Crystal structure of whiteite-(CaFeMg)

Capitelli, F., Chita, G., Cavallo, A., Bellatreccia, F. and Della Ventura, G. (2011): Crystal structure of whiteite-(CaFeMg) from Crosscut Creek, Canada. Z. Kristallogr. 226 (in press)
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The structural characterization of whiteite-(CaFeMg), a natural orthophosphate from Crosscut Creek, Yukon, Canada, with chemical composition (Ca0.86Na0.05)sum=0.91(Fe0.88Mn0.02)sum=0.90Mg2.17Al1.93(PO4)4(OH)2.8 H2O obtained by WDS-EMPA, was carried out by means of single crystal X-ray diffraction. The structure was solved in the P2/a monoclinic space group, with the following unit cell constants: a = 14.8700(15), b = 6.9785(5), c = 9.9268(10)A˚ , beta =110.110(1)°, V = 967.31(15)A˚ 3. Structure refinement lead to the crystal chemical formula (Ca0.95Na0.11)sum=1.06Fe0.90Mg2.01Al2.05 (PO4)4(OH)1.99.8.17 H2O. Phosphorus atoms display tetrahedral (PO4) coordination, while magnesium, iron(II) and aluminum display regular octahedral coordination; calcium displays a complex CaO8 coordination. MgO2(H2O)4, FeO6, AlO4(OH)2, CaO8 polyhedra and PO4 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.