X-ray diffraction and thermal studies of crystals from the outer and inner layers of human dental enamel

X-ray diffraction showed that crystals from the outer layer had larger crystallite sizes, a-axis length and peak intensities than the inner layer crystals. Upon heating, apatites in both layers showed a contraction of a-axis length and changed their crystallite sizes along the direction of the a-axis independently of changes along the c-axis. The contraction of the a-axis length is attributed to loss of structurally incorporated water from the apatite lattice, causing void formation along the c-axis and altering the crystallite size normal to the c-axis. Whitlockite was detected in samples from both layers at temperatures above 800 degrees C, and the temperature required for whitlockite formation was considered to be influenced by the experimental atmosphere. The peak shift of whitlockite with temperature indicates that the whitlockite initially formed was magnesium rich. As to the amount of structurally incorporated water in both layers' apatites, the a-axis contraction data were inconsistent with the a-axis length data. This inconsistency indicates a larger degree of magnesium substitution in the inner layer apatite. A larger degree of magnesium substitution in apatite would show a smaller crystallite size, a weaker apatite peak intensity and a larger amount of whitlockite formation.