Thermal decomposition of human tooth enamel.

Further insight into human tooth enamel, dense fraction (TE), has been obtained by following the change and loss of CO3(2-), OH-, structurally incorporated H2O, Cl-, and, indirectly, HPO4(2-) after TE had been heated in N2 or vacuum in the range 25-1000 degrees C. Quantitative infrared spectroscopic, lattice parameter, and thermogravimetric measures were used. Loss of the CO3(2-) components begins at much lower temperature (e.g., 100 degrees C) than previously recognized, which has implications for treatments in vitro and possibly in vivo. CO3(2-) in B sites is lost continuously from the outset; the amount in A sites first decreases and then increases above 200 degrees to a maximum at approximately 800 degrees C (greater than 10% of the possible A sites filled), where it is responsible for an increase in a lattice parameter. A substantial fraction of the CO3(2-) in B sites moves to A sites before being evolved, apparently via a CO2 intermediary. This implies an interconnectedness of the A and B sites which may be significant in vivo. No loss of Cl- was observed at temperatures below 700-800 degrees C. Structural OH- content increases approximately 70% to a maximum near 400 degrees C. Structurally incorporated water is lost continuously up to approximately 800 degrees C with a sharp loss at 250-300 degrees C. The "sudden" a lattice parameter contraction, approximately 0.014 A, occurs at a kinetics-dependent temperature in the 250-300 degrees C range and is accompanied by reordering and the "sharp" loss of approximately 1/3 of the structurally incorporated H2O. The hypothesis that structurally incorporated H2O is the principal cause of the enlargement of the a lattice parameter of TE compared to hydroxyapatite (9.44 vs 9.42 A) is thus allowed by these experimental results.