HIGH-TEMPERATURE EXPERIMENTS ON THE RECONSTRUCTION OF THE CRYSTALLIZATION PROCESS OF MELILITE – PSEUDOWOLLASTONITE/WOLLASTONITE – ANORTHITE – GLASS ASSEMBLAGE WITHIN ZN-PB SLAGS FROM RUDA ŚLĄSKA, POLAND

Slags from Ruda Śląska are the result of pyrometallurgical smelting of Mississippi Valley-Type deposits. They are characterized by a unique, even in the slag, phase and chemical composition. They are dominated by glass in which was found assemblage based on pseudowollastonite/wollastonite, melilite, and anorthite. Their chemical and phase composition place them as analogs of natural rocks similar to melilite hornfels, melilitolite, or some types of paralavas. High-temperature experiments were carried out to determine the temperature of beginning of crystallization of the assemblage, and to fully reproduce their crystallization process under controlled temperature gradient conditions. Obtained results should give an insight into the process of crystallization of natural rocks. Experiments were carried out with 3 temperature gradients (7.60°C/h, 15.20°C/h, 53.25°C/h) on 2 slag samples: RS-6 (wo-mel-an-glass) and RS-8 (wo-glass), which despite the different phase composition have a similar chemistry (RS-6 vs RS-8 respectively): 42.39 vs 43.28wt.% SiO₂; 0.50 vs 0.55 wt.% TiO₂; 12.68 vs 11.76wt.% Al₂O₃; 0.52 vs 0.66wt.% Fe₂O₃; 2.55 vs 4.84 wt.% MnO; 36.52 vs 37.12wt.% CaO; 1.17 vs 1.56 wt.% MgO; 0.03 vs 0.12wt.% Na₂O; 0.20 vs 1.27wt.% K₂O. Based on experiments, the beginning of crystallization was determined at approx. 1250°C (RS-8) and 1300°C (RS-6). Controlled crystallization experiments indicated that: (i) even slight chemical deviation may result in a completely different assemblage at the same crystallization conditions: wo-mel-an-glass (experimental RS-6) vs mel-glass (experimental RS-8); (ii) the role of the temperature gradient in similar rocks is limited. In the case of RS-8, the crystallization stops at ca. 30% of the crystal content regardless of the gradient. It is caused by the changing composition of melilites from åkermanite to gehlenite. The latter has higher solidus temperature, and move melt to subsolidus conditions and consequently prevent further crystallization. This phenomenon explains the unique domination of glass in studied slag. In the case of RS-6 at 15.20°C/h, complete assemblage with the eutectic composition of wo-mel-an was observed. Extending the crystallization time leads only to move the crystallization front further within the sample.

Acknowledgments

This study was supported by National Science Centre grant No. 2016/21/N/ST10/00838