The most abundant and widespread of the phosphate minerals, crystallizing in the hexagonal system, space group P6₃/m. The apatite structure type includes no less than 10 mineral species and has the general formula X₅(YO₄)₃Z, where X is usually Ca²⁺ or Pb²⁺ or As⁵⁺, and Z is F⁻, Cl⁻, or (OH)⁻. The apatite series takes X = Ca, whereas the pyromorphite series includes those members with X = Pb. Three end members form a complete solid-solution series involving the halide and hydroxyl anions and these are fluorapatite, Ca₅(PO₄)₃F; chlorapatite, Ca₅(PO₄)₃Cl; and hydroxyapatite, Ca₅(PO₄)₃(OH). Thus, the general series can be written Ca₅(PO₄)₃(F,Cl,OH), the fluoride member being the most frequent and often simply called apatite.