The crystal chemistry of malinkoite, nabsio(4), and lisitsynite, KBSi2O6, from the Khibina-Lovozero complex, Kola Peninsula, Russia

The crystal structure of malinkoite, NaBSiO4, a 13.8964(4) Angstrom, c 7.70 01(2) Angstrom. P6(3), Z= 18, D-x = 2.922 g cm(-3). from the Khibina-Lovoze ro complex, Kola peninsula. Russia, has been solved by direct methods and r efined to an R index of 3.8% using 2094 unique observed (\F-o\ > 4 sigmaF) reflections collected with a single-crystal diffractometer fitted with a CC D detector and MoK alpha X-radiation. Malinkoite is a framework borosilicat e with a structure consisting of six-membered rings of regularly alternatin g (SiO4) and (BO4) tetrahedra, and Na located in the large channels extendi ng along [001]. There are three unique Si sires, each occupied by Si and ea ch coordinated by four O atoms in a tetrahedral arrangement, with < Si-O> d istances of 1.625, 1.622 and 1.622 Angstrom, respectively. There are three unique B sites, each occupied by B and each coordinated by four O atoms in a tetrahedral arrangement, with <B-O> distances of 1.476, 1.476 and 1.473 A ngstrom, respectively. There are five unique Na sires, each completely occu pied by Na. Three of the Na sites are [9]-coordinated- with < Na-O > distan ces of 2.626, 2.642 and 2.630 Angstrom, respectively; the remaining two Na sites are [6]-coordinated, with <Na-O> distances of 2.444 and 2.440 Angstro m, respectively. The (SiO4) and (BO4) tetrahedra link to form six-membered rings of two different configurations based on the following sequences of u p (U and down (D) linkages: UDUDUD and UUDUDD in the ratio of 1:2. Nine-coo rdinated Na polyhedra are located in big cages formed by stacking of UDUDUD rings. Stacking of UUDUDD rings produces smaller cages occupied by [6]-coo rdinated Na. In accord with the ratio of two types of rings, there are twel ve Na octrahedra and six [9]-coordinated Na polyhedra per unit cell in mali nkoite. The framework topology is related to that of beryllonite NaBePO4. In the crystal structure of lisitsynite, KBSi2O6, four-membered rings consi st of two (SiO4) and two (BO4) tetrahedra and are linked into zig-zag chain s (puckered ladders) along [001]. There are two types of chemically distinc t six- and eight-membered rings: (4Si + 2B) and (5Si + B): (5Si + 3B) and ( 6Si + 2B). Two different pairs of six-membered and eight-membered rings. (4 Si + 2B) and (5Si + 3B), and (5Si + Bland (6Si + 2B), respectively, Form sh eets on (100) and (010). Stacking of sheets and zig-zag chains gives rise t o a three-dimensional framework. There are one-dimensional seven-ring chann els along the c direction, Lisitsynite, KBSi2O6, and synthetic "boroleucite ", K(Si2B)O-6, may be regarded as polymorphs with different patterns of SI- B order.