The thermal transformation under vacuum and the reduction behavior in hydrogen atmosphere of 2- and 6-line ferrihydrite (FeOOH center dot nH(2)O) as well as 2-line FeOOH center dot nH(2)O deposited onto silica are reported. The investigation methods include magnetization measurements, temperature programmed reduction, in-situ X-ray diffraction and Mossbauer spectroscopy. The thermal transformation of FeOOH center dot nH(2)O to hematite (alpha-Fe2O3) was monitored through changes in the magnetization as a function of temperature; it appears to proceed through the loss of the lattice water and sintering accompanied by improved crystallinity and structural changes. Such a transformation is initiated at T similar to 580 K for 2-line and 6-line FeOOH center dot nH(2)O and at T similar to 660 K for 2-line FeOOH center dot nH(2)O/SiO2, i.e., the presence of SiO2 appears to inhibit the transformation. SiO2 also tends to prevent the increase of the crystallite size above a certain threshold value. Reduction reactions are initiated at relatively lower temperatures (similar to 480 K) implying that a gaseous environment facilitates the thermal dehydration/dehydroxylation process. Three different reduction mechanisms of FeOOH center dot nH(2)O to metallic iron (alpha-Fe) are observed; a two-stage process via magnetite (Fe3O4) as an intermediate phase for 2-line FeOOH center dot nH(2)O, a three-step reduction involving Fe3O4 and wustite (FeO) as intermediate phases for 2-line FeOOH center dot nH(2)O/SiO2 and a thermal transformation to alpha-Fe2O3 followed by a two-step reduction via Fe3O4 intermediate for 6-line FeOOH center dot nH(2)O. It is inferred that SiO2 interacts with Fe species to form Fe-O-Fe structures which tend to inhibit the thermal transformation and resist the reduction. (C) 2014 Elsevier B.V. All rights reserved.