The crystal structure of a mineral related to paulkerrite

F. Demartin, T. Pilati, H. D. Gay und C. M. Gramaccioli

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: ao = 10.561(5) Å, bo = 20.585(8) Å, co = 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.

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