Quartz: amorphous silica, thermodynamic

The amount of H2O in amorphous silica (number n of H2O molecules per unit formula) varies between 0.14 and 0.83 (Frondel, 1962). Nevertheless, the thermodynamic properties of the phase are not particularly affected by the value of n (Walther and Helgeson, 1977). The molar volume of opal is 29 cm /mole. The same volume of a-quartz may be adopted for chalcedony see table 5.68 for the other polymorphs. 

Other crystalline polymorphs of Si02 have solubilities greater than that of quartz, the thermodynamically most stable (and thus least soluble) form of Si02, and less than that of amorphous silica, which is the least stable form of silica (cf. Dove and Rimstidt 1994). The only crystalline nonquartz silica polymorphs found in significant amounts at low temperatures are the minerals cristobalite and tridymite, which are both unstable relative to quartz at such temperatures (cf. Brown et al. 1978). The low- and high-temperature structural forms of the silica polymorphs are designated their a and JB forms, respectively. At 1 bar pressure, a-quartz is stable up to 573°C and / -quartz from... 

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