Coupling of diagenetic alterations and mechanical properties of Lower Permian siliciclastic sandstones: a pilot study

Initial detrital composition and authigenic alterations during diagenesis of three sandstone types are related to their mechanical properties. Sandstones were prepared for geotechnical standard tests [density, uniaxial compressive strength (UCS), Young’s modulus (E), strain at failure (ε)] and thin sections for petrographic analyses (point counting). UCS ranges from 3 to 62 MPa and positively correlates with density (1.75–2.35 g/cm3) and E (0.3–12.7 GPa). Optical porosity is controlling these mechanical parameters and was linked to diagenetic alterations. Diagenetic alterations affecting porosity reduction are the abundance of clay minerals, and the intensity of mechanical and chemical compaction. The latter is controlled by clay mineral coatings on contacts between detrital grains, and the occurrence of authigenic quartz and dolomite. Horizontal contact lengths of grains normalized to their respective particle diameter (effective contact ratio, ECR) and porosity are identified as a control on the mechanical properties UCS and E, reflected by the rock strength index SR. The results of this pilot study suggest that SR is able to predict UCS and E based on petrographic information obtained from the studied samples. These results enhance the understanding of the coupling between mineralogy and geomechanics and highlight the impact of diagenesis on geomechanical behavior.

Liquid–Solid Core-Shell Microcapsules of Calcium Carbonate Coated Emulsions and Liposomes

Micron-sized core-shell particles consisting of a calcium carbonate (CaCO3) mineral shell and a fluidic core were generated using a biomimetic approach, for the purpose of use as biodegradable microcapsules for release of active agents. Dinoflagellate cysts, unicellular organisms which deposit a protective hard mineral shell around their soft and fluidic cellular interior, served as our inspiration. Using the biomimetic polymer-induced liquid-precursor (PILP) mineralization process, calcium carbonate coatings were deposited on charged emulsion droplets and liposomes. Light microscopy, scanning electron microscopy, polarized light microscopy, X-ray diffraction, and confocal fluorescence microscopy were used to demonstrate that smooth CaCO3 mineral coatings can be deposited onto the high curvature surfaces of emulsions and liposomes to yield micron-sized microcapsules for the effective entrapment of both hydrophobic and hydrophilic active agents. These biodegradable and biocompatible CaCO3 microcapsules are novel systems for producing a powdered form of fluid-containing capsules for storage and transport of pharma/chemical agents. They may be used in lieu of, or in conjunction with, existing microcapsule delivery approaches, as well as providing a convenient foundation for which polymeric coatings could be further applied, allowing for more complex targeting and/or chemical-release control.

Geomicrobial Investigations of Colored Outer Coatings from an Ethiopian Rock Art Gallery

The open rock shelter of Yabelo in Ethiopia hosts diverse Holocene paintings of great cultural importance. The paintings are characterized by the presence of different mineral coatings, whose features have not been studied yet. Our goal was to understand whether different rock samples from the Yabelo paintings collected in close proximity may reveal coatings with different minerology and biology. Thus, elemental analyses combined with microscopic and molecular investigations were performed on two coatings, one whitish (sample 1) and one reddish (sample 2). Although both samples were dominated by heterotrophic bacteria, the two coatings showed distinct mineralogical and microbiological characteristics. Sample 1 contained higher amounts of Ca and P than sample 2, which was likely related to the presence of organic matter. Sample 1 hosted bacterial genera that are potentially involved in biomineralization processes, metal redox cycles and metal resistance. In contrast, sample 2 showed mainly pathogenic and commensal bacteria that are characteristic of animal and human microbiota, and other microorganisms that are involved in nitrogen and metal biogeochemical cycles. Overall, our results indicated that the bacterial communities were particular to the coating mineralogy, suggesting a potential role of the biological components in the crust genesis.

Role of bedrock weathering in microaggregate formation - limestone alteration in the aeration zone

<p>During pedogenesis, the development of the soil aggregate system may be strongly dependent on weathering of bedrock in the vadose zone. There, periodic drying and flushing by meteoric waters provides alternating hydro- and biogeochemical conditions for fluid-rock interactions and provokes the dissolution, displacement and placement of minerals as well as the release or adsorption of colloids. As a result, the seepage suspension is enriched with mobile mineral and organic matter that infiltrates into the bedrock void system thereby fueling aggregate forming materials and composite building units to exposed surfaces of the bedrock. We aim to elucidate related bedrock alteration processes in dependence on water composition, seepage vs. saturation and the fracture network during weathering.</p><p>Our study combines the investigation of the weathering rim of natural bedrocks (outcrop analogue) with the simulation of the natural conditions by column experiments in the laboratory. Study object are Triassic limestones (Upper Muschelkalk) of the Hainich area in Thuringia (central Germany). The columns were filled with fresh, unaltered material, crushed into coarse gravel fraction size and percolated with artificial rainwater or soil litter extract over a runtime of 6 months. In order to mimic natural conditions percolation periods changed with periods of drying. Geochemical data of the liquid phase resemble very well the alternating periods of drying and flushing by systematic changes of the element concentration and milieu parameters. Generally, dissolved elements in the seepage are higher in concentration when litter extract is used pointing towards a significant impact on dissolution kinetics, especially after periods of longer water-rock interaction. Weathered natural bedrock surfaces (bedrock clasts in the covering soil and fractures and voids in the bedrock) exhibit carbonate dissolution (edge pits and dissolution vugs) and the formation of clay mineral coatings, in part with iron oxides. The same holds true for rock clasts after the column experiments. The alteration is macroscopically visible by brownish and beige coatings on formerly greyish pristine surfaces. This feature seems more pronounced on clasts percolated with liquids from soil litter extracts than on clasts treated with artificial rainwater indicating the formation of organo-mineral associations during solid-liquid interaction.</p><p>Generally in both, nature and experiment, facial aging features include the dissolution of carbonates, the formation of clay minerals as well as oxides and hydroxides of iron, but also the appearance of organic constituents. Our results contribute to a better mechanistic understanding of the role of bedrock alteration during weathering for (a) the provision of microaggregate forming materials and (b) the formation of composite building units and microaggregates from pristine environments.</p><p> </p>