ISSN 1063-7745, Crystallography Reports, 2007, Vol. 52, No. 1, pp. 71–79. © Pleiades Publishing, Inc., 2007. Original Russian Text © O.V. Yakubovich, W. Massa, I.V. Pekov, P.G. Gavrilenko, 2007, published in Kristallografiya, 2007, Vol. 52, No. 1, pp. 73–81. 71 INTRODUCTION Tveitite-(Y) is a very rare mineral. It is a cation- ordered calcium yttrium lanthanide fluoride that is structurally related to fluorite. This mineral was origi- nally found in an amazonite granite pegmatite at Höy- dalen (Tördal, Telemark, Southern Norway). The thin polysynthetic twinning has complicated the acquisition of the structural and crystallographic data for this min- eral. In the first paper [1] concerned with the structural investigation of tveitite-(Y), Burgsetöl et al. [1] reported data only on the pseudocell (monoclinic, a' = 3.924 Å, b' = 3.893 Å, c' = 5.525 Å, β' = 90.26°) and proposed the simplified formula Ca 1 – x (Y,Ln) x F 2 + x (where Ln is a lanthanide and x ≈ 0.3). In [1], it was assumed that tveitite-(Y) is the product of a slow solid- phase transformation of highly yttrium-enriched fluo- rite, which is metastable at low temperature. The min- eral tveitite-(Y) was also found at Barringer Hill (Texas, United States) [2]; however, analytical data for tveitite-(Y) from this region are not available in the lit- erature. The crystal structure of tveitite-(Y) was ini- tially studied using a sample from Norway and inter- preted as a superstructure with respect to the fluorite structure type. Bevan et al. [3] showed that the structure of this mineral is described with space group (a = 16.692 Å, c = 9.666 Å) and proposed the idealized for- mula Ca 14 Y 5 F 43 . According to [3], the ordered distribu- tion of yttrium and calcium atoms over structural posi- R 3 tions is responsible for the rhombohedral distortion of yttrium-bearing fluorite and leads to the formation of the structure of tveitite-(Y). The large discrepancy fac- tor (R = 0.125) obtained in the refinement of the “ideal” structural model (without regard for the distribution of lanthanide atoms over structural positions) in the iso- tropic approximation and the ambiguity of the formula proposed for the mineral in [3] have given impetus to our refinement of the crystal structure of tveitite-(Y). It should also be noted that no analogue of the mineral tveitite-(Y) is known among numerous synthetic com- pounds in the CaF 2 –YF 3 –LnF 3 system. In the present work, we investigated the crystal structure of tveitite-(Y) from granite pegmatites of the rare-metal amazonite deposit of Rov-Gora Mountain (West Keivy, Kola Peninsula, Russia). This finding of tveitite-(Y) is likely to be the third known occurrence in the world. The untwinned structure of the tveitite-(Y) individuals makes it possible to perform a high-quality single-crystal X-ray experiment and to investigate the subtle crystal-chemical features of the mineral. DISCOVERY, GENERAL MINERALOGICAL CHARACTERISTICS, AND CHEMICAL COMPOSITION Tveitite-(Y) in Rov-Gora Mountain was found by one of the authors in a large vein pegmatite body of the STRUCTURE OF INORGANIC COMPOUNDS Dedicated to the 50th Anniversary of the Journal Crystal Structure of Tveitite-(Y): Fractionation of Rare-Earth Elements between Positions and the Variety of Defects O. V. Yakubovich a , W. Massa b , I. V. Pekov a , and P. G. Gavrilenko b a Moscow State University, Leninskie gory, Moscow, 119992 Russia e-mail: yakubol@geol.msu.ru b Philipps-Universität Marburg, Biegenstrasse 10, Marburg, D-35032 Germany Received February 2, 2006 Abstract—The crystal structure of the mineral tveitite-(Y) (Y 0.883 Na 0.106 ) · (Ca 0.841 LREE 0.159 )(Ca 0.716 Na 0.204 HREE 0.080 )(Ca 0.092 Na 0.074 )F 6.952 from amazonite pegmatites of Rov-Gora Mountain (Keivy, Kola Peninsula) is determined using X-ray diffraction (Stoe IPDS diffractometer, λMoK α , graphite monochromator, 2θ max = 63.5°, R = 0.051 for 1542 reflections). The main crystal data are as follows: a = 17.020(2) Å, c = 9.679(2) Å, V = 2428.2(4) Å 3 , space group , Z = 18, and ρ calcd = 4.00 g/cm 3 . The ide- alized structural formula of the mineral is represented as (Y,Na) 6 (Ca,LREE) 6 (Ca,Na,HREE) 6 (Ca,Na)F 42 (Z = 3). The defect structure of the mineral manifests itself in a mixed occupation of all four independent cation posi- tions and in a randomly disordered distribution of fluorine atoms over the majority of anion positions. It is shown that the crystal structure of tveitite-(Y) fulfills the function of an “Eratosthenes sieve” for yttrium cations and two groups of lanthanide cations, so that these cations are distributed over three different positions. PACS numbers: 61.66.Fn, 61.72.-yIi DOI: 10.1134/S1063774507010087 R3