The crystal structure of arapovite, U4+(Ca,Na)2(K1-x▭x) [Si8 O20], x≈0.5, a new mineral species of the steacyite group from the Dara-i-Pioz Moraine, Tien-Shan Mountains, Tajikistan

The crystal structure of arapovite, ideally U4+ (Ca,Na)(2) (K(1-x)square [Si-8 O-20], x approximate to 0.5, tetragonal, a 7.5505(4), c 14.7104(9) Angstrom, V 838.6(1) Angstrom(3), space group P4/mcc, Z = 2, D calc. 3.365 g/cm(3), was refined to an R-1 index of 2.9% based on 528 observed [F-0 > 4sigmaF] unique reflections measured with MoKalpha 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 = 1.617 Angstrom. There are two [8]-coordinated sites, A and B, occupied by (U-0.59(4+) Th-0.26 Ca-0.10 REE0.04) and (Ca-1.23 Na-0.68 REE0.08 Ba-0.01), with = 2.403 and = 2.490 Angstrom. There is one [12]-coordinated site, C, partly occupied by K (K-0.52 square(0.48)), with = 3.103 Angstrom. 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) (K(1-x)square(x)) Si-8 O-20 (H2O)(n) and steacyite, Th (Na,Ca)(2) (K(1-x)square(x)) Si-8 O-20, x approximate to 0.5. The topology of silicate minerals with [Si8O20](8-) units, including ekanite, Th Ca-2 Si8O20, and litidionite, Na2K2Cu2Si8O20, is discussed.