Crystal structure of whiteite-(CaFeMg) from Crosscut Creek, Canada

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Abstract
The structural characterization of whiteite-(CaFeMg), a natural orthophosphate from Crosscut Creek, Yukon, Canada, with chemical composition (Ca 0.86 Na 0.05 ) Σ = 0.91 (Fe 0.88 Mn 0.02 ) Σ = 0.90 Mg 2.17 Al 1.93 (PO 4 ) 4 (OH) 2 · 8 H 2 O obtained by WDS-EMPA, was carried out by means of single crystal X-ray diffraction. The structure was solved in the P 2/a monoclinic space group, with the following unit cell constants: a = 14.8700(15), b = 6.9785(5), c = 9.9268(10) Å, β = 110.110(1)°, V = 967.31(15) Å 3 . Structure refinement lead to the crystal chemical formula (Ca 0.95 Na 0.11 ) Σ = 1.06 Fe 0.90 Mg 2.01 Al 2.05 (PO 4 ) 4 (OH) 1.99 · 8.17 H 2 O. Phosphorus atoms display tetrahedral (PO 4 ) coordination, while magnesium, iron(II) and aluminum display regular octahedral coordination; calcium displays a complex CaO 8 coordination. MgO 2 (H 2 O) 4 , FeO 6 , AlO 4 (OH) 2 , CaO 8 polyhedra and PO 4 groups are arranged such as to form a three-dimensional framework via edge- and vertex-sharing arrangements. Strong O—H … O hydrogen bonds contribute to stabilize the array: such interactions were also investigated by means of Libowitzky formula (Libowitzky, 1999) application, owing to the results of IR spectroscopy.