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

Elena V. Sokolova^1,, Frank C. Hawthorne¹ and Alexander P. Khomyakov²

¹ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
² Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements, Moscow 121357, Russia

The crystal structure of malinkoite, NaBSiO₄, a 13.8964(4) Å, c 7.7001(2) Å, P6₃, 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 (SiO₄) and (BO₄) 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 (SiO₄) and (BO₄) 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 NaBePO₄.

In the crystal structure of lisitsynite, KBSi₂O₆, four-membered rings consist of two (SiO₄) and two (BO₄) 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, KBSi₂O₆, and synthetic "boroleucite", K(Si₂ B)O₆, may be regarded as polymorphs with different patterns of Si–B order.

Keywords: malinkoite, lisitsynite, new mineral species, borosilicate, crystal structure, Khibina, Lovozero, Russia.

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