Fluorapatite (FA) is formed when fluoride exchanges with the hydroxyl groups of present hydroxyapatite crystals in the bone, which is less soluble in acid.

Where does hydroxyapatite come from?

Hydroxyapatite is a naturally occurring mineral of biological and agricultural importance. Human and animal bones are composed of hydroxyapatite. In nutrient recovery processes, hydroxyapatite is generated through treatment of waste streams containing calcium.

How does hydroxyapatite turn into fluorapatite?

When hydroxyapatite dissolves under cariogenic (acidic) conditions, if fluoride is present, then fluorapatite will form. Because fluorapatite is less soluble than hydroxyapatite, it is also more resistant to subsequent demineralization when acid challenged (Figure 4).