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Vol. 126 (2017) No. 3 特集号:風化─ナノスケールからグローバルスケールまで─1. 微視的風化と基礎研究 p. 343-354




 This study aims to clarify the solution characteristics of carbonate rock tablets in a tropical savannah climate. The study site is located within the ruins of the Angkor Wat Temple in north western Cambodia. Two kinds of carbonate rock from Japan were used: middle Carboniferous limestone from the Akiyoshi-dai Plateau and marble from the Abukuma Mountains. The tablets used in the experiment were fashioned into discs 40 mm in diameter and 4 mm in thickness. They were then placed horizontally in a secure rack 120 cm above the ground, next to a weather station, and remained exposed from August 24, 2011 to March 15, 2014 (a duration of 934 days). The tablets were weighed before, during, and after the experiment in order to measure changes due to dissolution effects. The initial weights of the tablets were 13.624 g for limestone and 13.700 g for marble; on day 934, their dry weights were 13.523 g for limestone (99.26% of initial weight) and 13.630 g for marble (99.49% of initial weight). The experiment produced three major results: (1) limestone dissolves faster than marble; (2) solution occurred most rapidly during the initial test period, and thereafter progressed constantly but at a lower rate; (3) the observed solution rate under a tropical savannah climate was lower than previously reported values from a humid temperate climate derived from equivalent methods. The faster solution of limestone is thought to occur because limestone has a higher composition of fine-grained calcite than marble; therefore, it has a larger surface area, which is more conducive to solution reactions. In tandem with surface solution of post-test tablets, calcite surfaces are covered with recrystallized products, such as speleothems. Thus, under a tropical savannah climate with a clearly defined dry season, recrystallization associated with moisture loss is likely to reduce the solution rate. For solution processes to progress most effectively within carbonate rock, appropriate temperature conditions are not sufficient; continuous contact with water is also essential.

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