Directional solidification and microstructural evolution between diopside and anorthite in the system CaO-MgO-Al 2 O 3 -SiO 2

Abstract

Silicate mixtures close to the diopside–anorthite tie line have been unidirectionally solidified by float–zone melting in an arc image furnace and the microstructures examined by optical microscopy, scanning electron microscopy, electron microprobe analysis and from electron backscatter patterns. The structures which evolved have been interpreted by reference to available phase diagram data for the anorthite–diopside eutectic surface within the four–component oxide system, CaO–MgO–Al2O3–SiO2. It is demonstrated how the microstructures and segregation paths can be described in a visually explicit way within the quaternary system. The morphologies of primary diopside, anorthite and the eutectic mixture are described and discussed. In this silicate system the low liquid diffusion rates lead to glass formation at rapid cooling rates and at temperatures close to the freezing point, so that local equilibrium is only partially realized. The consequences are interpreted with reference to the quaternary phase diagram.