Zhaoliang Hou, Dawid Woś, Cornelius Tschegg, Anna Rogowitz, Alexander Hugh Rice, Lutz Nasdala, Florian Fusseis, Piotr Szymczak, Bernhard Grasemann

Manganese (Mn) dendrites are a common type of mineral dendrite that typically forms two-dimensional structures on rock surfaces. Three-dimensional (3D) Mn dendrites in rocks have rarely been reported, hence their growth implications have largely escaped attention. Here, we combine high-resolution X-ray and electron-based data with numerical modelling to give the first detailed description of natural 3D Mn dendrites (in clinoptilolite tuffs) and elucidate their growth dynamics. Our data show that 3D dendrite growth occurred by accretion of Mn oxide nanoparticles formed when Mn-bearing fluids mixed with oxygenated pore-water. The geometry of the resulting structures is sensitive to ion concentrations, the volume of infiltrating fluid, and the number of fluid pulses; thus 3D dendrites record the hydro-geochemical rock history.