Fig. 4.

Evidence for Fe vacancies in ferrihydrite. (A) Intensity increases for specific correlations (Red Arrows) during initial 8 h aging time are attributed to filling of vacant Fe sites. Shifts in other correlations (Blue *) are due to the decreasing a-dimension of the unit cell during the fh → ferrifh transition. (B) Element-specific and (C) site-specific partials calculated from the refined structure of ordered ferrifh (Fig. 3B). (Inset) Refined occupancies (%) from PDF analysis normalized for total Fe for three Fe sites in ferrihydrite during aging.