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Ferrihydrite occurs in soils undergoing rapid weathering, and in soils containing soluble silicate or organic anions which inhibit the formation of more crystalline iron oxides. Because of its very high specific surface area and adsorptive capacity (analogous to allophane), ferrihydrite can profoundly influence soil properties, even if present in only low concentrations. Ferrihydrite was recognised as a mineral by the International Mineralogical Association in 1975. Its structure and chemical formula, however, are not yet clearly understood. Most evidence to date indicates hexagonal‐close‐packed layers of O2′, OH, and H2O with Fe(III) occupying octahedral positions and giving a trigonal unit cell (a=0.508 nm; c=0.94 nm). Some samples appear to have only a partially ordered structure and uncertainty exists as to how to name such material. Natural ferrihydrites commonly contain up to about 9% Si and the role and location of silicate are subjects of active research. At concentrations greater than 5‐10%, ferrihydrite in soils can usually be identified by X‐ray diffraction. At lower concentrations, a combination of methods can be indicative. Acid‐oxalate‐extractable iron is a convenient and often useful indicator of the presence and quantity of ferrihydrite in a soil, though it cannot be regarded as a means of positive identification.

The structure of Silica Springs allophane from New Zealand has been studied by high-resolution 27A1 and 29Si NMR spectroscopy, IR spectroscopy and electron microscopy. Experimental NMR conditions were chosen to give a quantitative relationship between signal strength and A1 content. Aluminium is present in both octahedral (A1 vi) and tetrahedral (A1 iv) coordination, and the ratio A1 vi : A1 iv was obtained for three samples with different total AI : Si ratios. In all cases the A1 iv : Si ratio was 1 : 3, consistent with the ratio in ideal muscovite. The negative charge arising from A1 iv was apparently balanced by positive charge at adjacent A1 vi sites. Both NMR and IR spectra indicate that Si exists in a condensed network, probably a sheet, with Si atoms linked (through oxygen) to one, two and, possibly, three other Si atoms, but not four. The spectra are consistent with a mixture of Si~O-Si, Si-O-AI, and Si-OH linkages. A structure based on fragments of single curved 1 : 1 aluminosilicate layers, with the tetrahedral sheet on the outer surfaces, is suggested, the diameter of curvature being 2-3 nm. The tetrahedral sheet is more or less complete and has a A1 iv : Si ratio of 1 : 3; the octahedral AI vi sheet is incomplete. Electron micrographs suggest that, at least in some cases, the fragments form more or less complete spherules. This proposal seems to accord with Farmer's concept of "aluminous hydrous feldspathoids". The minimum pH at which Silica Springs allophane has precipitated corresponds to the initial presence of tetrahedrally coordinated A1 species in solution.Silica Springs allophane is the name given to the creamy-white deposit in the stream bed below the vents of Silica Springs on Mount Ruapehu, Tongariro National Park, New Zealand. The site and the allophane have previously been described in detail by Wells et al. (1977). Silica Springs water is derived from the addition of geothermal CO2 (and possibly H2S) to near-surface meteoric water and the chemical attack of this water on the andesitic rocks and soil through which it passes. The pH of the water at the spring vents is 5.45 and increases downstream with turbulence and loss of COa, reaching 5.90 at 100 m, where deposition of allophane first occurs, and 6-80 at 320 m which is below the region of maximum deposition. The deposit was described as "... a hydrous X-ray-amorphous, aluminosilicate (allophane)". The total A1 : Si atomic ratio was found to vary from -1.7 to ~1-0 with increasing distance downstream. Wells et al. (1977) provided no information about the coordination of ALl in the deposit.On the basis of the position of X-ray fluorescence A1-Ko~ lines, Parfitt & Henmi (1980) suggested that -40% of the A1 in a sample of Silica Springs allophane might occur in tetrahedral sites. They also suggested, from gas chromatography of trimethylsilyl ether derivatives, that the silicate tetrahedra in their Silica Springs allophane sample was more polymerized than in soil allophanes, consistent with infrared (IR) spectra.More recently, Good...

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.

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