When topical fluoride reacts with the hydroxyapatite of enamel, it forms a more acid-resistant mineral known as:

Fluoride ions substitute for hydroxyl ions in hydroxyapatite to form fluorapatite/fluorhydroxyapatite, which has a lower critical pH (about 4.5) and greater resistance to acid dissolution, enhancing remineralization.Other options: Calcium carbonate, sodium bicarbonate, and silica are not products of the fluoride-enamel reaction.Key exam takeaway: Fluoride plus hydroxyapatite forms fluorapatite (more acid-resistant, lower critical pH), the basis of fluoride's anticaries effect.