Morphological transitions in mineral dendrites

Mineral dendrites are tree-like, branched patterns commonly found on or within rocks, typically formed by the deposition of minerals like manganese or iron oxides. In natural systems these patterns exhibit diverse morphologies and shapes, varying in thickness or degree of branching. This study focuses on quasi-planar dendritic patterns growing along fractures and bedding planes.  In these dendrites, abrupt morphological transitions, where their thickness changes suddenly, are often observed. Understanding the mechanisms behind this phenomenon is the aim of this work.

 

Dendrites form through the infiltration of fractured rocks by manganese- or –iron-bearing fluids. When these fluids mix with oxygenated solutions, metal oxides precipitate, creating the dendritic patterns. The exact deposition mechanism remains debated. One model suggests that as the fluids mix, nanoscale particles of manganese or iron oxide are first formed. These nanoparticles then aggregate, resulting in the formation of mineral dendrites.

 

In such a scenario, the final dendrite morphology turns out to be highly sensitive to the initial concentrations of manganese (or iron) in the system. We show that morphological transitions can be triggered by subsequent infiltrations of metal-bearing fluids, characterized by different concentrations of manganese/iron ions. However, we also point to another factor that can induce morphological transitions in dendrites, this time related to the change in the aperture of the fracture or bedding plane along which they grow. We show that larger fracture apertures correlate with the formation of thicker dendritic structures. We analyze the characteristics of both transitions, focusing on the features that allow them to be distinguished from one another.

 

The ultimate goal would be to establish a link between the morphology of the dendrites and the physicochemical conditions in which they grew. This connection would allow for the decoding of the hydrogeochemical history of the dendrite-bearing rock strata.