MgO-CaO-Al2O3-SiO2 glassy and glass-ceramic systems: the effect of substitution of CoO or NiO in place of MgO or CaO

Nickel and cobalt were separately incorporated in MgO-CaO-Al2O3-SiO2 glasses Their effect on the crystallisation phenomenon of the amorphous materials into a glass ceramic was investigated by means of thermal analysis and x-ray powder diffraction. These two transition cations decrease the crystal growth of anorthite and diopside and nickel produces forsterite formation. The ionic state and the coordination geometry of Ni and Co ions in the vitreous and the devitrified states have been characterised by means of electronic spectroscopy. Divalent ions were observed for nickel and cobalt in both the glasses and the glass ceramics; in the glasses both Ni2+ and Co2+ are in tetrahedral symmetry, while in glass ceramics the symmetry is predominantly octahedral. Colour changes from glass to glass ceramics correlated well with the cations coordination. The atomic distribution of Ni and Co in the glass ceramics obtained by x-ray fluorescence dispersion spectroscopy complement the information about their coordination environment, the first being present in some crystalline areas and the second being mostly segregated in the residual glass at the grain boundary of crystalline zones.