Crystallization and properties of glasses based on diopside-Ca-Tschermak's-fluorapatite system

The crystallization characteristics of glasses based on compositions in the diopside [CaMgSi2O6]-Ca-Tschermak's [CaAl2SiO6]-fluorapatite [Ca-5(PO4)(3)F] system have been investigated. The effect of Ca-Tschermak's/diopside replacement, at constant Ca-5(PO4)(3)F content, on the crystallization characteristics of the glasses and the solid solution phases formed, as well as the resulting microstructure, are traced by differential thermal analysis (DTA), X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). Various pyroxene solid solutions together with fluorapatite phases are detected by XRD analysis. There is a preferential tendency for diopside to capture Ca-Tschermak's in its structure forming pyroxene solid solution of diopsidie type. The maximum concentration of CaAl2SiO6 that could be accommodated in the diopside structure was 25%. Above this percentage, gehlenite Ca2Al2SiO7 also developed. However, there was no solid solution formed between pyroxene and fluorapatite. The thermal expansion coefficients (alpha-values) and microhardness of the glasses and glass-ceramics were determined. The data of the glasses were correlated to the internal structure of the glasses, nature and role played by glass forming cations. However, the properties of the crystalline glasses were mainly attributed to different factors including the crystalline phases formed, residual glassy phase and the microstructures. (c) 2004 Elsevier Ltd. All rights reserved.