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.