Experimental investigation shows that amorphous silica is converted into quartz through cristobalite under hydrothermal conditions. The rate of transformation, essentially dependent on the reaction temperature, was studied on the basis of quantitative analyses of quartz in the products, and the variation of quartz content was elucidated by taking the induction period into consideration. The transformation is a consecutive reaction involving two first-order steps, from amorphous silica to low-cristobalite and from low-cristobalite to low-quartz. Kinetic data such as the rate constants and the energy of activation are obtained.
Mineralogical varieties of silica found in siliceous deposits suggest that the transformation probably takes place under natural conditions. Younger deposits contain amorphous silica or cristobalite, whereas older ones are invariably composed of quartz. By assuming that the transformation process observed experimentally holds in nature, kinetic data can be estimated. Since the process depends upon the thermal history, it is possible to follow the transformation process for a given model of a thermal history. An example of computed results is compared with the geological evidence, and it is concluded that the mineralogical variation of silica is accounted for by the transformation process.
Zonal distribution of silica minerals apparently corresponds to that of zeolite minerals, and the transformation of silica is considered to occur during diagenesis. Evidently, almost all diagenetic facies represent intermediate stages on the way to the ultimate equilibrium. The cristobalite stage described in this paper belongs to one of these stages, and is referable to the early stage of diagenesis. It is suggested that silica minerals can be used as clues to understand a progressive change in diagenesis.