Abstract
A crystal chemical investigation of a natural specimen of whitlockite, ideally Ca9Mg(PO4)6[PO3(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 (Ca8.682Na0.274Sr0.045)Σ9.000(Ca0.034□0.996)Σ1.000(Mg0.533Fe2+0.342Mn2+0.062Al0.046)Σ0.983(P1.006O4)6[PO3(OH0.968F0.032)Σ1.000]. Crystal-structure refinement, in the space group R3c, converged to R1 = 7.12% using 3273 unique reflections from NDP data and to R1 = 2.43% using 2687 unique reflections from XRD data. Unit cell parameters from NDP are a = 10.357(3) Å, c = 37.095(15) Å and V = 3446(2) Å3, and from XRD, the parameters are a = 10.3685(4) Å, c = 37.1444(13) Å and V = 3458.2(3) Å3. NDP results allowed a deeper definition of the hydrogen-bond system and its relation with the structural unit [PO3(OH)]. The FTIR spectrum is very similar to that of synthetic tricalcium phosphate Ca3(PO4)2 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 Ca3(PO4)2 is provided, based on the XRD/NDP and FTIR results.
Highlights
The study of calcium phosphates (CaP) is a topic of paramount interest for the scientific community, with these minerals being the natural counterparts of several biomaterials largely employed in biomedical applications such as cements, scaffolds and coatings of components of bone and teeth used in in orthopedics and dentistry [1]
The interest of life and material sciences in whitlockite is due to its presence in the human body, inside hyper-mineralized osteocyte lacunae in human alveolar bone [8], in arthritic cartilage and soft-tissue deposits [2], in dental calculi [9] and because it is closely related to the class of synthetic tricalcium phosphates, Ca3(PO4)2 (TCP) [10,11,12]
X-ray and Neutron Structural Studies The unit cell parameters from neutron and X-ray diffraction measured for the present sample are in the range of those observed in other natural samples from whitlockite and merrillite subgroups from the literature (Table 4)
Summary
The study of calcium phosphates (CaP) is a topic of paramount interest for the scientific community, with these minerals being the natural counterparts of several biomaterials largely employed in biomedical applications such as cements, scaffolds and coatings of components of bone and teeth used in in orthopedics and dentistry [1]. These applications are mostly based on the similarity of the density of these compounds with that of human hard tissues [2]. A whitlockite sample from the Palermo Mine, North Groton, New Hampshire (USA), has been characterized by X-ray and neutron single-crystal diffraction, electron microprobe analysis and (for the first time) infrared spectroscopy, with the aim of defining its crystal-chemical identity
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