Replacement of magnetite by hematite in hydrothermal systems: A refined redox-independent model

The replacement of magnetite by hematite is commonly observed in various geologic systems. In contrast to the formation of hematite by a solid-state oxidation, numerous experimental results have demonstrated that it can also occur by a redox-independent dissolution-reprecipitation reaction. However, the orientation relationship and the intermediate products between magnetite and hematite remain unknown during the redox-independent replacement. In this work, high-resolution transmission electron microscopy (TEM) was used to investigate porous hematite from the Baishiya skarn deposit, East Kunlun orogenic belt, aiming to build a refined growth model applicable to the replacement of magnetite by hematite in natural hydrothermal systems. Initially, hydrothermal leaching of Fe2+ from magnetite led to the formation of ferrihydrite that transformed to goethite and hematite nanocrystals successively. Oriented attachment of pseudocubic hematite nanoparticles induced by Cl along specific crystallographic directions formed hematite mesocrystals on the exposed dodecahedral facets of magnetite, leading to an orientation relationship between magnetite and adjacent hematite (i.e., (110)(magnetite)parallel to (012)(hematite)), which is different from that of oxidation (i.e., (110)(magnetite)parallel to (110)(hematite)). However, oriented attachment can be imperfect in some instances, and dislocations of adjacent nanoparticles result. The dislocations in the hematite mesocrystals have acted as a closed space to capture the remaining ferrihydrite. When the Si concentration in ferrihydrite was sufficient, the solid-state conversion of the remaining ferrihydrite to hematite was blocked. We propose that repeated dissolution and reprecipitation of hematite mesocrystals (i.e., defective crystals) are required to remove Si, and thereby form defect-free hematite crystals, providing a genetic link between the widespread hematitisation and related multistage fluid infiltration in some of the world's richest deposits (e.g., Olympic Dam and Bayan Obo deposits). This is the first time that Si and Cl have been verified to act as key factors to shape the hematite growth pathway during the ore-forming processes, challenging the `ion-by-ion' growth model that has underpinned our knowledge of mineral solubility, nucleation, and mass transfer from nano- to micron-scales in natural hydrothermal systems. (C) 2021 Elsevier B.V. All rights reserved.