The structure of fedorite: A re-appraisal

The crystal structures of fedorite from the type locality, the Turiy complex (Kola Peninsula, Russia) and the Little Murun complex (Sakha, Russia), have been refined from single crystals by CCD X-ray diffractometry. Both samples are triclinic, space group P (1) over bar; fedorite from Turiy has a 9.6300(7), b 9.6392(7), c 12.6118(9) Angstrom, alpha 102.422(1), beta 96.247(1), gamma 119.888(1)degrees [R1 = 4.87%], whereas that from Little Murun has a 9.6450(7), b 9.6498(7), c 12.6165(9) Angstrom, alpha 102.427(l), beta 96.247(1), gamma 119.894(1)degrees [R1 = 3.94%]. The structure is described as a modular centrosymmetric structure (stacking sequence OS2(S) over bar O-2) consisting of sheets of edge-sharing M phi (6) (M = Ca, Na; phi = O, F, Cl) polyhedra (O-sheet) linked to sheets of (Si,Al)O-4 tetrahedra (S-2 and S-2 sheets), with Na, K and H2O groups occurring as interlayer cations in partially occupied sites. Four distinct M phi (6) polyhedra characterized by different Na and Ca occupancies are recognized in the sheet of octahedra. The sheet of tetrahedra consists of two types of six-member rings of corner-sharing (Si,Al)O-4 units: a slightly distorted hexagonal ring of six upwardly pointing (relative to the center of symmetry) tetrahedra, and a strongly distorted hexagonal ring consisting of four upwardly and two downwardly pointing tetrahedra, The latter provide the link between the centrosymmetrically related S-2 and (S) over bar (2) sheets. This study provides significant improvements in the accuracy and precision of cell dimensions, bond lengths and pattern of cation order relative to previous studies. The ideal structural formula of fedorite is A(2-3)M(7)T(16)O(38)X(2). nH(2)O, where A = Na, K, Ba, Ca; M = Ca, Na, Mn2+, Fe2+; T Si, Al, Ti; X = F, Cl or OH, and n approximate to 3.5.