tveitite unit cell is rhombohedral, with hexagonal parameters a = 16.9620 (9), c = 9.6664 (8) A, Z = 57, and the space group is either R3 or R3. The cations are displaced very little from their sites in the ideal fluorite structure, as is the case for most known fluorite-related superlattices, but the anion sublattice has undergone a major rearrangement with respect to that of fluorite. In order to describe this, and to emphasize the structural relationship between tveitite and its fluorite-type parent structure, we dissect from the latter and focus attention on an element which is an octahedron of six cations grouped around a threefold axis. In fluorite, each cation is coordinated by a cube of eight anions, and these MX s cubes share edges, so the relevant group, shown in Fig. l(a), consists of six MX s cubes sharing edges to generate an empty X s cube in its centre. Such groups then further edge-share to fill space: this description is of course not new. In the tveitite structure, modified units, based on the same grouping of six cations around the now unique threefold axis, appear as discrete entities, and the modification is simply that the cation (yttrium) coordination polyhedra have transformed to square antiprisms which, by incorporation of additional anions, now share corners, as shown in Fig. l(b), to enclose a cuboctahedron of anions. The centre of this cuboctahedron, as ...
