Microstructural insights into the stabilization of β-cristobalite

A glass with the molar composition 6.75 CaO – 6.75 Al<inf>2</inf>O<inf>3</inf>– 86.5 SiO<inf>2</inf> was prepared and subsequently crystallized at temperatures between 1100 and 1400 °C. As main crystal phase, cristobalite forms. The higher the temperature of heat treatment, the greater the amount of α-cristobalite. Lower temperatures favor the formation of the β-phase. The high temperature phase (β-cristobalite), which is known for its comparatively low coefficient of thermal expansion at high temperatures, is chemically stabilized by the incorporation of Ca²⁺ and Al³⁺. However, microstructural insights and elemental analyses show that the mechanism of β-formation at room temperature is not a pure chemical stabilization. The maximum solubility of CaO and Al<inf>2</inf>O<inf>3</inf> within the cristobalite lattice was measured and is considerably lower than the composition of the base glass. A matrix, which is enriched in CaO and Al<inf>2</inf>O<inf>3</inf>, surrounds the crystals. Milling experiments, where the constraints from the matrix are removed, show that a pure chemical stabilization of β-cristobalite is not possible and is always supported by the (often undetected) surrounding matrix.