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The principal inorganic components of bone are phosphate and calcium ions, which nucleate to form apatite crystals (nanocrystals), most commonly hydroxyapatite represented by the chemical formula Ca₁₀(PO₄)₆(OH)₂. Significant amounts of bicarbonate, sodium, potassium, citrate, magnesium, carbonate, fluorite, zinc, barium, and strontium are also present. Infrared spectrometry shows the presence of different apatite molecules and carbonate substituting for both PO₄ and OH in many cases [22].

The precise form that the inorganic phase takes and its location relative to the collagen fibrils are poorly understood. Whether mineral forms within fibrils, outside them or a combination of the two remains contentious. There is evidence that mineral is initially deposited in the gaps within fibrils (between collinear collagen molecules) by a process of heterogeneous nucleation – a surface‐catalyzed or assisted nucleation process. However, there are those who argue that the data and the structural restraints imposed by collagen within the fibrils do not support or permit such an arrangement. Similarly, the morphology of the crystals is not universally accepted. There is evidence that mineral is deposited as needle‐like crystals, whereas others argue that it is really in the form of flakes or plates, which appear as needles when viewed from side on. There is general agreement though that the crystals are anisotropic: they are elongated along their crystallographic c‐axis, which is aligned parallel with the collagen fibrils. Schwarcz et al. [20] have recently proposed a model whereby mineral that is not in the form of apatite initially forms in the gap zones of fibrils. It then extends out into the extra‐fibrillar space where apatite crystals form sheets or lamellae that partially wrap around the fibrils (Figure 2.10). Several mineral lamellae may form around a single fibril, and lamellae surrounding one fibril and those of adjacent fibrils bind firmly together through strong bonds.

Figure 2.10 Schematic diagram showing progressive steps in the mineralization of collagen molecules in a single fibril, assuming that most mineral in bone is intrafibrillar. (a) Early mineralization in gap zones; (b) further mineralization extends into adjacent overlap zones.

Source: Landis et al. [23]. Reproduced with permission of Elsevier.