Figure 4. a, b) The lattice cell structure of hydroxyapatite. c) Types of Hydroxyapatite Vibrational Modes.
Carbonate
The biomineralization process of calcium carbonate is similar to that of hydroxyapatite, both occurring by interaction with biological substrates. Mostly as amorphous calcium carbonate (ACC), aragonite, and calcite occur in the shells of crustaceans and the teeth of sea urchins.[60,61,168], a portion of carbonate is also present in vertebrate enamel and bone. During the maturation of hydroxyapatite, carbonate ions replace the hydroxyl or phosphate in it, thus changing the stability and solubility of the mineral phase. The embedding of carbonate in the lattice causes a significant increase in crystal solubility and deterioration of crystallinity.[169–171] The result shown in the Raman spectrum is an increase in the ratio of ν1CO32-1PO43-, indicating a decrease in crystallinity (Table 2 ). A-type carbonate (OH substituted by CO32-) and B-type carbonate (PO43- substituted by CO32-) are both present in enamel and bone, but the exact spatial distribution is unknown.[172,173] It is worth affirming that the content of B-type carbonate dominates and is associated with increased B-type carbonate substitution (the ratio of B-type/A-type increased.) and deterioration in the mechanical properties of biomineralized tissues.[174–176] The increase in B-type substitution also leads to an increase in the half-peak width of ν1 PO43-, indicating an increase in hydroxyapatite crystal defects.
Table 1. Raman spectroscopic band assignments for biomineralization.