Hydroxylapatite or hydroxyapatite (HAP) is a calcium-phosphate-based mineral of the apatite family. Its chemical formula is Ca10(PO4)6(OH)2. It can be found widely in nature and is the major component of bone, enamel, and dentin in teeth, antler, ganoid fish scales (in alligator gar and Senegal bichir), turtle shells, and armadillo and alligator osteoderms. It exists in minute quantities in the brain (brain sand), without significantly affecting its function. Thus, the expression “having sand in the head” is not without reason. The density of HAP is 3.15 g/cm3. Nonstoichiometric minerals can exist with Ca10(PO4)6(OH, F, Cl, Br)2; if the OH group is replaced by F it is called fluoroapatite; if it is replaced by Cl, it is called chloroapatite. It can be occasionally used as a gem, and the cat’s eye is a commonly known use.
In this chapter, we will concentrate on bone and teeth with emphasis on their structure and mechanical properties. They are HAP–collagen composites and their mechanical properties are the result of the complex interplay and hierarchy built by these structures. Selected calcium-phosphate-based bony tissues with unique functionalities, such as antler, turtle shells, alligator osteoderms, and fish scales, will also be described in the second part of this chapter.