Which compound is the main mineral in the bones and teeth of vertebrates?

Hydroxyapatite is the main mineral component of vertebrate bones and teeth.

Hydroxyapatite is a calcium phosphate mineral with the formula Ca₅(PO₄)₃(OH), often written in a doubled form as Ca₁₀(PO₄)₆(OH)₂. It forms crystals that reinforce the flexible collagen framework of bone and contribute hardness to tooth enamel.

Natural bone is not made of perfectly pure, large hydroxyapatite crystals. Its mineral phase is small, imperfect, and can contain substitutions such as carbonate. This structure helps bone combine strength with a degree of resilience and allows ongoing remodeling by living tissue.

Tooth enamel contains a particularly high proportion of mineral. Acids produced by oral bacteria can dissolve some enamel mineral, contributing to decay. Fluoride can be incorporated into apatite crystals and generally makes them more resistant to acid attack, which is one reason fluoride is used in dental care.