THE CRYSTAL CHEMISTRY OF MALINKOITE, NaBSiO4, AND LISITSYNITE, KBSi2O6, FROM THE KHIBINA LOVOZERO COMPLEX, KOLA PENINSULA, RUSSIA

Abstract

The crystal structure of malinkoite, NaBSiO 4 , a 13.8964(4) Å, c 7.7001(2) Å, P6 3 , Z = 18, D x = 2.922 g cm -3 , from the Khibina-Lovozero complex, Kola peninsula, Russia, has been solved by direct methods and refined to an R index of 3.8% using 2094 unique observed (|F o | > 4F) reflections collected with a single-crystal diffractometer fitted with a CCD detector and MoK␣ X-radiation. Malinkoite is a framework borosilicate with a structure consisting of six-membered rings of regularly alternating (SiO 4 ) and (BO 4 ) tetrahedra, and Na located in the large channels extending along [001]. There are three unique Si sites, each occupied by Si and each coordinated by four O atoms in a tetrahedral arrangement, with <Si-O> distances of 1.625, 1.622 and 1.622 Å, 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 Å, respectively. There are five unique Na sites, each completely occupied by Na. Three of the Na sites are [9]-coordinated, with <Na-O> distances of 2.626, 2.642 and 2.630 Å, respectively; the remaining two Na sites are [6]-coordinated, with <Na-O> distances of 2.444 and 2.440 Å, respectively. The (SiO 4 ) and (BO 4 ) tetrahedra link to form six-membered rings of two different configurations based on the following sequences of up (U) and down (D) linkages: UDUDUD and UUDUDD in the ratio of 1:2. Nine-coordinated Na polyhedra are located in big cages formed by stacking of UDUDUD rings. Stacking of UUDUDD rings produces smaller cages occupied by [6]-coordinated Na. In accord with the ratio of two types of rings, there are twelve Na octahedra and six [9]-coordinated Na polyhedra per unit cell in malinkoite. The framework topology is related to that of beryllonite NaBePO 4 .In the crystal structure of lisitsynite, KBSi 2 O 6 , four-membered rings consist of two (SiO 4 ) and two (BO 4 ) tetrahedra, and are linked into zig-zag chains (puckered ladders) along [001]. There are two types of chemically distinct 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, (4Si + 2B) and (5Si + 3B), and (5Si + B) and (6Si + 2B), respectively, form sheets on (100) and (010). Stacking of sheets and zig-zag chains gives rise to a three-dimensional framework. There are one-dimensional seven-ring channels along the c direction. Lisitsynite, KBSi 2 O 6 , and synthetic "boroleucite", K(Si 2 B)O 6 , may be regarded as polymorphs with different patterns of Si-B order. SOMMAIRE Nous avons résolu la structure cristalline de la malinkoïte, NaBSiO 4 , a 13.8964(4), c 7.7001 (2) Å, P6 3 , Z = 18, D x = 2.922 g cm -3 , provenant du complexe de Khibina-Lovozero, péninsule de Kola, en Russie, par méthodes directes, et l'avons affiné jusqu'à un résidu R de 3.8% en utilisant 2094 réflexions uniques observées (|F o | > 4F), prélevées avec un diffractomètre pour cristaux uniques muni d'un détecteur CCD et avec rayonnement MoK␣. Il s'...

RIETVELD REFINEMENT OF SYNTHETIC MONOCLINIC NaBSiO4

“…The atom coordinates are listed in Table 2. The crystal structure of synthetic monoclinic NaBSiO 4 is closely related to that of the hexagonal mineral malinkoite from the Kola Peninsula, Russia, which has the same chemical composition (Sokolova et al 2001). There is, however, no group-subgroup relationship between the polymorphs (P2 1 /n and P6 3, respectively).…”

“…The structure refinement was started with the atom coordinates of beryllonite, NaBePO 4 , according to Golovastikov (1962), but replacing the atomic form factors of B for those of Be, and those of Si for those of P. Constraints were set so that the Si-O distances remained close to 1.620(1) Å, and the B-O distances, close to 1.470(1) Å, as reported for malinkoite (Sokolova et al 2001). The O-T-O tetrahedron angles were constrained to remain close to 109.5(2)°.…”

“…Furthermore, tridymite, nepheline and kalsilite exclusively contain six-membered rings of tetrahedra whose apices point in alternate direction, up (U) and down (D), when viewed along the hexagonal or pseudohexagonal axis. In both monoclinic NaBSiO 4 (with the beryllonite structure) and malinkoite (Sokolova et al 2001), only one third of the rings is of this UDUDUD type, whereas two thirds of the rings are of the UUDDUD type, with two pairs of adjacent tetrahedra per ring pointing in the same direction (Figs. 2a, b, 3).…”

“…The main difference in the frameworks of tetrahedra in the two phases lies in the way in which the two different rings are linked (cf. Sokolova et al 2001). The different linkages of these rings exclude the possibility of a simple displacive transformation between monoclinic and hexagonal NaBSiO 4 ; a phase transition would require rupture and reconstruction of bonds.…”

“…Recently, Sokolova et al (2001) described the crystal structure of malinkoite from the Kola Peninsula, which also has the formula NaBSiO 4 . It has a hexagonal symmetry and, following a personal communication with E. Sokolova (2004), is equivalent to the unnamed hexagonal NaBSiO 4 mineral from Mont Saint-Hilaire labeled UK53A by Chao et al (1990).…”