The crystal structure of a mineral related to paulkerrite

Abstract

Abstract An apparently new phosphate mineral (named “benyacarite”) showing evident relationships with paulkerrite and mantienneite, but richer in Mn+2, was found in granitic pegmatites in Argentina (Cerro Blanco, Tanti, Còrdoba) where it is associated with other phosphates such as triplite etc., and fluorides such as pachnolite. The unit-cell parameters are: a o = 10.561(5) Å, b o = 20.585(8) Å, c o = 12.516(2) Å, orthorhombic, space group Pbca, Z = 4, Dx = 2.37 gcm−3. The crystal structure has been refined to R(unweighted) = 0.034 and R(weighted) = 0.040 using 1023 independent reflections. It contains layers perpendicular to [010] of two kinds of octahedra, M(2)O5(O,F) and 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 sides of these layers there are PO4 tetrahedra, with one face approximately parallel to the layers; the three corners of this face are shared with octahedra centered on Ti and M(2), whereas the remaining one is shared with another M(1)O6 octahedron centered by Mn+2 atoms (in part replaced by Fe+2). This network of octahedra and tetrahedra leaves cavities occupied by K+ and water molecules. According to these data, and to microprobe analysis the chemical formula corresponds to (H2O0.78K0.16Na0.03)2Ti(Mn0.75 +2Fe0.21 +2Mg0.04)2(Fe0.68 +3Ti0.28 +4Al0.07)2(PO4)4-(O0.6F0.4)2 · 14H2O. On the basis of the knowledge of the structure of “benyacarite” and the strong similarities of its crystal chemistry with those of paulkerrite and mantienneite, a better characterization of these minerals is obtained. These species imply substitution of Mn+2 by Mg or Fe+2, and of Fe+3 by Ti+4, or Al, respectively.