Abstract--The unit-cell dimensions of synthetic, Al-substituted goethites showed that the c dimension is a linear function of A1 substitution in the range 0-33 mole % A1, but that the a dimension is variable over this same range. The b dimension is also linearly related to Al substitution but is slightly more variable than the c dimension for A1 substitutions of 20-33 mole %. The variability of the a dimension is postulated to be the result of structural defects. An improved procedure for estimating Al substitution from x-ray powder diffraction positions requires (1) calculation of the c dimension from the positions of the 1 l0 and 111 diffraction lines using the formula: c = (l/d(11 l) 2 -l/d(110)2) -'h, and (2) estimation of A1 substitution from the relationship: mole % A1 = 1730 -572.0c. The 95% confidence interval of the estimate is +2.6 mole % A1 when using this procedure, in contrast to _+4.0 mole % A1 when the position of the 111 reflection alone is used.

No abstract

Fifty-seven goethites, synthesized by a variety of procedures and with A1 substitutions of 0-33 mole%, were characterized by XRD, IR, DTA, TEM and chemical techniques. Most of the properties measured showed significant intercorrelations. Mole% A1 substitution (measured chemically) did not explain all the relationships among variables, but the inclusion of Aa, defined as the observed a dimension minus the a dimension predicted by the Vegard rule, explained much of the variation not explained by A1 substitution. OH stretching frequency, in particular, was better correlated with Aa than the AI substitution or other properties. The properties of the goethites could best be explained by a combination of A1 substitution and structural defects, with Aa being a measure of the defects. In general, the effect of structural defects was opposite to that of A1 substitution. Increase in AI substitution led to a decrease in all three unit-cell dimensions and OH stretching frequency and to an increase in the distance between the two OH bending vibrations (6OH-yOH) and the temperature of dehydroxylation. Increase in structural defects, on the other hand, caused the a dimension, Aa, and OH stretching frequency to increase and 6OH-yOH and the average temperature of dehydroxylation to decrease. Crystal size tended to decrease with increases in both A1 substitution and structural defects. Surface area was significantly correlated with the reciprocal of the mean crystal thickness in the a direction. Comparison of XRD and TEM data showed that many samples consisted of crystals with several coherently scattering domains. The nature of the defects, i.e. whether they occur primarily in the interdomain areas or whether they are also distributed throughout the coherently diffracting domains, could not be determined.The ionic substitution of A1 for Fe in the structure of goethite has a marked effect on goethite crystal properties. The most obvious structural change is-a decrease in unit-cell size (Thiel, 1963; Jdn~is & Solym~, 1970) caused by the slightly smaller A1 cation. This reduction is, however, not regular. Schulze (1984) has shown that only the c dimension decreases strictly linearly with increasing AI substitution, whereas the a dimension can be quite variable for samples with the same degree of A1 substitution and is always greater than the a dimension predicted by the Vegard rule. The b dimension also decreased linearly with AI substitution but it was slightly more variable than the c dimension.It might be expected that the changes in crystal chemistry (A1 instead of Fe) and unitcell size would also lead to a variation in the properties of Al-substituted goethites. Variations in crystal size and surface area, IR spectra, structural OH content, thermal and magnetic properties, phosphate adsorption, and dissolution behaviour in HCI have been

Ferrihydrite is a poorly crystalline, natural Fe3+ oxide which occurs in ochreous spring precipitates and hydromorphic soils of humid temperate climates. The identification of ferrihydrite in soils is complicated by its association with goethite, quartz, and layer silicates.The following criteria were used to identify ferrihydrite in Fe‐oxide accumulations from soils: high solubility in acid oxalate, five to six broad x‐ray diffraction lines, and the existence of a typical magnetic hyperfine field distribution at 4K in Mössbauer spectra rather than a discrete field value. Identification of low concentrations of ferrihydrite (≲ 20% oxalate‐soluble Fe) by x‐ray diffraction was made possible by subtracting diffraction data obtained after oxalate treatment from data obtained before such a treatment (differential XRD). Oxalate treatment preferentially dissolves ferrihydrite over goethite. This led to an increase in the quadrupole splitting observed in Mössbauer spectra from ∼0.10 to ∼0.23 mm s−1, resulted in a significantly narrower field distribution, and intensified the goethite DTA peak.

No abstract

No abstract

No abstract

No abstract