Fluoride catalyzes remineralization of dissolved calcium and phosphate atoms back to hydroxyapatite; this also happens without fluoride, but less efficiently. I

Fluoride catalyzes remineralization of dissolved calcium and phosphate atoms back to hydroxyapatite; this also happens without fluoride, but less efficiently. In addition fluorine ions can substitute the hydroxide ions in hydroxyapatite, forming fluorapatite which is more resistant against acid.