Chlorapatite in the context of "Fluoride"

Hydroxyapatite (IMA name: hydroxylapatite) (Hap, HAp, or HA) is a naturally occurring mineral form of apatite with the formula Ca₅(PO₄)₃(OH), often written Ca₁₀(PO₄)₆(OH)₂ to denote that the crystal unit cell comprises two entities. It is the hydroxyl endmember of the complex apatite group. The OH ion can be replaced by fluoride or chloride, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxyapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis.

Up to 50% by volume and 70% by weight of human bone is a modified form of hydroxyapatite, known as bone mineral. Carbonated calcium-deficient hydroxyapatite is the main mineral of which dental enamel and dentin are composed. Hydroxyapatite crystals are also found in pathological calcifications such as those found in breast tumors, as well as calcifications within the pineal gland (and other structures of the brain) known as corpora arenacea or "brain sand".

Fluorapatite, often with the alternate spelling of fluoroapatite, is a phosphate mineral with the formula Ca₅(PO₄)₃F (calcium fluorophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various colors (green, brown, blue, yellow, violet, or colorless), the pure mineral is colorless, as expected for a material lacking transition metals. Along with hydroxylapatite, it can be a component of tooth enamel, especially in individuals who use fluoridated toothpaste, but for industrial use both minerals are mined in the form of phosphate rock, whose usual mineral composition is primarily fluorapatite but often with significant amounts of the other.

Fluorapatite crystallizes in a hexagonal crystal system. It is often combined as a solid solution with hydroxylapatite (Ca₅(PO₄)₃OH or Ca₁₀(PO₄)₆(OH)₂) in biological matrices. Chlorapatite (Ca₅(PO₄)₃Cl) is another related structure. Industrially, the mineral is an important source of both phosphoric and hydrofluoric acids.