When enamel is exposed to preparations containing high concentrations of fluoride, the major reaction is:

The correct answer is fluoroapatite. Fluoroapatite is a mineral that forms when fluoride ions integrate into the crystal lattice of hydroxyapatite, the primary component of tooth enamel. This process occurs particularly when enamel is exposed to high concentrations of fluoride, resulting in a more acid-resistant and durable form of enamel.

Fluoroapatite has enhanced structural stability compared to hydroxyapatite, making it less susceptible to demineralization from acidic environments. This characteristic is particularly beneficial in preventing dental caries, as it strengthens the enamel and reduces the likelihood of decay.

Other substances like sodium fluoride, calcium fluoride, and stannous fluoride are associated with different reactions or forms of fluoride in dental care but do not lead to the same beneficial conversion to fluoroapatite. Sodium fluoride primarily acts as a source of fluoride ions, while calcium fluoride can provide a reservoir of fluoride without engaging in the transformation into a stable crystalline structure like fluoroapatite. Stannous fluoride offers antibacterial benefits and some remineralization potential, but it is the formation of fluoroapatite that plays a crucial role in enhancing enamel resistance to decay.