Crystallization and microstructure of glasses in the system Na{sub 2}O/MnO/SiO{sub 2}/Fe{sub 2}O{sub 3}

Glasses with the compositions (100 - x)(0.16Na{sub 2}O/0.10MnO/0.74SiO{sub 2})/xFe{sub 2}O{sub 3}, (x = 5-30) and 16Na{sub 2}O/10MnO/(74 - y)SiO{sub 2}/yFe{sub 2}O{sub 3} (y = 5-30) were studied using X-ray diffraction and scanning electron microscopy. The effect of the chemical composition and the thermal history on the phase formation and the resulting microstructure was investigated. During cooling, the precipitation of ferrimagnetic solid solutions Fe{sub 3}O{sub 4}/Mn{sub 3}O{sub 4} was observed. These crystals show dendritic or platelet shape, whereby the platelets are ferromagnetic and the dendrites - mainly paramagnetic. The tendency towards crystallization can be suppressed by increasing the Na{sub 2}O-concentration. In contrast to glasses without manganese oxide, the precipitation of hematite is not observed. Therefore, the addition of reducing agents is not required, in order to crystallize large volume concentrations of the ferrimagnetic phase.