THE CRYSTAL-STRUCTURE OF SINKANKASITE, A COMPLEX HETEROPOLYHEDRAL SHEET MINERAL

The crystal structure of sinkankasite, Z = 2, Mn2+(H2O)(4)[Al(PO3OH)(2)(OH)](H2O)(2), triclinic, a = 9.590(2), b = 9.818(2), c = 6.860(1) Angstrom, alpha = 108.04(3), beta = 99.63(3), gamma = 98.87(3)degrees, V = 590.7(2) Angstrom(3), space group P (1) over bar, has been solved by direct methods and refined to an R index of 7.2% and an R(w) index of 7.6% using 2170 unique observed reflections (\F-0\ greater than or equal to 5 sigma(F)) collected with MoK alpha X-ray radiation. The structure contains one distinct Mn2+ position, which is octahedrally coordinated by two O atoms and four H2O groups. There are two distinct Al positions; each is octahedrally coordinated by four O atoms and two OH groups. The two distinct P positions are each tetrahedrally coordinated by three O atoms and one OH group. There are 15 unique H positions, which have been located and refined using soft constraints. The structure consists of corner-sharing chains of Al phi(6) octahedra, decorated on both sides by P phi(4) tetrahedra. These heteropolyhedral chains are linked through the Mn phi(6) octahedra to form continuous sheets of composition MnAlP2O6(OH)(3) . 4H(2)O. There are two interlayer anion positions, both of which are occupied by H2O groups. There are eight unique H bonds between the interlayer cations and the sheets on either side, and no other bonding occurs between the sheets. A total of seven unique H bonds occur between anions of the same heteropolyhedral sheet.