Abstract

Mineralogical change of mudstone due to surficial weathering influences on engineering properties related to construction and geologic disaster. In this study the Poronai mudstone distributed in a landslide area of the central Hokkaido, was examined to understand the relationship between mineralogical change and deterioration of mudstone in weathering environment.

Mudstone of the Poronai formation is marine sedimentary siltstone of Eocene age. Major constituent minerals are quartz, plagioclase, K-feldspar, pyrite, calcite and clay minerals such as chlorite, illite, kaolinite and expandable clay minerals (smectite, mixed-layer minerals). The weathered mudstone can be divided into four classes: highly weathered, moderately weathered, slightly weathered and fresh mudstones. The slake durability and the seismic velocity of mudstone samples are concordant with the macroscopic classes of the mudstone weathering and show a positive correlation, suggesting that these tests are a useful tool in understanding the mudstone weathering.

In highly weathered mudstone of the Poronai mudstone, pyrite which is a common constituent mineral disappears. Chlorite, one of the ubiquitous constituent minerals of the mudstone, shows a characteristic reduction in quantity relating to the macroscopic classes of mudstone weathering, probably due to the chlorite dissolution by the sulfuric acid solution derived from pyrite oxidation. Mixed-layer minerals dominated in slightly weathered and fresh mudstones and smectite dominated in highly and moderately weathered mudstones can be understood to be alteration product from chlorite during weathering in the Poronai formation. These mineralogical changes would promote the deterioration of the Poronai mudstone, and could be a feasible factor controlling the landslide activity in this area.

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