COLORING INORGANIC OXIDES IN MGO-CAO-AL2O3-SIO2 GLASS-CERAMIC SYSTEMS

Glasses in the system MgOCaOAl2O3SiO2, in which CuO, Fe2O3 and MnO were substituted for CaO, were prepared and crystallized in absence of nucleating agents. Surface nucleation dominated for all compositions, and was followed by growth of only anorthite and diopside. The relative amount of each crystal was influenced by the chemical composition and the nature of the transition metal cation. Cu and Mn segregated at the grain boundary during crystallization and Fe did not. Changes in the coordination symmetry and oxidation states of the transition metal ions in the glassy and glass-ceramic phases are as follows: CuII is sixfold coordinated with regularly octahedral symmetry in glass and distorted in glass-ceramic, while the ratio CuII/CuI is decreased from 5 to 2 during crystallization. FeIII is fourfold coordinated with a symmetry C2V and FeII is present, more in the glass-ceramic than in the glass, with an octahedral symmetry. MnII is in cubic site symmetry with a rhombic distortion, more visible in the glass; MnIII is sixfold coordinated with octahedral symmetry in a constant ratio (MnIII:MnII is 2:1) during crystallization.