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As can be seen in this figure, the pattern shows a poorly crystalline hydroxylapatite but no other phase is present. These results imply that the first and only crystalline phase to precipitate in all experiments presented here is hydroxylapatite.

The observation of HAP precipitation in this study is in agreement with the results of Boskey and Posner (11), who investigated HAP homogeneous precipitation at low supersaturation (saturation degree between 5 and 9) at pH=7.4 and ionic strength 0.15 M. Their experiments were carried out by rapidly mixing a calcium and phosphate solution to the desired final supersaturation degree. After nuclei formation the crystals were allowed to grow keeping the pH constant during this time. The precipitate was identified using electron microscopy and was found to be identical, even though poorly crystalline, to hydroxylapatite. They concluded the precipitation of HAP without the formation of a precursor phase.

Nancollas and Tomazic (21) investigated heterogeneous precipitation of calcium phosphate onto HAP seeds at different levels of supersaturation. Precipitation in seeded experiments can take place at lower supersaturations than homogeneous nucleation. The range of supersaturation degree for calcium phosphate to precipitate in the presence of seeding material in their study was between 7 and 11. The saturated solutions were prepared by mixing calcium and phosphate solutions in which precipitation was initiated by the addition of a seed. The pH during the experiments was kept constant. The precipitate was studied using X-ray diffraction and infrared spectroscopy. They found that at high supersaturation (SI=11) an amorphous calcium phosphate (ACP) was formed as a precursor phase. At a low supersaturation degree (SI=7) the study found formation of hydroxylapatite as the first phase to precipitate. These previous studies (11, 21) worked with a solubility constant for HAP of 1.8*10-58  (log K = -57.74) which is similar to the solubility constant used here. Also Koutsoukos et al (22) reported the direct precipitation of a highly crystalline hydroxylapatite onto seeding material using a constant composition method and low supersaturation (SI=7). The precipitate was studied using X-ray diffraction.

A comparison of these studies with the results presented here indicates that the formation of ACP may depend on the supersaturation degree. In seeded experiments at high supersaturation, similar to the supersaturation degree necessary for homogeneous nucleation to take place (SI = 11), it is possible for ACP to form as precursor. Another possibility might be that both ACP and HAP will form simultaneously at a degree of supersaturation high enough for homogeneous precipitation to occur.

It should be noted that even though no precursor phase to HAP was observed here, and although every care

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