The Conversion of Quartz Into Cristobalite Below 1000°c, and Some Properties of the Cristobalite Formed*

An investigation of the conversion of quartz into the higher temperature forms of silica was made in order to establish whether cristobalite or tridymite is first formed in the temperature range 850 to 1050deg;C. The results of over 800 determinations established the fact that sodium silicate promotes the conversion and is materially superior to other fluxes reported in the literature. Cristobalite forms before tridymite in the conversion of quartz at 890 to 900°C. The rate of conversion is dependent upon time and temperature. The cristobalite in the temperature range studied was metastable and slowly converted to tridymite. The inversion of the low to the high form of cristobalite occurred at 250 to 260°C on heating. The refractive index was found to be in agreement with that for cristobalite formed at higher temperatures. The conversion is believed to be one of a solid‐liquid type, and a theory to explain the formation of cristobalite before tridymite is offered, based on the crystal lattice structure changes required to develop the two high‐temperature forms of silica from quartz.