Structures of biological minerals in dental research

Abstract

Structural features of some calcium phosphates of biological interest are described. Structure of hydroxyapatite (OHAp), considered as the prototype for the inorganic component of bones and teeth is discussed with respect to the kinds and locations of ionic substitutions. Octacalcium phosphate (OCP), is a probable precursor in biological mineralization. OCP has a layer type structure, with one layer quite similar to that of OHAp and the other, a hydrated layer consisting of more widely spaced Ca, and PO4 ions and the water molecules. The closeness of fit in the apatitic layers of OCP and OHAp accounts for the epitaxial, interlayered mixtures formed by these compounds and the in situ conversion of OCP to OHAp. Possible roles of OCP in biological mineralization are discussed.

Calcium orthophosphates (CaPO 4 ): Occurrence and properties

“…An anhydrous phase diagram CaO-P 2 O 5 at temperatures within 200-2200°C is shown in Fig. 3 (Kreidler and Hummel 1967;Carayon and Lacout 2003). Table 4 comprises crystallographic data of the existing CaPO 4 (Elliott 1994;White and Dong 2003;Mathew and Takagi 2001). The most important parameters of CaPO 4 are the ionic Ca/P ratio, basicity/acidity and solubility.…”

“…However, at temperatures above *250°C, there is a monoclinic to hexagonal phase transition in HA (space group P6 3 /m) (Elliott 1994(Elliott , 1998Mathew and Takagi 2001;Rangavittal et al 2000;Kim et al 2000a). The structural difference between the two modifications represents the ordered, head-to-tail arrangement of OH groups located in the center of every other Ca 2 triangle in the monoclinic lowtemperature symmetry and the disordered arrangement of OH groups, where the head-to-tail and tail-to-head arrangements alternate throughout the channel in the hexagonal high-temperature symmetry.…”

“…Namely, the crystal structure of HA is well described elsewhere (Elliott 1994;White and Dong 2003;Mathew and Takagi 2001), the detailed analysis of the electronic structure, bonding, charge transfer, optical and elastic properties is also available Rulis et al 2004;Snyders et al 2007;Ching et al 2009), while the readers interested in Posner's clusters are referred to still other papers (Treboux et al 2000;Kanzaki et al 2001). A shell model was developed to study the lattice dynamics of HA (Calderin et al 2005), while a cluster growth model was created to illustrate its growth (Onuma and Ito 1998).…”

“…A hierarchical structure for FA was proposed (Dorozhkin 2007a). The crystal structure of FA for the first time was studied in 1930 (Mehmel 1930;Naray-Szabo 1930) and is well described elsewhere (Elliott 1994;White and Dong 2003;Mathew and Takagi 2001). Computer simulations of the FA structure were performed as well (Li et al 2015).…”

“…To finalize the description of individual CaPO 4 , one should mention on an interesting opinion, that all types of CaPO 4 listed in Table 1 might be classified into three major structural types (Mathew and Takagi 2001;Chow and Eanes 2001). They comprise: (1) the apatite type, Ca 10 (PO 4 ) 6 X 2 , which includes HA, FA, OA, CDHA, OCP and TTCP; (2) the glaserite type, named after the mineral glaserite, K 3 Na(SO 4 ) 2 , which includes all polymorphs of TCP and, perhaps, ACP; (3) the Ca-PO 4 sheet-containing compounds, which include DCPD, DCPA, MCPM and MCPA.…”