Types of "H2O" in human enamel and in precipitated apatites

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 a-axis dimensions) and in the character of the IR absorption bands are correlated with weight losses at pyrolysis temperatures of 100 degrees to 400 degrees C and with effect of rehydration and reignition of previously ignited samples. This study demonstrated that the loss of "H2O" below 200 degrees C is reversible and causes no significant change in the lattice parameter of these apatites, whereas loss of "H2O" between 200 degrees and 400 degrees C is irreversible and causes a contraction in the a-axis dimension. It is proposed that two general types of "H2O" are present in these apatites: (a) adsorbed H2O--characterized by reversibility, thermal instability below 200 degrees C, and lack of effect on lattice parameters; and (b) lattice H2O--characterized by irreversibility, thermal instability between 200 and 400 degrees C, and induction of expansion in the a-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 P2O74- group) in precipitated apatites containing small amounts of CO32-.