Transformational changes of clay minerals, zeolites and silica minerals during diagenesis



Diagenetic transformation of clay minerals, zeolites and silica minerals in Cretaceous and Tertiary argillaceous rocks from deeply drilled wells in Japan were studied. Transformations of these minerals during diagenesis were as follows: in clay minerals, montmorillonite → montmorillonite-illite mixed-layer mineral → illite; in zeolites, volcanic glass → clinoptilolite → heulandite and/or analcite → laumontite and/or albite; in silica minerals, amorphous silica → low-cristobalite → low-quartz. Maximum overburden pressures and geothermal temperatures corresponding to these transformations in each well studied were calculated. For clay minerals, a pressure of approximately 900 kg cm−2 and a temperature of about 100°C are necessary for the transformation from montmorillonite to mixed-layer mineral and 920 kg cm−2 and 140°C for mixed-layer mineral to illite. Transformation from kaolinite to other minerals requires much higher pressures and temperatures than from montmorillonite to mixed-layer mineral. For zeolites, 330 kg cm−2 and 60°C are required for the transformation from volcanic glass to clinoptilolite, 860 kg cm−2 and 120°C for clinoptilolite to heulandite and/or analcite, and 930 kg cm−2 and 140°C for heulandite and/or analcite to laumontite and/or albite. For silica minerals, 250 kg cm−2 and 50°C are necessary for the transformation from amorphous silica to low-cristobalite and 660 kg cm−2 and 70°C for low-cristobalite to low-quartz. Based on these diagenetic mineral transformations, seven mineral zones are recognized in argillaceous sediments. On the other hand, from the porosity studies of argillaceous sediments in Japan, the process of diagenesis is classified into the following three stages. The early compaction stage is marked by shallow burial and viscous rocks with more than 30% porosity. The late compaction stage is characterized by intermediate burial and plastic rocks with 30-10% porosities. The transformation stage is marked by deep burial and elastic rocks with less than 10% porosity.