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Tissues and minerals
On a quantitative basis, the most important of the calcium phosphates (Table 1⇓) is an apatite closely related to hydroxylapatite (HAP). This is better described as an impure carbonate-containing apatite (CO3Ap) and forms the inorganic component of bones and teeth. Table 1⇓ also includes calcium phosphates (including two pyrophosphates) that occur in pathological mineralizations and those that are used for the repair of mineralized tissues. Unlike the well-controlled process of normal mineralization in bones and teeth (see later), pathological mineralizations are usually poorly controlled with the result that several calcium phosphates may occur together. In addition, their crystallographic orientations are often random.
The solubility isotherms of the calcium phosphates in the system Ca(OH)2-H3PO4-H2O at 37°C are shown in Figure 1⇓. Monetite occurs in the phase diagram, although its occurrence in normal or pathological calcifications has never been confirmed, one contributing reason being that its nucleation and growth is more difficult than brushite under biological conditions, so brushite forms in preference, even though brushite is less stable. β-Ca3(PO4)2 is a high-temperature phase that does not precipitate directly in aqueous systems; however, it is sufficiently stable in water for a solubility product to be determined so that an isotherm can be calculated, which is the origin of the isotherm in Figure 1⇓. However, if the aqueous system contains Mg2+ (1 or 2 mmol/l), the solubility product of “Ca3(PO4)2” is dramatically reduced (more than a thousand-fold) with the result that the structurally related whitlockite can easily precipitate (Hamad and Heughebaert 1986, LeGeros et al. 1989). Fe2+ ions have a similar effect. Whitlockite has superficially the same X-ray diffraction (XRD) …