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) A, c 7.7001(2) A, P 6 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 | > 4σ F ) reflections collected with a single-crystal diffractometer fitted with a CCD detector and Mo K α 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 A, 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, 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 A, respectively; the remaining two Na sites are [6]-coordinated, with Na –O> distances of 2.444 and 2.440 A, 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.