Mineral reaction fringes contain a wealth of information that can be used to decipher the conditions during transformation. One of the most important parameters for the development of fringe microstructures is the presence of water. Reaction fringes have the potential to ‘notice’ variations in the amount of water at grain boundaries, allowing them to be used as sensitive ‘geohygrometers’. Furthermore, the effect of water on the relative layer thicknesses may allow to distinguish between water-unsaturated and water-saturated conditions during transformation (Franke et al. 2023).
