Types of “H2O” in human enamel and in precipitated apatites

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Abstract
Types of “H2O” in human enamel and in precipitated apatites are characterized using X-ray diffraction, infrared (IR) absorption spectroscopic and thermogravimetric analyses. Changes in lattice parameters (principally in the α-axis dimensions) and in the character of the IR absorption bands are correlated with weight losses at pyrolysis temperatures of 100° to 400°C and with effect of rehydration and reignition of previously ignited samples. This study demonstrated that the loss of “H2O” below 200°C is reversible and causes no significant change in the lattice parameter of these apatites, whereas loss of “H2O” between 200° and 400°C is irreversible and causes a contraction in the α-axis dimension. It is proposed that two general types of “H2O” are present in these apatites: (a)adsorbed H 2O—characterized by reversibility, thermal instability below 200°C, and lack of effect on lattice parameters; and (b)lattice H 2O—characterized by irreversibility, thermal instability between 200 and 400°C, and induction of expansion in the α-axis dimensions of human enamel and precipitated apatites. Lattice H2O is assumed to be due to H2O-for-OH and/or HPO4-for-PO4 substitutions in these apatites. Loss of adsorbed H2O caused sharpening of the OH absorption bands in the spectra of these apatites. Loss of lattice H2O caused the appearance of P−O−P absorption bands (due to the presence of P2O7 4− group) in precipitated apatites containing small amounts of CO3 2−. The observed larger α-axis of human enamel apatite, i.e., 9.445±0.003A, compared to that of the mineral or synthetic (prepared at 1000°C) OH-apatite, i.e., 9.442A, may be attributed to the presence of lattice H2O, Cl-for-OH, and concerted substitutions of larger cations (e.g., Sr, Ba, Pb, K) for Ca in this apatite.