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CC-SS - coupled crack-seal simulation

Subject Area Palaeontology
Mechanical Properties of Metallic Materials and their Microstructural Origins
Term from 2018 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 391503625
 
Final Report Year 2023

Final Report Abstract

The first part of this work provides a detailed description and interpretation of the development of different vein microstructures that are common in micritic carbonate rocks. Microtextural observations from natural samples with numerical modeling of syntaxial crystal growth in fractures are compared. For example, wide-blocky vein morphologies form in fluid-filled open fractures by rapid epitaxial growth on a few grains along the fracture surface, such as freshly broken cleavage planes of calcite, whereas growth on wall-rock grain boundaries is slow due to the presence of inert second phases, such as clays, organics or dust particles that covers the growth surfaces. Therefore, the morphology and mineralogy of an initial fracture surface has major implications on vein sealing rates and resulting crystal morphologies. Depending on the fracture type (intergranular, transgranular or mixed type) and the fracture aperture, different crystal morphologies are observed, such as equidimensional-stretched/ blocky (in small veins), elongated-blocky/ elongated-stretched, and wide blocky. The simulated vein microstructures show many similarities with natural veins in limestone and overall are in agreement with previous numerical studies. Additionally, a systematic phase-field study on the formation of multi-crack-seal veins in quartz microstructures is presented. The results reveal that with small fracture apertures little growth competition between the fractured grains occurs, which is resulting in no crystallographic preferred orientation (CPO) inside the vein. With an increasing aperture increment growth competition is possible and leads to larger crystal width with a CPO inside the vein. Furthermore, when a oblique opening trajectory is applied sheared crystals form which show into the direction of minimal principal stress (similar as in antitaxial veins). Moreover, natural microstructures with serrated grain boundaries (radiators-fin structures) in thick veins and their transition to delocalized fracturing with thin vein bundles are recreated in the phase-field simulations and extended by probabilistic simulations. The simulated structures show many similarities with natural veins in quartz microstructures and give valuable insight in the crack-seal mechanism. An extension of the phase-field model is presented which incorporates a supersaturation dependent driving force by including the concentration equation and Navier-Stokes equations into the modeling approach. This results in locally different crystal growth velocities in the case of potash alum in a flow channel, where the supersaturation decreases along the flow direction due to a depletion of the fluid. The studies with potash alum give a sound agreement with experimental data, however, this approach might be limited to mesoscopic length scale and crystal systems with high solubilities and fast crystallization rates. The presented simulations demonstrate the capability of the phase-field method to model crystal growth in veins, considering a large number of variables that are expected to influence vein formation in nature. The results obtained during the course of the project are published in various renowned journals as individual and joint papers, presented at international conferences, and led to doctoral and master theses. Further, the findings also revealed many interesting future directions of research.

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