Atmospheric oxidation of the pyritic waste rock in Maardu, Estonia, 2: an assessment of aluminosilicate buffering potential

The assessment of the aluminosilicate buffering potential during acid weath ering of the Estonian alum shale is provided. It is found that the stoichio metric interaction between dissolved pyrite oxidation products and illite o f the shale best describe the buffering process and are consistent with ear lier field studies. The scheme includes incongruent dissolution of illite w ith smectite and K-jarosite precipitating. This complex mechanism involves buffering of 8% of the acidity by K+ and temporary precipitation of 25% of the acidity as K-jarosite. Dissolution proceeds at a low pH (1.5-3) until a ll pyrite in the shale particle is oxidised. Hence, if the total amount of illite present is larger than needed for stoichiometric interactions, only part of it is involved in a buffering process, neutralising a certain perce ntage of acidity. The next stage in shale weathering is the incongruent dis solution of K-jarosite with the release of the precipitated acidity and the formation of ferric oxyhydroxide.