For the synthesis of hydroxyapatite crystals from aqueous solutions three preparation methods were employed. From the experimental processes and the characterization of the crystals it was concluded that aging and precipitation kinetics are critical for the purity of the product and its crystallographic characteristics. The authentication details are presented along with the results from infrared spectroscopy, X-ray powder diffraction, Raman spectroscopy, transmission and scanning electron photographs, and chemical analysis. Analytical data for several calcium phosphates were collected from the literature, extensively reviewed, and the results were grouped and presented in tables to provide comparison with the data obtained here.

Synthesis and characterization of hydroxyapatite crystals: A review study on the analytical methods

The apatite-group minerals of the general formula, M10(ZO4)6X2 (M = Ca, Sr, Pb, Na…, Z = P, As, Si, V…, and X = F, OH, Cl…), are remarkably tolerant to structural distortion and chemical substitution, and consequently are extremely diverse in composition (e.g., Kreidler and Hummel 1970; McConnell 1973; Roy et al. 1978; Elliott 1994). Of particular interest is that a number of important geological, environmental/paleoenvironmental, and technological applications of the apatite-group minerals are directly linked to their chemical compositions. It is therefore fundamentally important to understand the substitution mechanisms and other intrinsic and external factors that control the compositional variation in apatites.

The minerals of the apatite group are listed in Table 1⇓, and representative compositions of selected apatite-group minerals are given in Table 2⇓. Also, more than 100 compounds with the apatite structure have been synthesized (Table 3⇓). Phosphate apatites, particularly fluorapatite and hydroxylapatite, are by far the most common in nature and are often synonymous with “apatite(s)”. For example, fluorapatite is a ubiquitous accessory phase in igneous, metamorphic, and sedimentary rocks and a major constituent in phosphorites and certain carbonatites and anorthosites (McConnell 1973; Dymek and Owens 2001). Of particular importance in biological systems, hydroxylapatite and fluorapatite (and their carbonate-bearing varieties) are important mineral components of bones, teeth and fossils (McConnell 1973; Wright et al. 1984; Grandjean-Lecuyer et al. 1993; Elliott 1994; Wilson et al. 1999; Suetsugu et al. 2000; Ivanova et al. 2001).

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 Table 1. 
Summary of the apatite-group minerals



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 Table 2. 
Representative compositions and unit-cell parameters of selected apatite-group minerals.



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 Table 3. 
Formulas of selected synthetic compounds with the apatite structure.



Following Fleischer and Mandarino (1995), Table 1⇑ also includes melanocerite-(Ce), tritomite-(Ce), and tritomite-(Y), the compositions …

Compositions of the Apatite-Group Minerals: Substitution Mechanisms and Controlling Factors

Apatites in biological systems

Numerous biological and chemical studies involve the use of calcium hydroxyapatite (HA), Ca10(PO4)6(OH)2. In this study detailed physicochemical characterization of HA, prepared from an aqueous solution, was carried out employing different methods and techniques: chemical and thermal analyses, x-ray diffraction, infrared and Raman spectroscopies, scanning and transmission microscopies, and Brunauer, Emmett, and Teller (BET) surface-area method. The contents of calcium (Ca(2+)), phosphate (PO4 (3-)), hydroxide (OH(-)), hydrogenphosphate (HPO4 (2-)), water (H2O), carbonate (CO3 (2-)), and trace constituents, the Ca/P molar ratio, crystal size and morphology, surface area, unit-cell parameters, crystallinity, and solubility of this HA were determined. This highly pure, homogeneous, and highly crystalline HA is certified as a National Institute of Standards and Technology (NIST) standard reference material, SRM 2910.

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C. H. Amberg

Papers

Nature of deficiency in nonstoichiometric hydroxyapatites. II. Spectroscopic studies of calcium and strontium hydroxyapatites