Apatite needlelike crystals

After nucleation in the apatite-leucite glass-ceramic, however, the apatite grows anisotropicaily in a preferred orientation into needlelike apatite (Holand et al., 1995). Previously, these crystals had only been produced under hydrothermal conditions (Newesely 1972 Jaha et al., 1997). Their morphology corresponds to that of the hydroxyapatite in natural teeth (enamel). 

Figure 2-33 Number (N) and length (L) of needlelike apatite crystals as a function of time for apatite-leucite glass-ceramics.

As reported the mechanisms of the nucleation and crystallization of needlelike apatite in apatite-leucite glass-ceramics is very difficult. Therefore, the mechanism was investigated in detail and the main results can be explained by the following two investigations. 

The primary crystals of apatite were determined at 700 C for 8 h heat treatment (Fig. 2-38). But these crystals did not show a needlelike habit. Needlelike fluoro-... 

Figure 2-38 Crystallization of needlelike apatite in the volume of the glass-ceramic after heat treatment at 700 C/8h aixl additionally at 1050°C h. SEM (10 sec, 2.5% HF). SEM micrograph of a representative microstructure arrows indicate the needlelike apatite and its special circled environment (diffusion area).

Heat treatment of glass powders results in the precipitation of leucite from the surface of the glass-ceramics and the volume crystallization of needlelike apatite. 

Another significant difference between needlelike apatite in glass-ceramics and other apatites is found in the fluoride content. The crystal structure of fluoroapatite compared with other apatites is shown in Section 1.3.2. The crystal structure of fluoroapatite is shown in Appendix Fig. 19. 

A typical apatite-leucite glass-ceramic is presented in Section 2.4.6. This section shows that the characteristic part of phase formation in the glass-ceramic is the controlled crystallization of the leucite and apatite crystal phases. The formation of the crystals and the typical microstructure of the glass-ceramic with the needlelike apatite crystals are described in Sections 2.4.6 and 3.2.12. 

Holand W, Rheinberger V, and Frank M., "Mechanism of Nucleation and Controlled Crystallization of Needlelike Apatite in Glass-Ceramics of the Si02-Al203-K20-Ca0-P205 Systems," / Non-Cryst. Solids, 253, 170-77 (1999). 

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