One-Step Route Synthesis of Siliceous Six-Line Ferrihydrite: Implication for the Formation of Natural Ferrihydrite

This paper presents a one-step route to the production of siliceous six-line ferrihydrite (6LFh) by oxidizing the aqueous Fe 2+ –silicate metal complex in solution. Synthetic 6LFh is analogous to naturally occurring 6LFh based on X-ray diffraction and transmission electron microscopy results. We also synthesized siliceous ferrihydrites in different conditions to understand the formation mechanism. The experimental results show the continuous series of siliceous ferrihydrite between two and six lines by increasing the Fe/Si molar ratio, temperature, and reaction time. The results suggest that the presence of Si in solution is the main factor of ferrihydrite formation. Without dissolved silica, only crystalline iron oxyhydroxides, such as goethite and akaganéite, form. Silica inhibits the transformation of ferrihydrite to the more crystalline iron oxyhydroxides. The synthetic experiments can explain the ferrihydrite formation at the Chocolate Pots hot spring in Yellowstone National Park, which represents similar ferrous iron concentration and circumneutral pH thermal spring environments. In comparison to Si-free ferrihydrite, synthetic siliceous ferrihydrite can provide the actual physical and chemical properties of ferrihydrite in natural environments because natural ferrihydrite always contains Si. The new synthesis method will help us better understand the ferrihydrite formations in various geological environments.