Abstract
The formation process of the clay minerals in clay veins (Seam or gouge) found in caves of dam-site at Nukui, Kake-cho, Yamagata-gun, Hiroshima Prefecture were investigated by X-ray diffraction, DTA, TEM, the chemical analysis and so on. The clay mineral, montmorillonite mineral and the interstratified mineral of mica-swelling layer accompanied with a small amount of kaolinite. Mica clay mineral and the interstratified mineral are found in veins which are situated at the lower level (lower than 260 m in altitude), while montmorillonite mineral exists dominantly in the upper lever (upper than 260 m in altitude). The water table is situated at about 250-260 m in altitude. The change of clay minerals in the lateral direction of clay veins are not recognized.
Mica clay minerals are identified to be 1 Md polytype. The differential thermal curves give one endothermic peak at 600-700°C. The crystal forms are well developed elongated plates or irregulary particles. The interstratified minerals are produced by two component stacking of 40-60% mica clay mineral layer and 60-40% swelling-layer. The differential thermal diagram of the interstratified mineral shows double endothermic peaks in the range of 500-700°C. Their crystal forms are similar to those of mica clay minerals. The electron diffraction pattern shows hexagonal network pattern overlapping with ring patterns, which is similar to that of hydrobiotite. The 14Å basal spacing of montmorillonite mineral expands to 17Å with ethylen glycol treatment, and reduces to 10Å, which is similar to the peak of mica clay mineral, with heating at 500°C. The differential thermal diagram of montmorillonite mineral exhibits two endothermic peaks at 500-700°C. Though montmorillonite minerals show as a general feature very thin irregular lamellae of different sizes which are partly folded, some of them are recognized as lath shaped particles.
Chemical compositions of the interstratified mineral and montmorillonite mineral are similar to those of mica clay mineral except K2O and CaO contents infered to be interlayer cations. The exchangeable cations in montmorillonite and the interstratified mineral are almost Ca2+ ion. The K2O content in clay minerals decreases linearlly with increasing the proportion on the swelling layer, while the CaO content in clay mineraisincreases in proportion to the decreases of the K2O content.
These results indicat that mica clay minerals in clay veins were converted to montmorillonitelike mineral through the interstratified mineral of mica-swelling layer by weathering. The change of clay minerals in the vertical direction of the veins were caused by the change of the level of the ground water table.
