ABSTRACT

Hydrothermal experiments treating amorphous silica (opal-A) in an alkaline solution have disclosed. that opal-CT is formed from opal-A or silica-X and is transformed to quartz directly or through silica-K. During the transformation of silica, the d(101) spacing of cristobalite decreases continuously from d(101) = 4.11 A to 4.05 A, which suggests a progressive ordering of opal-CT. The decreasing rate of d(101) spacing is approximately expressed by equation (4) given in the text, irrespective of chemical conditions and the transformation sequence of silica. The rate constant is strongly dependent on the reaction temperature but more or less affected by the chemical condition and by the transformation sequence of silica. On the basis of the experimental results, the ordering of opal-CT is discussed in relation to the diagenetic process of siliceous sediments.