Abstract
The Poronai formation area located in Central Hokkaido is mainly composed of Eocene Tertiary marine mudstone. This mudstone can be classified into four different zones depending upon the degree of weathering: a highly weathered, a moderately weathered, a slightly weathered and an unweathered zone. The crystallite size (by XRD) of chlorite in the mudstone shows a variation with the degree of weathering of the mudstone. In addition, the amount of pyrite and calcite in mudstone decreases with increases in the degree of weathering. These mineralogical changes in mudstone may be due to the sulfuric acid produced by pyrite oxidation. Particularly, in highly weathered zones, pyrite and calcite have dissolved and been removed. Weathered mudstone contains cracks, deteriorates in slaking tests and breaks easily. The chlorite mineralogy and the mudstone chemistry may be used as a weathering index for the Poronai mudstone.
