Abstract

The crystal structure of faheyite, ideally Mn2+Fe3+2[Be2(PO4)4](H2O)6, trigonal, a 9.404(7), c 15.920(11) Å, V 1219(2) Å3, Z = 3, space group P3121, has been solved and refined to an R1 index of 4.4% with single-crystal X-ray diffraction data collected from a twinned fiber. There are two P sites that are tetrahedrally coordinated by O atoms with <P–O> distances of 1.52 and 1.54 Å, respectively, one Be site tetrahedrally coordinated by O atoms with a <Be–O> distance of 1.63 Å, one Mn site occupied by Mn2+ coordinated by four O atoms and two (H2O) groups with a <Mn–O> distance of 2.22 Å, and one Fe site occupied by Fe3+ coordinated by four O atoms and two (H2O) groups with a <Fe–O> distance of 2.01 Å.

Each vertex of the Be tetrahedron is shared with a vertex of a neighboring P tetrahedron, and two vertices of each P tetrahedron are shared with neighboring Be tetrahedra to form a corner-sharing [Be(PO4)2] chain, with P tetrahedra flanking the Be tetrahedra of the central spine in the sequence -P(1)/P(1)-Be-P(2)/P(2)-Be-. Faheyite has a chiral structure, with the [Be(PO4)2] chain twisting about the c-axis in a clockwise direction for the refined P3121 enantiomer. The Mn octahedron lies along the 31 screw axis within the core region of the [Be(PO4)2] chain, forming [MnBe2(PO4)4] spires that are wrapped by Fe octahedra that share vertices with P tetrahedra. The crystal structures of fransoletite and parafransoletite also contain beryllophosphate chains topologically identical to that found in faheyite, although the [Be(PO4)(PO3OH)] chain in fransoletite and parafransoletite is straight, whereas the [Be(PO4)2] chain in faheyite forms a helix about the central c-axis.

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