The coal mining waste dump Anna I of the Anna black coal underground mine in Alsdorf (Germany) – in operation between 1854 and 1983 – is characterized by internal combustion. Volatile products of this process are emitted though vent-like holes and fractures on the top of the dump. Surface temperatures are heterogeneous within a dm-scale and partly exceed 210 degrees centigrade. The pH of condensed vapour ranges between 4.5 and 2. Along the surface of the vents the Carboniferous sandstones and shales are altered to sulphate-bearing mineral associations. These are quartz-muscovite-kaolinite-chlorite; quartz-muscovite-(kaolinite, hematite); quartz-jarosite-alunite-(muscovite), which can be interpreted in terms of increasing degree of alteration controlled by decreasing pH. The central parts of the vents show different sulphates such as Godovikovite, Mascagnite and Tschermigite, and other sulphates which were formed in a reaction with altered rock (Voltaite, Alunogen and Gypsum).
Chloride minerals (ammonium chloride, kremersite) and sassoline (B(OH)3) are less common. Rocks affected by alteration have high concentrations of Se and Hg. The concentration of Se, which was volatilized due to combustion together with S from the Carboniferous sedimentary rocks, corresponds to the degree of alteration. Highest Se concentrations (<2730 ppm) are given in the jarosite-alunite bearing samples. The less altered samples have Se concentrations between 32 and 149 ppm. Concentrations of Hg are variable between 1 and 290 ppm and do not correspond to variation of mineral associations. Other heavy metals (V, Cr, Zn, Cd, Tl, Pb) and As are not significantly mobilized. Enhanced concentrations of Se and Hg are also detected in moss and knotgrass. Sindern, S., Warnsloh, J.M., Witzke, T., Havenith, V., Neef, R. and Etoundi, Y. (2005) Mineralogy and geochemistry of vents formed on the burning coal mining waste dump Anna I, Alsdorf, Germany. Beih. z. Eur. J. Mineral., Vol. 17, No. 1, 130.