Crystallisation of silicate glasses and melts with chemical compositions in primary phase region of gehlenite

Crystallisation behaviour has been investigated on samples with two compositions: (A) 42CaO-37SiO₂-21Al₂O₃ and (B) 43CaO-27SiO₂-19Al₂O₃-11CaF₂ (all mass%). Composition (A) is within the primary phase region of gehlenite and composition (B) is just between gehlenite and cuspidine on a mass basis. Two types of sample were prepared from glassy and molten states for each composition. These samples were characterised by differential thermal analysis, X-ray diffraction analysis and electron probe microanalysis. - Crystallised samples A from glassy state: It contained gehlenite and wollastonite in the XRD profile, and in BE images gehlenite crystals grew more largely than wollastonite, suggesting that gehlenite precipitates in preference to wollastonite. - Crystallised samples B from glassy state: It contained cuspidine and gehlenite in the XRD profile. In BE images, cuspidine precipitated but gehlenite was not observed. - Crystallised samples A from molten state: It contained only gehlenite in the XRD profile, and in BE images there was dendritic gehlenite developed across the sample. - Crystallised samples B from molten state: It contained cuspidine and CaF₂ as well as gehlenite in the XRD profile. In BE images, gehlenite comprised the major part, and it is likely that gehlenite crystallises in preference to cuspidine. It is also likely that the pseudo-binary system of gehlenite and cuspidine forms eutectics and that the eutectic composition is rather closer to cuspidine. Consequently, the increase in Al₂O₃ concentration would little affect crystallisation of the glassy portion in actual mould fluxes containing CaF₂ but strongly affects crystallisation of the liquid portion.