Formation of baryte and celestine during supergene processes on sedimentary rock outcrops

Extraordinarily abundant baryte and rare celestine microcrystals have been recognised during a routine inspection of claystone and siltstone samples from rock outcrops in the northeastern part of the Czech Republic. They occur in geological units of various ages and settings, i.e., the Bohemian Massif (Upper Carboniferous), Outer Carpathians (Jurrasic to Paleogene), and their Quaternary sedimentary cover. Both minerals were described previously in soils from arid regions, which is not the case of the studied area, situated in middle latitudes with a rather oceanic climate. In an area of ca. 1800 km 2 , we gathered 147 samples, 81.6% of which yielded baryte, 5.4% baryte and celestine, and 2.7% only celestine. None of these minerals was detected only in 10.2% of samples. Both mineral phases were documented by electron microscopy using backscattered electron imaging (BSE) and semiquantitative energy-dispersive X-ray spectroscopic analysis (EDS). We documented various morphologies of minerals from the baryte–celestine series. The origin of both phases can be ascribed to the release of Ba during rock weathering. Barium (and strontium) concentrations, required for the equilibrium crystallisation of baryte (and celestine) from solutions, is limited by the common-ion effect. Sulfate ions released during the dissolution of co-forming gypsum strongly limit the Ba concentrations in the solution. However, the minimum solubility of the baryte occurs at temperatures close to 0 °C, so the supersaturation of the solution and subsequent crystallisation can also occur by cooling. Baryte and celestine are probably common products of rock weathering but are often overlooked due to their submicroscopic size and the methods of studies—they are unlikely to be found on polished samples.