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

Abstract

The structural characterization of whiteite-(CaFeMg), a natural orthophosphate from Crosscut Creek, Yukon, Canada, with chemical composition (Ca0.86Na0.05)Σ = 0.91(Fe0.88Mn0.02)Σ = 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 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) Å3. Structure refinement lead to the crystal chemical formula (Ca0.95Na0.11)Σ = 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.

* Correspondence address: CNR - Institute of Crystallography (IC), via Salaria Km 29.300, 00015 Monterondo (Rome), Italien, francesco.capitelli@ic.cnr.it

Published Online: 2011-05-06

Published in Print: 2011-09