Michal Filippi

A variety of speleothems are present in crevice and boulder caves developed in Cretaceous sandstones of the Elbe River Canyon in northern Czech Republic. A set of complementary instrumental mineralogical methods was applied to characterize the speleothems and cave dripwaters, including X-ray powder diffraction, scanning electron microscopy and microanalysis, Raman spectroscopy and optical emission spectrometry. Four morphological types were distinguished and characterized in terms of their mineral and chemical composition: 1, rusty brown mud-dominated coatings with micro-gours, composed of a mixture of clay minerals; 2, white “chalky” coatings (moonmilk) composed of calcite with minor gypsum; 3, cauliflower-shaped coralloids composed of calcite and silica in a layered structure, with gypsum layers in apical parts; 4, knob coralloids, dark gray-brown with smooth surfaces and distinctly layered structures, composed of silica (quartz, opal-A) and Si–Al phases (kaolinite) and including phosphate-rich laminae (sasaite, vashegyite, taranakite). Only modest microbial mediation of silica precipitation was observed in cauliflower-shaped coralloids while no clear signs are present in knob coralloids despite organic enrichment in the topmost layer. White “chalky” coatings and cauliflower-shaped coralloids precipitated from weakly acidic Ca-, Mg- and sulphate-rich deeper sandstone percolates. These forms are probably still active, much like the micro-gours, produced by particulate clay deposition. Formation of knob coralloids combined clay deposition and the dominant silica precipitation from pore waters similar to the present shallow acidic percolates under changing climatic conditions, probably in the Pleistocene. It was favored by specific rock lithology (quartzose sandstone with kaolinite admixture), which explains the scarcity of similar forms in sandstone caves. Concentration of knob coralloids along protruding vertical edges and the presence of wind-guided forms suggests that silica precipitation was driven by evaporation under a constant air flow.

Cavernous weathering (honeycombs, tafoni) is a common weathering feature of both natural and artificial exposures. Honeycombs are known from various environments but are best developed in coastal areas. There are several theories as to their origin, with salt weathering currently being the most favoured by the geomorphological community. To test if the drying pattern of salt‐laden moisture results in honeycombs (the theory of Huinink et al., Earth Surface Processes and Landforms, 29(10), 1225–1233, 2004), coastal honeycombs in the metasandstone of Tuscany (Italy) were studied both in the field and with a laboratory evaporation experiment. The depth of the evaporation front was measured by the ‘uranine‐probe’ method in the honeycomb pits and lips. The evaporation rate was calculated from the depth of the evaporation front as well as the climatic conditions at the study site. Lastly, the amounts of precipitated salts were estimated based on the evaporation rate of seawater. In the evaporation experiment, the evaporation front retreated faster in the lips than in the pits, and the field measured evaporation front was closer to the surface in the pits (2 mm) than in the lips (7 mm). Thus, the calculated evaporation rate was higher in the pits than in the lips (16.1 and 4.6 mm/yr, respectively). Similarly, the amount of salts precipitated was also higher in the pits (0.7 kg/m²/yr compared to 0.2 kg/m²/yr in lips). Faster salt deposition in the pits as well as the evaporation front position fits well with the theory of Huinink et al. Based on surface tensile strength measurements, case hardening is not protecting the honeycomb lips.

The labyrinthine world of arsenic minerals has piqued the curiosity of many researchers in mineralogy, geochemistry, chemistry, and environmental sciences. Arsenic was known to the ancient civilizations; there are written Greek, Roman, and Chinese reports about minerals and substances of this element (Emsley 2001). The discovery of elemental arsenic is attributed to Albertus Magnus (1193–1280) who prepared it by reduction of As₂O₃. The common public association of arsenic and poison is the heritage of a long history of eliminating unwanted and unloved ones with compounds of this element. Mary Ann Cotton (1832–1873) was charged...