Allotropy in Silica

There are 15 allotropes of silica. Most of them are metastable, and their transformations do not bring about much change in volume. 

At 1470°C, tridymite transforms to cristobalite. During this transformation, there is a contraction of 1.95%. Again this transformation is a slow process. Once transformed, cristobalite becomes stable up to the melting point, which is 1723°C. 

On cooling, P-cristobalite transforms to tridymite at 1470°C. This transformation is a slow process. Hence, unless the cooling is very slow, tridymite continues to remain in the same state up to 275°C. In the temperature range of 275°C-200°C, it transforms to a-cristobalite, which is a metastable state. This transformation is accompanied by a 3.15% decrease in volume. The highest temperature form of tridymite is P4. Because its equilibrium transformation to P-quartz is a slow process, it readily undergoes metastable transformations when cooled. These are P3 at 476°C, P2 at 210°C, Pj at 163°C, and a at 117°C. The contraction in volume for the P2 to Pi transformation is 0.18% and for Pj to a is 0.45%. The other two metastable transformations have negligible volume changes. 

Because of the many transformations involved in silica, as well as the volume changes, heating and cooling of silica bricks should be conducted very slowly. This ensures equilibrium transformations and will not result in any damage in the form of cracking and spalling. There are certain exceptions with respect to the actual behavior of silica bricks. Tridymite is less dense than vitreous silica. Stable tridymite is possible only with the help of impurities. 

The most important property of silica bricks is its very high refractoriness under load (RUL). This is due to interlocking tridymite needles. These bricks 

---