Cristobalite

Cristobalite, the highest-temperature polymorph of silica, was named after the place where it was discovered, the San Cristobal mountain in Mexico. Interestingly, silicate phases including cristobalite have also been found in cosmic dust collected by space vehicles [9]. The high cosmic and terrestrial abundance of silicas makes knowledge of their physical and chemical properties especially important in fields such as geology, chemistry, and physics. Cristobalite, like tridymite and keatite, is isostructural with ice polymorphs (i. e. cubic ice Ic).

Cristobalite has the Si atoms located as are the C atoms in diamond, with the O atoms midway between each pair of Si [13]. Like other crystalline polymorphs of silica, cristobalite is characterized by corner-shared SiO4 tetrahedra. In addition, Liebau [9] noted that cristobalite, like quartz, exists in two forms having the same topology, with variations mainly in the Si-O-Si bond angles. Thermodynamic variables (such as pressure and temperature) and kinetic issues will determine which of these phases is formed. The interconversion of quartz and cristobalite on heating requires breaking and re-forming bonds, and consequently, the activation energy is high. However, the rates of conversion are strongly affected by the presence of impurities, or by the intro­duction of alkali metal oxides or other “mineralizers.”

Cristobalite has been well-characterized since the late fifties [7,9]. The high – cristobalite structure is characterized by a continuously connected network of (SiO4)4- tetrahedra and is summarized in Table 3. The atomic model of the high-cristobalite structure in Fig. 6 [11] was generated with Accelrys Catalysis 3.0.0. Also, the Si-O distances have been noted to range between 0.158 nm and 0.169 nm.