A MODEL FOR THE STRUCTURE OF THE HYDRATED ALUMINUM PHOSPHATE, KINGITE DETERMINED BY AB INITIO POWDER DIFFRACTION METHODS

Abstract

The crystal structure of kingite, Al3(PO4)2(F,OH)2·8(H2O,OH), a secondary mineral from a Cambrian-Precambrian phosphate deposit at Tom’s Quarry, near Kapunda, South Australia, has been determined from a powder sample using synchrotron X-ray diffraction data. The structure was determined ab initio by direct methods and refined to RBragg = 0.022 and Rwp = 0.039 using the Rietveld method. The triclinic structure was solved and refined in the space group P1̅, a = 9.377(1), b = 10.113(1), c = 7.138(1) Å, α = 97.60(1), β = 100.88(1), γ = 96.01(1)°, V = 653.0(1) Å3 , Z = 2. The structure of kingite contains finite strings of three corner sharing Alϕ6 octahedra (where ϕ represents O, OH−, F−, or H2O). These strings are cross-linked via PO4 tetrahedra to produce layers that are perpendicular to [100]. The layers are linked via hydrogen bonding through H2O located in the interlayer space. Kingite is shown to have a different stoichiometry to that reported earlier. The relationship of kingite to the structures of wavellite, Al3(PO4)2(OH)3·5H2O, and mitryaevaite, Al5(PO4)2[(P,S)O3(OH,O)]2F2(OH)2(H2O)8·6.48H2O, are briefly discussed.