Abstract
Apatites are properly considered as a strategic material owing to the broad range of their practical uses, primarily biomedical but chemical, pharmaceutical, environmental and geological as well. The apatite group of minerals has been the subject of a huge number of papers, mainly devoted to the mass crystallization of nanosized hydroxyapatite (or carboapatite) as a scaffold for osteoinduction purposes. Many wet and dry methods of synthesis have been proposed. The products have been characterized using various techniques, from the transmission electron microscopy to many spectroscopic methods like IR and Raman. The experimental approach usually found in literature allows getting tailor made micro- and nano- crystals ready to be used in a wide variety of fields. Despite the wide interest in synthesis and characterization, little attention has been paid to the relationships between bulk structure and corresponding surfaces and to the role plaid by surfaces on the mechanisms involved during the early stages of growth of apatites. In order to improve the understanding of their structure and chemical variability, close attention will be focused on the structural complexity of hydroxyapatite (HAp), on the richness of its surfaces and their role in the interaction with the precursor phases, and in growth kinetics and morphology.
Highlights
As described in two excellent review papers [1,2] the term “Apatite” represents a group of minerals of a very wide interest
Concerning the research in the health field, it is worth outlining that two important members of apatites (Hydroxyapatite, HAp, and Carbonated apatite, CAp) represent both the inorganic pillar building hard tissues such as bones, theet, antlers of mammals and their pathological calcified tissues
To do this: (i) the character of the {hkl} forms was identified, through the search of the most important periodic bond chains (PBC) building up the crystal structure; (ii) the corresponding crystal slices of thickness dhkl were considered, respecting the space group constraints; (iii) all the possible surface profiles related to every dhkl slice were drawn, respecting the conditions of both electro-neutrality and vanishing of the dipole moment component perpendicular to the hkl plane; (iv) the specific surface energy was calculated, for each hkl surface profile, to draw the equilibrium shape (ES) of the single crystal, by applying the selective rules required by the Gibbs-Wulff’s theorem [3]
Summary
As described in two excellent review papers [1,2] the term “Apatite” represents a group of minerals of a very wide interest. Having considered that apatites are studied since more that two centuries and according to their wide field of interest, it is not surprising that the total amount of currently available publications on the “Apatite” subject exceeds 40,000 with the annual increase for, at least, 2000 papers. Minerals 2017, 7, 139 in order to improve the knowledge of the interfaces that form when the crystal interacts with its growth medium. This way of methodological thinking represents the unavoidable consequence ofthe scale-change which affects the crystals when going from geological (cm, mm) to industrial (mm-μm) to biomaterial (μm-nm) size. A sketch of the apatite bulk structure, drawn according the most realistic related symmetry space groups, focusing mainly on Hydroxyapatite (HAp) and, secondarily, on its structurally derivated, the Carbonated apatite (CAp).
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