| Summary: | A crystal chemical investigation of a natural specimen of whitlockite, ideally Ca<sub>9</sub>Mg(PO<sub>4</sub>)<sub>6</sub>[PO<sub>3</sub>(OH)], from Palermo Mine (USA), was achieved by means of a combination of electron microprobe analysis (EMPA) in WDS mode, single-crystal neutron diffraction probe (NDP) and single-crystal X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. The crystal-chemical characterization resulted in the empirical formula (Ca<sub>8.682</sub>Na<sub>0.274</sub>Sr<sub>0.045</sub>)<sub>Σ9.000</sub>(Ca<sub>0.034</sub>□<sub>0.996</sub>)<sub>Σ1.000</sub>(Mg<sub>0.533</sub>Fe<sup>2+</sup><sub>0.342</sub>Mn<sup>2+</sup><sub>0.062</sub>Al<sub>0.046</sub>)<sub>Σ0.983</sub>(P<sub>1.006</sub>O<sub>4</sub>)<sub>6</sub>[PO<sub>3</sub>(OH<sub>0.968</sub>F<sub>0.032</sub>)<sub>Σ1.000</sub>]. Crystal-structure refinement, in the space group <i>R</i>3<i>c</i>, converged to <i>R</i>1 = 7.12% using 3273 unique reflections from NDP data and to <i>R</i>1 = 2.43% using 2687 unique reflections from XRD data. Unit cell parameters from NDP are <i>a</i> = 10.357(3) Å, <i>c </i>= 37.095(15) Å and <i>V</i> = 3446(2) Å<sup>3</sup>, and from XRD, the parameters are<i> a</i> = 10.3685(4) Å, <i>c </i>= 37.1444(13) Å and <i>V</i> = 3458.2(3) Å<sup>3</sup>. NDP results allowed a deeper definition of the hydrogen-bond system and its relation with the structural unit [PO<sub>3</sub>(OH)]. The FTIR spectrum is very similar to that of synthetic tricalcium phosphate Ca<sub>3</sub>(PO4)<sub>2</sub> and displays minor band shifts due to slightly different P-O bond lengths and to the presence of additional elements in the structure. A comparison between whitlockite, isotypic phases from the largest merrillite group, and its synthetic counterpart Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> is provided, based on the XRD/NDP and FTIR results.
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