The crystallisation of glasses from the ternary CaF2-CaAl2Si2O8-P2O5 system

Abstract

A study of glasses from the ternary system CaF2-CaAl2Si2O8-P2O5 has been carried out. It has been shown that glasses with low phosphorus contents and high fluorite contents crystallise to fluorite. Fluorine reduces the glass transition temperature and is also required for the formation of fluorapatite (FAP). In the absence of fluorine in the glass no apatite phase is formed. Bulk nucleation of FAP is favoured for glasses with Ca:P ratios close to the apatite stoichiometry of 1.67 and with low crosslink densities. Thermal gravimetric analysis showed significant weight losses attributable to the formation of volatile silicon tetrafluoride to occur on crystallisation of the aluminium containing phases, anorthite and mullite, which supports the view that silicon tetrafluoride formation is hindered by fluorine bonding to the aluminium atoms of the glass network. Anorthite crystallisation always occurred by a surface nucleation mechanism and appeared to be favoured by the higher silicon to aluminium ratio in these glasses compared to previously studied glass compositions.