1983 Volume 78 Issue 12 Pages 459-466
Siliceous hard shales, which were found from the Gamanosawa Formation of Tertiary age, Aomori Prefecture, Japan, are mostly composed of opal-CT. The name of the opal-CT rock is used here instead of the hard shale. The content of SiO2 is in the range of 83 to 94 wt %. Broad diffused X-ray diffraction patterns correspond to the peaks of opal-CT. The spacing of 4 Å peak is in the range of 4.10 Å to 4.11 Å. Bulk density is in the range of 0.90 to 1.25, and apparent porosity in the range of 40 to 60 volume %. Crystallinity of the opal-CT rock is too poor to reveal phase transition. However, when it is heated over 1250°C in air, the crystallinity is improved and reveal phase transition at temperatures ranging from 182°C to 220°C. The spacing of 4 Å peak progressively decreased from 4.11 Å to 4.05 Å by heating it at high temperatures. By hydrothermal treatments at 500°C and 1000 kg/cm2 in a high pressure vessel, the spacing of 4 Å peak also decreased from 4.11 Aring; to 4.05 Å, and opal-CT gradually transformed into quartz. The opal-CT rocks may be a transitional stage from opal-CT to quartz due to diagenesis in nature.