Crack-seal microstructure evolution in bi-mineralic quartz-chlorite veins in shales and siltstones from the RWTH-1 well, Aachen, Germany

► Analysis of bi-mineralic, syntectonic crack-seal veins. ► A new, previously unknown microstructure within a shear vein, called fin microstructure. ► A model explaining serrated grain boundaries of fibers in bi-mineralic veins. ► Veinlets inside a high-angle dilational vein can be used to infer the stress orientation inside the vein. ► Model of vein formation: transition from single veinlets towards a bundle of veinlets into a fully developed vein.