Ferrihydrite, an antiferromagnetic iron oxyhydroxide with resulting magnetization due to uncompensated spins, is of great importance for the cycling of many trace metals in the environment. Four ferrihydrite samples prepared with 1.3 to 3.5 wt% of Si at different synthesis temperatures (7.5°C, 22°C, 50°C, and 75°C) were studied by temperature-dependent hysteresis loops, ZFC/FC susceptibility curves, ac susceptibility and Mössbauer spectroscopy. The incorporation of Si into the ferrihydrite during synthesis changed the properties of this mineral. Interestingly, seven sharp lines were observed in the X-ray diffraction pattern of the ferrihydrite samples prepared at 50°C and 75°C. In general, both XRD and magnetism demonstrate that particle size decreased (from 23 nm to 2 nm) and particle size distribution narrowed as the synthesis temperature was lowered. Those samples prepared between 7.5°C and 50°C showed the expected superparamagnetic behavior of ferrihydrite below 300 K. The ferrihydrite prepared at 75°C was unusually coarse-grained and had a blocking temperature above 300 K. Extrapolation of induced magnetization from the largest particles with the highest crystallinity allowed an estimate of a ferrihydrite Néel temperature of around 422 K. We also present XRD and magnetic data from large natural Si-ferrihydrite collected from a marine shallow-water hydrothermal area that formed at a temperature of approximately 88°C.