Hydroxylapatite in the context of "Hexagonal (crystal system)"

In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.

The geological definition of mineral normally excludes compounds that occur only in living organisms. However, some minerals are often biogenic (such as calcite) or organic compounds in the sense of chemistry (such as mellite). Moreover, living organisms often synthesize inorganic minerals (such as hydroxylapatite) that also occur in rocks.

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.