インタクトな花崗岩類の拡散性 ・ 透水性研究における 鉱物粒内変形微小構造の意義 Significance of Deformation Microstructures of Mineral Grains in the Study of Diffusivity and Permeability in Intact Granitoids
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インタクトな花崗岩類の場合でも, 構成鉱物粒内には多かれ少なかれ変形微小構造が形成されている. それらの微小変形構造は, 微小割れ目形成, 微小断層, 微小褶曲, 回復, 再結晶という過程を通じて形成される. これらの微小変形構造の中のサブミクロンサイズの割れ目やガウジはインタクトな花崗岩類の拡散や透水の通路になっている. 実際に, 過マンガン酸カリウム水溶液を用いた毛管実験によって, いわゆるマイクロクラックやグレインバウンダリー以外の鉱物粒内にも水溶液の通った痕跡がEPMAにより確認される. 変形微小構造内にサブミクロンサイズの割れ目やガウジが多く作られていることから判断して, インタクトでマイクロクラックの少ない花崗岩類の拡散や透水を評価する上で, 偏光顕微鏡による変形微小構造の種類や量の評価は重要な視点となろう.
Microscopic observation of intact granitoids reveals that different kinds of deformation microstructures within mineral grains result from various processes; deformation microstructures due to microcracking, microfaulting, microfolding, recovery and recrystallization. Some samples of intact granitoids exhibit only the undulose extinctions of quartz, plagioclase and biotite, while the others show abundant deformation microstructures, indicating that they contain many submicron-sized apertures in them. These deformation microstructures may be important as a possible cause of diffusivity and permeability of intact granitoids.<BR>Water pathways within intact granitoids are detected in an experiment based on the capillarity of a KMnO<SUB>4</SUB> solution. The EPMA images show that MnO<SUB>2</SUB> is present on the polished surfaces of orthoclase, plagioclase and biotite grains after the experiment. MnO<SUB>2</SUB> occurs in three ways : on grain-boundaries and/or intragranular cracks, in clustered submicron-sized pores, and dispersed on plagioclase and biotite grains. These modes of occurrence suggest that water pathways in intact granitoids are not only grain-boundaries and/or intragranular microcracks, but may also be mineral grains themselves that are permeated with submicron-sized apertures possibly having formed in deformation microstructures.<BR>It follows that deformation microstructures should be most apparent at sites where abundant submicron-sized apertures are present. There are differences of deformation grade even in intact granitoids. This means that some intact granitoids include relatively few deformation microstructures, whereas others include many deformation microstructures. Since deformation microstructures are possible pathways for water, it is very likely that the diffusivity and permeability of a granitoid sample correlates with the nature and abundance of its included deformation microstructures. These microscopic observations and experiment suggest that deformation microstructures, as well as microcracks, should be studied much more thoroughly with regard to the diffusivity and permeability of intact granitoids.
- Journal of the Japan Society of Engineering Geology
Journal of the Japan Society of Engineering Geology 40(3), 159-165, 1999-08-10
Japan Society of Engineering Geology