THE CRYSTAL-STRUCTURE OF A MINERAL RELATED TO PAULKERRITE

An apparently new phosphate mineral (named ''benyacarite'') showing ev ident relationships with paulkerrite and mantienneite, but richer in M n+2, was found in granitic pegmatites in Argentina (Cerro Blanco, Tant i, Cordoba) where it is associated with other phosphates such as tripl ite etc., and fluorides such as pachnolite. The unit-cell parameters a re: a(o) = 10.561(5) angstrom, b(o) = 20.585(8) angstrom, c(o) = 12.51 6(2) angstrom, orthorhombic, space group Pbca, Z = 4, D(x) = 2.3 7 gcm -3. The crystal structure has been refined to R(unweighted) = 0.034 an d R(weighted) = 0.040 using 1023 independent reflections. It contains layers perpendicular to [010] of two kinds of octahedra, M(2)O5(O,F) a nd TiO5(O,F), the first essentially centered by Fe+3 atoms, with some Ti+4; the fluorine atom shares vertices of both octahedra. On both sid es of these layers there are PO4 tetrahedra, with one face approximate ly parallel to the layers; the three corners of this face are shared w ith octahedra centered on Ti and M(2), whereas the remaining one is sh ared with another M(1)O6 octahedron centered by Mn+2 atoms (in part re placed by Fe+2). This network of octahedra and tetrahedra leaves cavit ies occupied by K+ and water molecules. According to these data, and t o microprobe analysis the chemical formula corresponds to g0.04)2(Fe0. 68+3Ti0.28+4Al0.07)2(PO4)4-(O0.6F0.4)2 . 14H2O. On the basis of the kn owledge of the structure of ''benyacarite'' and the strong similaritie s of its crystal chemistry with those of paulkerrite and mantienneite, a better characterization of these minerals is obtained. These specie s imply substitution of Mn+2 by Mg or Fe+2, and of Fe+3 by Ti+4, or Al , respectively.