A model for the structure of the hydrated aluminum phosphate, kingite determined by ab initio powder diffraction methods
Date
2003
Authors
Wallwork, K.
Pring, A.
Taylor, M.
Hunter, B.
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Advisors
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Journal article
Citation
American Mineralogist, 2003; 88(1):235-239
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Abstract
The crystal structure of kingite, Al<inf>3</inf>(PO<inf>4</inf>)<inf>2</inf> (F,OH)<inf>2</inf>·8(H<inf>2</inf>O,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 R<inf>Bragg</inf> = 0.022 and R<inf>wp</inf> = 0.039 using the Rietveld method. The triclinic structure was solved and refined in the space group P̄1, 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) Å<sup>3</sup>, Z = 2. The structure of kingite contains finite strings of three corner sharing AlΦ<inf>6</inf> octahedra (where Φ represents O, OH<sup>-</sup>, F<sup>-</sup>, or H<inf>2</inf>O). These strings are cross-linked via PO<inf>4</inf> tetrahedra to produce layers that are perpendicular to [100]. The layers are linked via hydrogen bonding through H<inf>2</inf>O 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, Al<inf>3</inf>(PO<inf>4</inf>)<inf>2</inf>(OH)<inf>3</inf>·5H<inf> 2</inf>O, and mitryaevaite, Al<inf>5</inf>(PO<inf>4</inf>)<inf>2</inf>[(P,S)O<inf>3</inf> (OH,O)]<inf>2</inf>F<inf>2</inf>(OH)<inf>2</inf> (H<inf>2</inf>O)<inf>8</inf>·6.48H<inf>2</inf>O, are briefly discussed.