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

Abstract

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) A, V 838.6(1) A3, space group P 4 /mcc , Z = 2, D calc. 3.365 g/cm3, was refined to an R 1 index of 2.9% based on 528 observed [ F o > 4σ F ] unique reflections measured with Mo K α radiation on a Bruker P 4 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 A. 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 = 2.403 and = 2.490 A. There is one [12]-coordinated site, C , partly occupied by K (K0.52 □0.48), with = 3.103 A. 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.