Gothic-Arch Calcite from Speleothems of the Bohemian Karst (Czech Republic): Its Occurrence, Microscopic Ultrastructure and Possible Mechanism of Growth

Gothic arch calcite, a specific crystallographic variety of calcite known from some hot springs and tufa streams, has been newly recognized in the Koněprusy Caves. The gothic-arch calcite occurs on the exteriors of exotic coralloid speleothems where it coexists with scalenohedral (dogtooth) spar crystals. The crystals exhibit microscopic ultrastructural features including deeply eroded topography, etch pits, and spiky and ribbon calcite crystallites, pointing to its extensive natural etching. Many gothic-arch calcites originated as late-stage, secondary overgrowths on older, etched dogtooth calcite crystals. Its characteristic outward curvature resulted from the recrystallization of etching-liberated fine carbonate grains and newly formed needle-fiber calcite laths, which were accumulated and bound on the faces and at the bases of corroded crystals. These intimately coexisting destructive and constructive processes of carbonate crystal corrosion and growth were probably mediated by bacteria, fungi, or other microorganisms. Fluid inclusions embedded in calcite crystals point to a vadose setting and temperatures below ~50 °C. This, combined with the wider geological context, indicates that the gothic arch calcite crystals originated only during the late Pleistocene to Holocene epochs, when the cave, initially eroded by hypogene fluids in the deeper subsurface, was uplifted to the subaerial setting and exposed to the meteoric waters seeping from the topographic surface. The radiocarbon analysis shows that gothic-arch calcite crystals are generally older than ~55,000 years, but the surface layers of some crystals still reveal a weak 14C activity, suggesting that microbiologically mediated alterations of the speleothems may have been occurring locally until now.

Sustainable forestry and iron compounds in karstic soils: qualitative and semi-quantitative results focused on the occurrence of Fe-compounds on mineral particles

Relic karstic soils in nine localities in the Dinaric Karst in Slovenia, five localities in the Moravian Karst and four localities in the Bohemian Karst were sampled for soil scientific, mineralogical and petrological studies focused on the presentation of descriptive aspects of particular iron compounds. The macroscopy and microscopy of Fe²⁺ and Fe³⁺compounds were determined and an interpretation of these data was performed aimed at describing sources and their palaeotransports. The presented results show that the studied karstic soils have a heterogeneous petrographical and mineralogical composition when, depending on circumstances, hematite does not dominate and goethite prevails over it or it is an opposite. Results from the chosen methods reinforce sources of the new materials as the crucial factor for the studied karstic soils.