Tooth enamel, when treated with dilute H3PO4 solutions, dissolved incongruently with formation of large CaHPO4 · 2HO2O crystals. Equilibrated solutions were saturated with respect to CaHPO4 · 2H2O, and a mineral more soluble than well-crystallized, synthetic Ca5(PO 4)3OH, probably an impure, defective apatite. The CaHPO 4·2H2O crystals formed at considerably higher pH values than expected because of enhanced solubility of the apatitic phase in enamel. Pyrolysis of carious enamel revealed the presence of acidic calcium phosphate presumed to be CaHPO4·2H2O.
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