The transitional siliceous rocks from the Belchatow lignite deposit belong to the deposits with heterogeneous petrographic composition. The research allows us to identify among others, opoka-rocks and gaizes. The mineralogical-chemical analysis proves that the main ingredients of the studied rocks commonly used as building material are minerals of the SiO₂ group. Laboratory tests show that the nature of siliceous mineral phases has several effects on the geomechanical parameters of the studied transitional rocks. They are a reduction in water content and rock porosity, which leads to the transition of opal type A to opal type crystobalit and trydymit and then to quartz or microquartz. Their density and strength parameters are increased.

In the Carboniferous rock mass of the Upper Silesian Coal Basin, large changes in the geomechanical conditions often occur over relatively short distances. These conditions relate to rock properties that are primarily responsible for the occurrence of geodynamic phenomena in the rock mass. The main factor influencing the manifestation of these phenomena is tectonic stress developed during Variscan and subsequent Alpine orogenesis. This stress contributed to creating tectonic structures in the Carboniferous formations and influenced the properties of the rocks themselves and the rock mass they form. As a result of the action of the stresses, compaction zones (main stresses were compressive) were formed, along with zones in which one of the main stresses was tensile. For the compaction zones in the Carboniferous rocks, the following geomechanical parameters have been calculated: uniaxial compressive strength, Young’s modulus and post-critical modulus. The local stress field was determined according to the focal mechanism in selected areas (Main and Bytom troughs) to characterize changes in geomechanical properties of the rocks that are responsible for high-energy tremors (E ≥ 106 J, ML ≥ 2.2).

This paper presents laboratory analyses of the influence of an acidic environment, salinity and temperature change are able to exert on the geomechanical properties of the opoka-rocks. This rock material, deriving from four sites in East–Central Poland, was found to be variously resistant to factors like the destructive action of water-soluble salts and the effects of an acidic environment, on account of the actuallydiverse nature of the rocks in question. Ultimately, the work offered a basis for a distinction to be drawn between the light opoka-rocks present at the Annopol and KazimierzWielki sites, and the heavy opoka-rocks from Bochotnica and Krasnobród, in terms of both textural and physical-mechanical features. The heavy opoka-rocks from Krasnobród proved least resistant to an acidic environment, which left strength reduced significantly. This kind of rock also experiences both an increase in porosity and absorbability and a decrease in weight. Furthermore, the influence of an acidic environment on aesthetic features of the examined rocks was in all cases negative, salts formed a patina on surfaces that obscured original structural and textural features. None of the tested types of rock presented resistance to the crystallization pressure such salt is able to exert.