Diversity of structures, textures, and mineral and chemical compositions is among the most characteristic features of tonsteins. This diversity is observed in lateral and vertical directions in hand specimens and under the microscope. The main goal of this study was to explain the origin of the observed diversity in tonsteins. On the basis of chemical analyses and observations in hand specimens and under the optical microscope, supported by selected instrumental techniques, it has been established that the petrographic diversity of tonsteins in GZW was caused by: — local physical and chemical conditions of sedimentation and diagenesis in the coal-forming environment;
— size, type, and mineral composition of grains in pyroclastic materials deposited in the coal-forming environment
There are several tonstein horizons in the Upper Silesian Coal Basin (tab. 1). The major petrographic differences between them consist in the occurrence of:
— moderately kaolinized rocks with a variabel mineral composition. They contain numerous
restites, well developed sedimentary textures, and primary mineral assemblages.
— heavily kaolinized rocks with simple, uniform mineral composition. They contain small
amounts of restites. The primary mineral composition is difficult to recognize.
Both moderately and heavily kaolinized tonsteins formed due to alteration of rhyolitic tuffs and tufiites. Tonsteins of the paralic series are usually moderately kaolinized, though locally they may be kaolinized to a larger degree. Tonsteins of limnic series are heavily kaolinized, though locally they may be moderately kaolinized.
The extent of kaolinization of the primary pyroclastic material in tonsteins is reflected in their structures and textures. There is also a characteristic trend of changes in the chemical composition. The molecular ratio of SiO2/Al2O3 to K2O/A12O3 is the best criterion to classify tuffogenic intercalations kaolinized to a different degree (fig. 47). The plot of SiO2/Al2O3 vs. K2O/A12O3 indirectly shows also the influence of plants vegetation on the alteration of the pyroclastic material deposited- in the coal-forming environment Some trace elements (Sr, Ni, Rb, Th, U) are sensitive indicators of the degree of kaolinitization of the pyroclastic material.
Microscopic analysis of restites (pyroquartz, biotite, sanidine, heavy minerals, volcanic glass, fragments of volcanic rocks) and forms of occurence of clay minerals (mainly kaolinite and mixed-layer minerals) in tonsteins of GZW suggest that heavily kaolinized tonsteins formed due to alteration of the pyroclastic intercalations, the texture and mineral composition of which were similar to the initial material for moderately kaolinized tonsteins.