A Structural Model for N a T U R a L Siliceous Ferrihydrite

-X-ray diffraction of four natural samples of ferrihydrite indicates the presence of crystalline domains within the primary particles. The average diameter of the primary particles (determined from low-angle powder patterns) decreases from 4.1 nm to 2.5 nm as the domain size in the xy-plane (determined by applying the Scherrer equation to the broad [110] XRD peak at 0.26-0.27 nm) decreases from 1.0 nm to 0.77 nm. The Si content (measured by acid-oxalate extraction) increases from 4.1% to 6.1% as both the domain and particle sizes decrease; other factors, however, are likely to be important in influencing particle size. For one sample of ferrihydrite, the smallest possible domain (i.e., c = 0.94 nm in the z-direction) contains 36 O atoms and three Si atoms. A model for ferrihydrite is suggested in which silicate bonds to, and bridges, the surfaces of the domains. Th~ model can account for several aspects of the behavior of siliceous ferrihydrites. Key Words--Domain size, Ferrihydrite, Low-angle X-ray diffraction (LAXRD), Particle size, Silicate, Structure, X-ray diffraction (XRD). I N T R O D U C T I O N Ferrihydrite is a hydrous Fe oxide which, because of short-range structural order, gives broad X-ray diffraction (XRD) peaks (e.g., Schwertmann, 1988). Up to five peaks are usually observed for natural ferrihydrites though seven have been recorded for synthetic samples. These can be indexed to a hexagonal unit cell with a = 0.508 nm and c = 0.94 nm. There appears to be a range of naturally occurring ferrihydrites from those that show at least five peaks and may be considered "well ordered," down to those which show only two and which have sometimes been tentatively called "protoferrihydrite" or "two-line ferrihydrite" (Childs, 1992). Various formulae have been proposed for ferrihydrite. These include 5Fe203 "9H20 (Fleischer et al., 1975), FesHOs '4H20 (Towe and Bradley, 1967) and Fe2032FeOOH. 26H20 (Russell, 1979). The structure of ferrihydrite is not as well understood as those of other Fe oxide minerals (Childs, 1992). The IMA (Fleischer et al., 1975) adopted a structural model of hexagonal-close-packed layers of O 2-, OH , and H20 with Fe In occupying octahedral positions. Relative to the distribution of Fe atoms in hematite, some of those in ferrihydrite are vacant and, therefore, the repeat period is four planes of O atoms rather than six as in hematite (Towe and Bradley, 1967). Recently, a different structural model for synthetic ferrihydrite based on a trigonal cell with 2/3 octahedral and about 1/3 tetrahedral Fe was proposed by Eggleton and Fitzpatrick (1988), though Manceau et al. (1990), in response, claimed that the evidence from X-ray absorption spectroscopy pointed to the absence of tetrahedral Fe. Copyright 9 1992, The Clay Minerals Society Naturally occurring ferrihydrites from soil-related environments contain Si, typically of the order of 5% (w/w), although this varies considerably from sample to sample (Childs, 1992). Evidence of Si -O-Fe bonds is observed in infrared spectroscopy (Carlson and Schwertmann, 1981; Childs et aL, 1986). Whether silicate is " in the structure" or "adsorbed on the surface" of ferrihydrite is still a subject of debate and research (e.g., Vempati and Loeppert, 1989). The presence of Si may be responsible for the slow rate of transformation of natural ferrihydrite to more crystalline Fe oxides. In this paper we use low-angle X-ray powder diffraction (LAXRD) analyses to measure the average diameter of the primary particles of four naturally occurring ferrihydrite samples, and the Scherrer equation (Brindley, 1980) is applied to the (110) broad peak observed in conventional XRD patterns to help estimate the size of the crystalline domains, i.e., portions of the structure that diffract X-rays coherently.