Crystal characteristics of tooth enamel and dentin were investigated using XRD, SEM, and EPMA methods. The results show that the mineral phase in enamel is HA and in dentin is HA and minor whitlockites. The dentin HA and the enamel HA have different crystallinity, the crystallinity of enamel HA is much higher than that of dentin HA. The average particle size of the enamel HA and dentin HA are 897 A and 309 A, respectively. The HA in enamel is regularly arranged, and in dentin the arrangement of HA is different from the enamel HA in the same section. Both the enamel and the dentin are mainly. consisted of Ca, P, O, and C, and the trace elements Mg, Sr, Al, Na, and K. The dentin contains more trace elements than the enamel. However, the incorporation of trace elements in both dentin and enamel are very limited. Other impurities such as F and Cl are less than their detection limit. The a and c values of enamel HA are 9.433 A and 6.896 A, and those of the dentin HA are 9.498 A and 6.896 A, respectively. The expansion in a value results from those the larger size of [ CO3 ]^2- group substituing for the smaller [ OH ] ^- group in the channel, and replacement of [ OH ]^ - by [ CO3 ]^2- dominates the change in cell parameter, taking into account of other trace elements.
The surface properties of kaolinite were determined using density functional theory discrete variational method （DFT-DVM） and Gaussian 03 program. A SiO4 tetrahedral hexagonal ring with two A1 octahedra was chosen to model the kaolinite crystal. The total density of states of the kaolinite cluster are located near the Fermi level at both sides of the Fermi level. Both the highest occupied molecular orbit （HOMO） and the lowest unoccupied molecular orbit （LUMO） of kaolinite indicate that kaolinite system can not only readily interact with electron-acceptor species, but also readily interact with electron-donor species on the edge surface and the gibbsite layer surface, and thus, shows amphoteric behavior. Substitution of Al3^＋ for Si4＋ in the tetrahedral site linking the vacant Al3^＋ octahedra does not increase the surface chemical reactivity of kaolinite, while substitution of Al3^＋ for Si^4＋ in the tetrahedral site with the apex O linking Al3^＋ octahedra increase the surface chemical reactivity of the siloxane surface of kaolinite, especially acting as electron donors. Additionally, substitution of Al3^＋. for Si^4＋ in the tetrahedral site results in the re-balance of charges, leading to the increase of negative charge of the coordinated O atoms of the AlO4 tetrahedra, and therefore favoring the formation of ionic bonds between cations and the surface O atoms in the basal plane.
