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Dedicated to Professor G. BECHERER on the occasion of his 65th birthday The addition of NiO t o a MgO-Al,O,-SiO, basic glass has an influence on the phase separation and crystallization mechanism. The differentiation of the components already detectable in the amorphous state continues in the crystallization process of the nickelrich glasses. The formation of Ni-Al-spinel in the bulk restricts the precipitation of high quartz crystals to the skin. The result is a high strength glass-ceramic with clearly distinguishable bulk and surface regions.