The presence of Si up to a Si/(Si + Fe) mole ratio (X-Si) of 0,270, added during or after the preparation of a 2-line ferrihydrite, raised the temperature of its transformation to hematite (T-t), as determined by differential thermal analysis, from 340 degreesC without Si to 740 degreesC at X-Si = 0.270. Deformation of the hematite unit cell and the greater suppression of crystal growth along a as compared to along c its X-Si increased from 0 to 0.0403 were probably caused by structural incorporation of Si, Rietveld fits of X-ray diffraction (XRD) data showed a consistent Fe deficit of up to 1 Fe atom (from the ideal of 12.0) in the hematite unit cells. The magnetic hyperfine field (B-hf) at room temperature of the hematites heated to 800 degreesC Cull regularly from 51.65 to 51.16 T as X-Si increased from 0 to 0.0679. No separate SiO2 phase was detected by XRD, and the Morin transition was suppressed even at 4.2 K. At X-Si greater than or equal to 0.134, however, B-hf returned to 51.40 T, a value similar to that obtained at X-Si similar to 0.03. This increase indicates that part of the Si was ejected from the structure and, as seen by a weak Mossbauer doublet, formed a Si-Fe-O phase with a quadrupole split of 0.92 mm s(-1). This phase gave a broad XRD feature centered at 0.36 nm.