Crystal structure of perloffite

Reference:
▪ Elliott, P. & Willis, A.C. (2011): The crystal structure of perloffite. Mineralogical Magazine, 75, (in press).

Abstract:
The structure of perloffite, ideally BaMn2+2Fe3+2(PO4)3(OH)3, has been solved and refined from single-crystal X-ray data collected on a Sr-bearing sample from the Spring Creek copper mine, near Wilmington, South Australia. A fragment of the crystal used for the collection of the X-ray intensity data was analysed by electron microprobe. The resulting unit formula is (Ba1.00,Sr0.03)Σ1.03 (Mn2+1.15,Fe2+0.76Ca0.05Mg0.02,Na0.01)Σ1.99(Fe3+1.94,Al0.06)Σ2.00(PO4)2.99(OH)3.04. The structure is monoclinic, space group P21/m, a = 9.2425(18), b = 12.470(3), c = 5.002(1) Å, Β = 100.19(3)°, V = 567.41(19) Å3, Z = 2, and was refined to R1 = 4.71% for 1568 unique observed reflections. The X site is [11]-coordinated and is occupied by dominant Ba and minor Sr, with an <X–O> distance of 2.953 Å. The M1 site is octahedrally coordinated and is occupied by Mn2+and Fe2+ plus minor Ca, Mg and Na with an observed <M1–O> distance of 2.208 Å. The M2 site is also octahedrally coordinated and is occupied by Fe3+ plus minor Al with a <M2–O> distance of 2.018 Å. Perloffite is isostructural with other members of the bjarebyite group. Edge-linked [M22O6(OH)4] dimers link via corners to form chains along [010]. Chains are decorated with PO4 tetrahedra and link in the a direction via [M1O5(OH)] octahedra and (Ba,Sr) atoms to form a complex framework. The H2 atom hydrogen-bonds strongly to the O3 anion, providing additional linkage between chains.