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The crystallographic structure of surface coatings produced by three topical fluoride agents deposited by a single application on intact human enamel surfaces was investigated by light- and dark-field transmission electron microscopy, as well as by electron and x-ray diffraction. The agents studied were an acidic silane fluoride lacquer, a neutral NaF lacquer, and an APF gel. The smallest coherently diffracting particles were CaF2 microcrystallites 4-15 nm in diameter. Large apatite-like crystals, approximately 1 micron in length, were also observed in the outer surface coating produced by the silane fluoride lacquer.
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Nelson et al. (1984) studied this question.
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