REFINEMENT OF THE CRYSTAL-STRUCTURE OF BURANGAITE

The crystal structure of burangaite, .02)Sigma(1.00)Al(4.89)(PO4)(4.06 )(OH)(6)(H2O)(2), a 25.099(2), b 5.0491(7), c 13.438(1) Angstrom, beta 110.88(1)degrees, V = 1591.1(3) Angstrom(3), C2/c, Z = 4, has been re fined to an R index of 2.8% for 2004 observed reflections measured wit h MoK alpha X-radiation. Burangaite is isostructural with dufrenite, c onsidered by Moore (1970) to have a composition between Ca0.5Fe2+Fe53(PO4)(4)(OH)(6)(H2O)(2) and square Fe3+Fe53+(PO4)(4)(OH)(6)(H2O)(2). T he fundamental building block (FBB) of the structure of burangaite is a face-sharing triplet of octahedra [Al phi(6) - Fe phi(6) - Al phi(6) ] corner-linked to two Al phi(6) octahedra and two PO4 tetrahedra. The FEB is polymerized parallel to the c axis to form dense slabs in the {100} plane. Alternating Al phi(6) octahedra and Na phi(8) polyhedra f orm a chain parallel to the c axis, and the dense slab and chain alter nate along the a axis.