A Structural Model for Natural Siliceous Ferrihydrite

Abstract

Abstract--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.

The influence of silicate on transformation pathways of synthetic 2-line ferrihydrite

“…A total mass loss of FHYD-000 is 21.28 mass% and remains similar for all the samples studied. The losses are comparable with earlier measurements of both synthetic [5,17,35,37,39] and natural ferrihydrites [16,25,49], but are explicitly higher than theoretical water content (*17 mass%), due to a high amounts of physisorbed water. A bend on DTA curves of low-Si samples at ca.…”

“…For all the samples containing Si, an additional band attributable to Si-O-Fe stretching [16,25,41] occurs. Its shift from 930 up to 1000 cm -1 with increasing Si/Fe molar ratio indicates partial silicate polymerization [25,46]. Similar features were reported previously [18,27,40].…”

“…These ions affect ferrihydrite composition, surface chemistry, reactivity and sorption properties [14,[17][18][19][20][21][22]. Silicate, the most important impurity, lowers crystallinity of this nanomineral and modifies its morphology, magnetic ordering and solubility [15,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39], though the ferrihydrite structure itself is not directly altered [30]. Reducing crystallinity has been thought to result from the formation of soluble Fe-Si complexes during the oxyhydroxide precipitation.…”

“…Also, synthetic ferrihydrites analyzed so far exhibited usually low Si/Fe molar ratios, and the data for higher Si/Fe ratios are scarce [44]. The reason for this is probably a quite low silica content (up to several mass percent) in most of natural ferrihydrites [1,25,[45][46][47][48], but in some environments, such as modern seafloor hydrothermal vents, higher Si/Fe ratios (up to ca. 3) were reported [15,16,49,50].…”

Inhibition of Sintering by Si During the Conversion of Si-Rich Ferrihydrite to Hematite

“…On the other hand, Yoshinaga and Kanasaki (1993) precipitated ferrihydrite in the presence of Si and Ge, which have significantly different ionic radii, and concluded from the similarity in d-values that these elements did not substitute in the ferrihydrite structure. Parfitt et al (1992) proposed a model where silicate bridges the surfaces of ferrihydrite domains, which aggregate to form primary particles. The rearrangement of Fe octahedra to form hematite is inhibited by the Si in those particles.…”