[en] In most nuclear cratering and cavity formation applications, the working fluid in the expanding cavity consists primarily of vaporized silica and steam. The chemical reaction products of silica and steam under these conditions are not known, although it is known that silica is very volatile in the presence of high-pressure steam under certain geologic conditions and in steam turbines. A review is made of work on the silica-steam system in an attempt to determine the vapor species that exist, and to establish the associated thermo-dynamic data. The review indicates that at 600-900 deg K and 1-100 atm steam pressure, Si(OH)₄ is the most likely silicon-containing gaseous species. At 600-900 deg. K and 100-1000 atm steam, Si₂O(OH)₆ is believed to predominate, whereas at 1350 deg K and 2000-9000 atm, a mixture of Si(OH)₄ and Si₂O(OH)₆ is consistent with the observed volatilities. In work at 1760 deg. K in which silica was reacted either with steam at 0.5 and 1 atm, or with gaseous mixtures of H₂/H₂O and O₂/H₂O at 1 atm total pressure, only part of the volatility could be accounted for by Si(OH)₄. Hydrogen was found to greatly enhance the volatility of silica, and oxygen to suppress it. The species most likely to explain this behavior is believed to be SiO(OH). A number of other species may also be significant under these conditions. Thermodynamic data have been estimated for all species considered. The Si-OH bond dissociation energy is found to be ∼117 kcal/mole in both Si(OH)₄ and Si₂O(OH)₆. (author)