Slags from Ruda Śląska, Poland as a large-scale laboratory for the crystallization of rare natural rocks: melilitolites and paralavas

Zinc and lead smelting slags from Ruda Śląska are unique in their chemistry and phase composition, which resemble rare natural rocks such as paralavas and melilitolites. Moreover, considering its size, we can treat a pyrometallurgical slag dump as a geological body. In slags from Ruda Śląska the assemblage melilite ± pseudowollastonite ± wollastonite ± plagioclase was discovered in glassy slag. High-temperature experiments were performed to determine the temperature conditions and to reconstruct the crystallization of such an assemblage. Two slag samples were subjected to complete melting and crystallization with controlled thermal gradients of: 53.25 °C/h, 15.20 °C/h and 7.60 °C/h. The results showed that crystal nucleation started at temperatures of 1250-1300 °C depending on the fluctuations of chemical composition. In both samples the thermal gradient only partly influenced the phase differentiation, being rather responsible for the disappearance of the primary glass. Moreover, even slight differences in chemical composition resulted in changes in phase assemblages under the same temperature conditions (mll + gls vs pwol+pl + mll + gls). It was proven that it is due to the combination of undercooling conditions and solidus dependences in the åkermanite – gehlenite solid solution. The occurrence of such phenomena should be considered in natural rocks with similar composition. In the case of the Ruda Śląska slags it explains the dominance of glassy slag in that location. The experiments gave us an opportunity to observe and precisely analyze crystallization in real time, providing new insights into the creation of slags and their natural analogues. However, the study has also shown that possible variations of the original crystallization should always be assessed.