ABSTRACT

Opal-CT in sediments is formed from opal-A and is converted to quartz during diagenesis. In that process, opal-CT becomes progressively ordered as is distinctly indicated by the decrease in d(101) spacing. Based on a kinetic equation experimentally found by KANO (1979) and KANO and TAGUCHI (1982), the decreasing rate of d(101) spacing has been calculated from previously reported d(101) spacings (MITSUI, 1975) and thermal history of sediments in a borehole, MITI Hamayuchi. The calculated rate makes a subtle difference between the reported and calculated d(101) spacings, suggesting the availability of the equation applied. However, even if compared at the same temperature, the rate is much slower than that in hydrothermal alkaline conditions. Thus, the ordering process of opal-CT is more or less chemically controlled. Taking the thermal history of sediments into account, a model based on the kinetic equation well fits the previously reported relationship between the d(101) spacing of Neogene siliceous sediments and the maximum temperature to which the sediments have been subjected (IIMIMA and TADA, 1981), suggesting that the ordering process of opal-CT in diagenetic conditions is controlled mainly by temperature. The features of distribution of d(101) spacing in sediment piles have been discussed by using the model.