- journal_title：The Canadian Mineralogist
- Contributor：Yulia A. Uvarova ; Elena Sokolova ; Frank C. Hawthorne ; Atali A. Agakhanov ; Leonid A. Pautov
- Publisher：Mineralogical Association of Canada
- Date：2004-
- Format：text/html
- Language：en
- Identifier：10.2113/gscanmin.42.4.1005
- journal_abbrev：Can Mineral
- issn：0008-4476
- volume：42
- issue：4
- firstpage：1005
- section：Articles
The crystal structure of arapovite, ideally U4+ (Ca,Na)2 (K1–x□x) [Si8 O20], x ≈ 0.5, tetragonal, a 7.5505(4), c 14.7104(9) Å, V 838.6(1) Å3, space group P4/mcc, Z = 2, D calc. 3.365 g/cm3, was refined to an R1 index of 2.9% based on 528 observed [Fo > 4σF] unique reflections measured with MoKα radiation on a Bruker P4 diffractometer with a 1K CCD area detector. In the crystal structure of arapovite, there is one tetrahedrally coordinated site solely occupied by Si, with <Si–O> = 1.617 Å. There are two [8]-coordinated sites, A and B, occupied by (U0.594+ Th0.26 Ca0.10 REE0.04) and (Ca1.23 Na0.68 REE0.08 Ba0.01), with <A–O> = 2.403 and <B–O> = 2.490 Å. There is one [12]-coordinated site, C, partly occupied by K (K0.52 □0.48), with <C–O> = 3.103 Å. In the crystal structure of arapovite, (SiO4) tetrahedra form a double four-membered ring: [Si8O20]. Eight-coordinated A and B polyhedra share common edges to form a (001) sheet. The sheets are connected through [Si8O20] groups to form a framework; C atoms are situated in large cages within the framework. The topology of the arapovite structure is identical to that of turkestanite, Th (Ca,Na)2(K1–x□x) Si8 O20 (H2O)n, and steacyite, Th (Na,Ca)2 (K1–x□x) Si8 O20, x ≈ 0.5. The topology of silicate minerals with [Si8O20]8–units, including ekanite, Th Ca2 Si8O20, and litidionite, Na2 K2 Cu2 Si8O20, is discussed.