磷灰石是一类含钙的磷酸盐矿物总称，其化学成分为Ca₅(PO₄)₃(F,Cl,OH)，其中含CaO为55.38%，含P₂O₅为42.06%，含F为1.25%，含Cl为2.33%，含H₂O为0.56%。最常见的磷灰石矿物种是氟磷灰石Ca₅(PO₄)₃F，其次是氯磷灰石Ca₅(PO₄)₃Cl、羟磷灰石Ca₅(PO₄)₃(OH)、氧硅磷灰石Ca₅[(Si,P,S)O₄]₃(O,OH,F)和锶磷灰石Sr₅(PO₄)₃F等。

磷灰石有三种生成方式，分别生成于火成岩、沉积岩和变质岩中，生成于火成岩中的为内生磷灰石，一般作为副产物在基性或碱性岩石中富集；在沉积岩中为外生磷灰石，是由生物沉积或生物化学沉积形成的，一般为结核状；在变质岩中生成的是经区域变质生成的。

Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite and chlorapatite, with high concentrations of OH, F and Cl ions, respectively, in the crystal. The formula of the admixture of the four most common endmembers is written as Ca₁₀(PO₄)₆(OH,F,Cl)₂, and the crystal unit cell formulae of the individual minerals are written as Ca₁₀(PO₄)₆(OH)₂, Ca₁₀(PO₄)₆(F)₂ and Ca₁₀(PO₄)₆(Cl)₂.