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Abstract Pressure-solution creep is one of the most common crustal deformation mechanisms, inducing changes in the porosity and permeability of rocks. For a variety of rock types undergoing pressure solution, it has been shown that the presence of phyllosilicates may significantly enhance the rate of the pressure-solution process. In this experimental investigation, we present 4-dimensional (three dimensions + time) X-ray microtomographic data that contrast deformation by pressure-solution of a pure NaCl aggregate with that of a mixture of NaCl and biotite. The results show that for mixed samples (NaCl+biotite), phyllosilicates induce a marked reduction in porosity and pore connectivity and contribute to an increase in the local strain rates by an order of magnitude over pure NaCl samples. At the same time, phyllosilicates do not induce strain localization in the sample. We discuss various possible explanations for these observations including a possible positive feedback between the porosity distribution and pressure solution. Our study yields novel insights into the local effects of phyllosilicates during pressure-solution creep and provides full 4-dimensional imaging and characterization of the coupled evolution of porosity and pore connectivity over previously unprecedented experimental time scales. Highlights • Local porosity around biotite grains is reduced by ∼100% during pressure-solution. • Biotite increases the local strain rates by an order of magnitude. • Pressure-solution is enhanced due to geochemical self-organization. [ABSTRACT FROM AUTHOR]

Stylolites form by the transportation and precipitation of dissolved materials in pore fluids due to concentration gradients that develop in rocks under stress. This is a coupled chemical-hydraulic-mechanical (CHM) process, resulting from the interplay between chemical dissolution, pore fluid flow and stress during stylolite growth. Although the CHM feedbacks that develop around stylolite seams lead to porosity compartmentalization, the dynamic processes controlling porosity evolution during stylolite growth remain unknown. Here, thrust-induced stylolites in micritic limestones of the Eilean Dubh Formation that crop out in the footwall of the Caledonian Ullapool/Moine Thrust in NW Scotland are used to investigate the dynamic processes controlling stylolite growth. Stylolite microstructures observed in detail, using a backscatter electron image mosaic (102,600 × 18,239 pixels; 0.17μm/pixels), both reveal stylolite seams (core zones) surrounded by zones of increased-porosity (damage zones) and suggest a four stage self-organization process for stylolite growth and evolution. In the first stage, stress concentration in the micritic matrix led to the partial dissolution of the matrix, forming new pores adjacent to pyrites grains. In the second stage, the new pores facilitated local dissolution, which in turn promoted their growth, establishing a positive feedback loop between texture and solubility that enlarged the local porous zone by self-organization. In the third stage, the porous zones facilitated the influx of externally derived fluids and differentiated into core zones, with syn-kinematic precipitation of secondary pyrites and clay minerals, and damage zones, with ongoing dissolution. In the fourth stage, the porous zones propagated perpendicular to the shortening direction and became interlinked, forming mature stylolites with a core zone in the centre surrounded on both sides by damage zones with high porosities. The fluid flux along the stylolites during their formation was likely controlled by episodic movements along the Ullapool Thrust. • Stylolites localize along impurities like pyrites. • Secondary minerals like pyrites or clays can precipitate within the porosity. • Stylolites consist of a core zone surrounded by a highly porous damage zone. • Stylolite growth is characterized by a self-organized porosity increase. [ABSTRACT FROM AUTHOR]

Plaster, made of calcium sulfate hemihydrate (CaSO 4 •½H 2 O) and/or γ -anhydrite (CaSO 4) is obtained by dehydration of gypsum (CaSO 4 ·2H 2 O) at 100–200 °C. When mixed with water, it dissolves while new gypsum crystals precipitate. Plaster's microstructures affect its reactivity and setting properties, but are poorly understood due to confusion between crystals and polycrystalline structures in the literature. Through a multi-scale approach combining X-ray microtomography, optical and scanning electron microscopy, we revealed size, morphology and orientation of pores and crystals formed by dehydrating single crystals at T = 105–130 °C and p H2O = 0–40 hPa. The expulsion of water from the (010) interlayers results in a three-level fragmentation: cracks form parallel to (010), then the solid is divided into [101] polycrystalline needles (5–10 x 30–300 μm) formed of micron-sized crystals arranged in a fishbone pattern. The same behavior is observed whatever T and p H2O , however the crystallites size increases with p H2O. Gypsum dehydration-induced microstructure: the fishbone pattern is the smallest feature in a multi-scale system of cracks induced by strains in the crystal structure. [Display omitted] • First study of gypsum single crystal dehydration using X-ray absorption tomography. • Evidence and mechanism of a multi-scale crack pattern due to structure change. • How stress-induced shearing generates the "fishbone" patterns. [ABSTRACT FROM AUTHOR]