MURATAITE, A UB12 DERIVATIVE STRUCTURE WITH CONDENSED KEGGIN MOLECULES

T. SCOTT ERCIT
Research Division, Canadian Museum of Nature, Ottawa, Ontario K1P 6P4

FRANK C. HAWTHORNE
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2

Abstract

The crystal structure of murataite, space group F3m, a 14.886(2) Å, has been solved by Patterson methods and refined to an R of 4.91 (wR 4.71%) for 265 observed reflections (MoK radiation). The ideal formula is (Y,Na)6(Zn,Fe)5Ti12O29(O,F)10F4 with Z = 4, but its simple appearance conceals extensive cation disorder within the structure. There are four distinct cation sites: the X site is [8]-coordinated and contains (Y,HREE,Na,Ca,Mn); the T site is tetrahedrally coordinated and contains (Zn,Si); the M1 site is octahedrally coordinated and contains (Ti,Nb,Na); the M2 site is [5]-coordinated by a triangular bipyramid and contains (Zn,Fe3+,Ti,Na). Three M1 octahedra share edges to form a compact M3 13 group. Four of these M3 13 groups link by sharing corners to form a tetrahedral cage in the center of which is the T site. The resulting Keggin-structured [M12T 40]n- unit may be considered as the fundamental building block of the structure. The net formed by linkage of the M1 polyhedra is topologically the same as the B net of UB12, which accounts for the similarity of X-ray properties of murataite and this compound.

Keywords: murataite, crystal structure, Ti-oxide mineral, rare-earth elements, peralkaline granitic pegmatite, Keggin structure, UB12.