泥質岩の風化指標としての微細黄鉄鉱抽出法

Applying to the indicator of mudstone weathering, an extracting method of pyrite micrograins and the secondary materials formed by oxidation and hydroxidation was examined using water diluted HF (3%). Based on the condition of extracted materials, the weathering grade of the drilling-cores of the Lower Cretaceous Fujikawa Shale Formation was subdivided into Oxidized- (weathered), Fresh-Oxidized Coexistence-, and Fresh- (non-weathered) zones in the order from the ground surface. The Fresh Zone is identified by only occurrence of non-altered primary pyrite micrograins that originate a bio-chemical process in sedimentary stage, and are characterized by aggregation of framboidal (raspberry-like), tubal (pellet-like), cubic and dodecahedral shapes. In the Fresh-Oxidized Coexistence Zone, pyrite micrograins and the secondary materials (hematite, goethite and limonitic hydrous ferric oxides) formed by oxidation and hydroxidation of pyrite co-exist, whereas the Oxidized Zone is characterized by the occurrence of ferric secondary materials without primary pyrites. A pyrite-alternation model was proposed in relation with the progressing of slaking. The Extracting-method of pyrite micrograins is useful even the case that the pyrite peak is not detected by X-ray analysis because of lower containing percentage.