Weathering of Some Norwegian Alum Shales

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Abstract
The investigations have shown that Norwegian alum shales have the capacity of producing large amounts of acidity. The amounts produced, though strongly correlated with the amount of pyrite in the material, also depends upon the quality of the pyrite, i.e. its weatherability. A shale sample collected from a road-cut near Brumunddal (BRUM 1) contains much higher amounts of pyrite than samples from Storting and Munchsgate, Oslo (STORT and MUNCH respectively), but the relationship between % pyrite remaining and acid produced is, for example, much stronger in STORT than in BRUM 1. The pyrite in STORT and MUNCH appears as minute grains, finely distributed in the material, whereas in BRUM 1 it appears more as concretions. This is a determining factor in the rate of weathering and potentiality of acid production, with finer grained pyrite weathering more easily than coarser grained. Another important factor in the weathering of shales is the presence of carbonates and other buffering components. It is demonstrated in these investigations that calcareous shales like the MUNCH sample weather at a slower rate than acid shales like STORT & BRUM1, because the acid generated, which is mainly responsible for the weathering, is consumed at a faster rate and more completely in the former than in the two latter samples. As such, small amounts of acid, Fe and Al were found in solution for MUNCH during the 100-day weathering period, contrary to what was found in the acid samples. The liming trial further demonstrates this fact.