THE CRYSTAL STRUCTURE OF MOSKVINITE-(Y), Na2 K (Y,REE) [Si6O15], A NEW SILICATE MINERAL WITH [Si6O15] THREE-MEMBERED DOUBLE RINGS FROM THE DARA-I-PIOZ MORAINE, TIEN-SHAN MOUNTAINS, TAJIKISTAN

Abstract

The crystal structure of moskvinite-(Y), ideally Na 2 K (Y,REE) [Si 6 O 15 ], from the moraine of the Dara-i-Pioz glacier, the Alai mountain range, Tien-Shan mountains, northern Tajikistan, a 10.623(2), b 14.970(2), c 8.553(1) Å, V 1359.8(4) Å 3 , space group Ibmm, Z = 4, D (calc.) = 2.850 g.cm -3 , has been solved by direct methods and refined to an R 1 index of 1. 7% based on 936 observed [| F o | > 4F] unique reflections collected with single-crystal diffractometer with MoK␣ X-radiation. Electron-microprobe analysis gave the composition Na 2.06 K 0.95 (Y 0.77 Dy 0.09 Gd 0.04 Er 0.04 Ho 0.02 Sm 0.02 Nd 0.01 Tb 0.01 ) ⌺1.00 Si 6 O 15 based on 15 O apfu. In the crystal structure of Na 2 K (Y,REE) [Si 6 O 15 ], there are two tetrahedrally coordinated T sites occupied by Si with <Si-O> = 1.623 Å. The (SiO 4 ) tetrahedra form three-membered rings, [Si 3 O 9 ], that link by sharing apical vertices to form a doublering of the form [Si 6 O 15 ]. There are two [6]-coordinated sites, one occupied by (Y 0.77 + REE 0.23 ) with <(Y,REE)-O> = 2.255 Å, and the other occupied by Na with <Na-O> = 2.438 Å. There is a [10]-coordinated site occupied by K with <K-O> = 3.062 Å. The {(Y,REE)O 6 } and (NaO 6 ) octahedra share common edges to form sheets parallel to (010); these sheets connect through [Si 6 O 15 ] double-rings to form a framework. Channels extend along [001] and contain K. The topology of the Na 2 K Y [Si 6 O 15 ] framework is identical to that of synthetic Na 3 Y [Si 6 O 15 ], but the arrangement of the channel cations is different. The crystal structures of double-ring silicates and [Si 6 O 18 ]-chain structures are discussed. Keywords: moskvinite-(Y), Y-silicate, crystal structure, [Si 6 O 15 ] double ring, Dara-i-Pioz, Tajikistan. SOMMAIRENous avons résolu la structure cristalline de la moskvinite-(Y), dont la composition idéale est Na 2 K (Y,REE) [Si 6 O 15 ], provenant d'une moraine du glacier Dara-i-Pioz, dans la chaîne Alai des montagnes Tien-Shan, dans le nord du Tajikistan, a 10.623(2), b 14.970(2), c 8.553(1) Å, V 1359.8(4) Å 3 , groupe spatial Ibmm, Z = 4, D (calc.) = 2.850 g.cm -3 , par méthodes directes; la structure a été affinée jusqu'à un résidu R 1 de 1.7% en utilisant 936 réflexions uniques observées [| F o | > 4F], prélevées avec un diffractomètre pour monocristal et un rayonnement MoK␣. Une analyse à la microsonde électronique a donné Na 2.06 K 0.95 (Y 0.77 Dy 0.09 Gd 0.04 Er 0.04 Ho 0.02 Sm 0.02 Nd 0.01 Tb 0.01 ) ⌺1.00 Si 6 O 15 sur une base de 15 atomes d'oxygène par unité formulaire. La structure de Na 2 K (Y,REE) [Si 6 O 15 ] contient deux sites T à coordinence tétraédrique qu'occupe le Si, avec <Si-O> = 1.623 Å. Les tétraèdres (SiO 4 ) sont agencés en anneaux à trois membres, [Si 3 O 9 ], liés par partage de vertex apical pour former un anneau double [Si 6 O 15 ]. Il y a deux sites à coordinence [6]; l'un d'eux contient (Y 0.77 + REE 0.23 ), avec <(Y,REE)-O> = 2.255 Å, et l'autre contient le Na, avec <Na-O> = 2.438 Å. Il y a de plus un site à coordinence [10] qu'occupe le K, avec <K-O> = 3.06...

Condensation of [Si3O9]6- Anions in the Solid State to the Dimeric Cyclotrisilicate Anion [Si6O17]10-

“…The existence of such a dimeric anion (“double-ring”) has been predicted by Liebau, who introduced the general formula {uB, 2r}[ Pr Si 2 p O 6 p - l ] (4 p -2 l )- with p = Pr ≥ 3 and 1 ≤ l ≤ p for unbranched (uB) double-ring (2r) silicates, , and the anion [Si 6 O 17 ] 10- is the first representative of this class of silicates with p = 3 and l = 1. So far, only double-ring silicates of the formula {uB, 2r}[ Pr Si 2 p O 5 p ] 2 p - have been described; an example of which with p = 3 is found in the silicate Na 3 KY[Si 6 O 15 ] (Figure c). In contrast to the situation in [Si 6 O 17 ] 10- where only one out of three [SiO 4 ] tetrahedra within the ring is connected to the second ring, in [Si 6 O 15 ] 6- , all tetrahedra are interconnected, thereby forming a sandwichlike structure.…”

Sazhinite-(La), Na3LaSi6O15(H2O)2, a new mineral from the Aris phonolite, Namibia: Description and crystal structure

“…The depletion of these elements in sazhinite-(La) from Aris is caused by competition from the concomitant growth of other minerals such as eudialyte and korobitsynite, which deplete the fluid in small-radius cations (<0.8 Å ). Haile and Wuensch (1997) Sokolova et al, 2003], and a framework silicate (K 2 Ce[Si 6 O 15 ]; Karpov et al, 1976). Haile et al (1995) proposed that the topology of the silicate units in [Si 6 O 15 ] compounds is controlled by the electronegativity of the cation at the M site rather than by difference in radius.…”

First occurrence of moskvinite-(Y) in the Ilímaussaq alkaline complex, South Greenland – implications for rare-earth element mobility

“…Moskvinite-(Y), ideally Na 2 K(Y,REE)Si 6 O 15 , was first described from the Dara-i-Pioz moraine in Tajikistan (Agakhanov et al, 2003) and Sokolova et al, (2003) described the structure in detail. Until now, Dara-i-Pioz is the only locality where moskvinite-(Y) has been observed.…”