Evaporite minerals from the coal mines of the Czech part of the Upper Silesian Basin, including a new mineral králíkite (BaCl$_2$·2H$_2$O)

The Czech part of the Upper Silesian Basin is still a home of bituminous coal mining in Europe. The underground mines there feature rich assemblages of evaporite minerals that precipitate from the water flowing through the mines. The minerals occur in stalactites, rarely as cave pearls. The predominant mineral of the stalactites is halite (NaCl). The first group consists of halite stalactites with sylvite (KCl), carnallite (KMgCl$_3$·6H$_2$O), and rare gypsum (CaSO$_4$·2H$_2$O) and baryte (BaSO$_4$) on their surface. Orange coloration may be caused by lepidocrocite or akageneite (both Fe-oxyhydroxides). In the second group, halite is covered by carnallite, králíkite (BaCl$_2$·2H$_2$O), and SrCl$_2$ hydrates. In the third group, halite is associated with burkeite [Na$_6$(CO$_3$)(SO$_4$)$_2$], blödite [Na$_2$Mg(SO$_4$)$_2$·4H$_2$O], kainite [KMg(SO$_4$)Cl·3H$_2$O], thénardite (Na$_2$SO$_4$), and trona [Na$_3$H(CO$_3$)$_2$·2H$_2$O]. Some of the stalactites are dominated by sulfates, with starkeyite, hexahydrite, epsomite (all representing MgSO$_4$ hydrates), or thénardite. The fourth group comprised a single sample, a stalactite with α‑calcium formate and králíkite. ... mehrAll these minerals precipitate from brines that acquire their dissolved load primarily from Early Badenian groundwater bodies, frequently in direct contact with the Carboniferous basement rocks. These are fossil marine waters with total mineralization >10 g·l$^{−1}$, rich in NaCl, locally also in CH$_4$ and CO$_2$, and enriched in Sr$^{2+}$, Ba$^{2+}$, I$^−$ and Br$^−$. These types of water also led to precipitation of the newly described mineral králíkite. Owing to the low solubility of baryte, králíkite can only form from water strongly depleted in sulfate. The brines are locally mixed with sulfate-rich acid mine drainage water, resulting in some sulfate-rich compositions observed in this work. Our study highlights some processes tightly linked to coal and coal mining, namely formation of secondary mineralization underground and its possible environmental impact during and after the mining. As the coal mines are being successively shut down, this aspect should be prominent for the middle- to long-term elimination of hazards related to recent and current coal mining.