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, D x = 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)O 5 (O,F) and TiO 5 (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 PO 4 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)O 6 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 (H 2 O 0.78 K 0.16 Na 0.03 ) 2 Ti(Mn 0.75 +2 Fe 0.21 +2 Mg 0.04 ) 2 (Fe 0.68 +3 Ti 0.28 +4 Al 0.07 ) 2 (PO 4 ) 4 -(O 0.6 F 0.4 ) 2 · 14H 2 O. 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.
Zeitschrift für Kristallographie - Crystalline Materials – de Gruyter
Published: Jan 1, 1993
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