Acid Mine Drainage Chemistry
November 21, 2003
Pyrite (iron sulfide) is a common constituent of lignites and coals. This pyrite oxidizes as it is exposed to the atmosphere in the course of mining, leading to the generation of sulfuric acid and dissolved iron (acid mine drainage). Besides the detrimental effect that the acidity has on the environment, this process often leads to an extensive generation of ochreous precipitates.
In the example shown here, waters draining a sulfide-rich lignite seam in the Czech Republic precipitate under different conditions. Initially a bright orange precipitate, consisting mainly of the sulfate-containing mineral schwertmannite, forms under acid conditions. After merging with alkaline waters (entering from the right center of the picture), iron that has remained in solution precipitates almost instantaneously as ferrihydrite, a poorly-crystalline, sulfate-free iron oxyhydroxide, leading to a sudden change of precipitate color to reddish brown. The variations in color can thus serve as indicators for the precipitate composition, and thereby for the conditions under which the precipitates formed.
For more information on this locality, see the article by Murad and Rojík in American Mineralogist vol. 88, 2003, pp. 1915-1918.