Effect of the redox state and concentration of iron on the crystallization behavior of iron-rich aluminosilicate glasses

Two glasses from the SiO2–Al2O3–MgO–CaO–FeO system were investigated concerning the effect of the iron content and the redox state of iron on the crystallization behavior. Thermal analysis of the samples was made using a differential scanning calorimeter. Systematic heat-treatments on the two glasses were conducted in the temperature range where the crystallization process takes place. The crystalline phases formed in the glasses were identified by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It is found that the concentration and the redox state of iron have a significant impact on both the thermal and the crystallization behavior. The crystal phases identified in the reduced and oxidized glasses are augite/diopside, and traces of anorthite and akermanite are also seen in the oxidized glasses. The morphology of the crystals in the oxidized glasses is dendrite-like whereas in one of the reduced glasses star-like crystals are formed. In the other reduced glass with low iron concentration, no clear crystal pattern is seen but cracks have developed.