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Human dental calculi are biological calcium phosphates, which consist of an organic phase and an inorganic or mineral phase. In the latter phase, spectrochemical analyses have revealed the presence of several different magnesium and calcium phosphates. As the crystalline structure of the calculus passes through several stages during its allocation in the mouth, special attention is paid to some elements, such as zinc, that can modify the mineralization process. Several in vitro studies relating to the dental calculus mineralization process have been performed so far, but there is a lack of data obtained from biologically synthesized samples. The knowledge of the zinc distribution and incorporation in biological calcium phosphates is of great interest in providing more information about the biological process of calculus formation. In this paper we present surveys of the elemental distribution and incorporation of zinc in human dental calculus, by using a combination of different techniques: x-ray microfluorescence using synchrotron radiation, scanning electron microscopy and x-ray absorption spectroscopy. One-dimensional x-ray microfluorescence of zinc and magnesium measurement shows that there is a high accumulation of both elements in the sub-gingival region of the calculus and a strong correlation of their spatial distribution. Experimental Ca/P molar ratios were determined by energy-dispersive spectroscopy to identify different calcium phosphate phases, the sub-gingival region being composed of a mixture of highly and poorly calcified phosphates and the supra-gingival region composed mainly of carbonated hydroxyapatite. Finally, x-ray absorption measurements were carried out at the zinc K edge on synthetic and biological samples. The Zn—O distance and coordination number of the synthetic samples and the supra-gingival calculus show that zinc is adsorbed on these structures, whereas in the sub-gingival samples it is allocated in a cation site. The results are indicative of the active participation of zinc in the calcification process of sub-gingival calculus. Copyright © 2003 John Wiley & Sons, Ltd.

The first stages of crystallization have been investigated for a glass in the system SiO2-Al2O3-Li2O with small amounts of titanium and zirconium as nucleating elements. The studies were performed by electron diffraction and electron microscopy in high-resolution mode. It has been found that the two phases forming the final glass ceramic crystallize successively. The first one gives small crystallites (less than 5 nm) randomly dispersed. The lattice plane images in several orientations are in good agreement with TiZrO4 crystal lattice. At 780 degrees C the average crystal size increases with increasing time of the heat treatments from 2.5 nm at 0.5 h to 4.5 nm at 2 h and then remains nearly constant. The crystallization of the second phase, a silicate solid solution with beta-quartz structure, occurs after complete crystallization of TiZrO4.