Aluminum-substituted hematites (Fe2−xAlxO3) were synthesized from Fe-Al coprecipitates at pH 5.5, 7.0, and in 10−1, 10−2, and 10−2 M KOH at 70°C. As little as 1 mole % Al suppressed goethite completely at pH 7 whereas in KOH higher Al concentrations were necessary. Al substitution as determined chemically and by XRD line shift was related to Al addition up to a maximum of 16–17 mole %. The relationship between the crystallographic a0 parameter and Al substitution deviated from the Vegard rule. At low substitution crystallinity of the hematites was improved whereas higher substitution impeded crystal growth in the crystallographic z-direction as indicated by differential XRD line broadening. At still higher Al addition crystal growth was strongly retarded. The initial Al-Fe coprecipitate behaved differently from a mechanical mixture of the respective “hydroxides” and was, therefore, considered an aluminous ferrihydrite.

Ferrihydrite was transformed to goethite and/or hematite at various temperatures, [OH], and [Al]. Increasing temperature and [Al] favored hematite, increasing [OH] favored goethite. A given [Al] induces hematite more effectively at lower [OH]. Al substitution in goethites increased linearly with log[Al], but was independent of temperature. At a given [Al], substitution increased with decreasing [OH]. In a plot of Al/Fe in the goethite against [Al]/[Fe(OH)4−] in solution a straight line was obtained for all preparations independent of [OH].

The different types of iron oxide phases associated with the surfaces of two suites of kaolins from Georgia, U.S.A., and from the Southwest Peninsula of England, have been identified using electron spin resonance (ESR) spectroscopy combined with magnetic-filtration, thermal, and chemical treatments. It has been shown that the English kaolins are coated with a lepidocrocitelike phase, which is readily removed by de Endredy's method of deferrification, while the Georgia kaolins are coated with a hematite- or goethitelike phase, which is not removed by this treatment. Throughout the course of this study, the effects of the various physical and chemical treatments on the brightness values of the kaolins were examined.

Hydroxy-carboxylic acids inhibit the crystallization of ferrihydrite in the pH range 9–11 in the order

citric > meso tartaric > L-tartaric ≫ lactic

and favor hematite formation relative to goethite in the order

L-tartaric > citric > meso tartaric > lactic.

The crystal shape of hematite can change from hexagonal plates to acicular in the presence of these acids. The influence of the acids on the crystallization rises with increasing concentration and with falling pH.

The effectiveness in suppressing crystallization depends on whether and how strongly the acid adsorbs on ferrihydrite and how strongly it complexes with Fe3+ in solution. Inhibition of crystallization of hematite is believed to be due to the di- and tricarboxylic acid linking ferrihydrite particles in an immobile network. Goethite formation is suppressed by the acid complexing with Fe in solution and hindering nucleation; strongly adsorbing acids also adsorb on the nuclei and hinder further growth. Certain acids can induce hematite formation because they contain a group which acts as a template for nucleation of hematite.

A series of mixed iron and titanium oxide coprecipitates ranging in composition between 0 < Ti/Ti + Fe < 1 was synthesized and aged under varying conditions of pH, temperature and time in order to establish a working model for pedogenic titanium and titano-ferric oxides. X-ray powder diffraction (XRD), selective chemical dissolution, magnetic susceptibility, charge distribution and electron optical data indicate that the freshly prepared Fe-Ti oxides consist of an Fe-rich (Ti-ferrihydrite) phase (Ti/Ti + Fe ⩽ 0.70) having pH-dependent positive charge and a Ti-rich phase (Ti/Ti + Fe ⩾ 0.7) with permanent and pH dependent negative charge.

Synthetic Ti-ferrihydrite and amorphous TiO2 were completely soluble in acid ammonium oxalate (2 hr extraction in the dark) whereas poorly crystalline anatase (width at half height, WHH > 2.0°2θ) was partly oxalate soluble. NH4-oxalate soluble Ti was particularly high in soils developed under a cool montane climate (afro-alpine) and lower in soils of warmer subtropical climate, which contain anatase and rutile.

Several mixed Fe-Ti crystalline phases were identified after aging NH3 coprecipitates of Fe and Ti nitrate at 70°C and pH 5.5 for 70 days:

1. (1) goethite and hematite in the composition range 0 < Ti/Ti + Fe ⩽ 0.20; at low Ti concentrations (<5 mole %) goethite was favored and/or hematite inhibited

2. (2) microcrystalline pseudorutile in the composition range 0.20 ⩽ Ti/Ti + Fe ⩽ 0.70

3. (3) anatase and ferriferous anatase in the range 0.70 ⩽ Ti/Ti + Fe < 1.0; with decreasing proportion of Ti the crystal-Unity of anatase decreased.

The results suggest that secondary or pedogenic Ti-Fe oxides can form by coprecipitation and crystallization in the weathering solution, and emphasize the essential role of water (as opposed to dry oxidation) in the alteration of primary titaniferous minerals.

The dissolution of synthetic magnetite, maghemite, hematite, goethite, lepidocrocite, and akaganeite was faster in HCl than in HClO4. In the presence of H+, the Cl− ion increased the dissolution rate, but the ClO4− ion had no effect, suggesting that the formation of Fe-Cl surface complexes assists dissolution. The effect of temperature on the initial dissolution rate can be described by the Arrhenius equation, with dissolution rates in the order: lepidocrocite > magnetite > akaganeite > maghemite > hematite > goethite. Activation energies and frequency factors for these minerals are 20.0, 19.0, 16.0, 20.3, 20.9, 22.5 kcal/mole and 5.8 × 1011, 1.8 × 1010, 7.4 × 107, 5.1 × 1010, 2.1 × 1010, 3.0 × 1011 g Fe dissolved/m2/hr, respectively. The complete dissolution of magnetite, maghemite, hematite, and goethite is well described by the cube-root law, whereas that of lepidocrocite is not.

Adsorption of p-hydroxybenzoate anion on synthetic Fe oxides, hydroxides, and oxyhydroxides (hereafter referred to as oxides) was measured at pH 5.5, and the organic-oxide interaction was characterized using diffuse-reflectance Fourier-transform infrared (DRIFT) spectroscopy. Surface complexes with ferrihydrite, hematite, goethite, and noncrystalline Fe oxide were investigated. Infrared (IR) spectra of the oxides after separation from p-hydroxybenzoate solutions showed the organic to be coordinated by an inner-sphere mechanism to the oxide surface through the carboxylate group. Bidentate binding of the carboxylate was identified to be the dominant type of complexation on the oxides. Although the IR study suggested that goethite formed the strongest surface iron-carboxylate bond, the total amount of organic adsorbed was the least on this oxide. This result suggested that bond energy was less important than adsorption site density in determining the amount of organic anion adsorption. Increasing ionic strength had little effect on adsorption by the noncrystalline Fe oxide, but dramatically decreased adsorption on hematite and goethite. This difference might have been due to anion competition for binding sites. At pH 5.5, the amount of organic adsorbed per unit weight of oxides (in 0.05 M NaClO4) followed the order: ferrihydrite > noncrystalline Fe oxide > hematite ≫ goethite. Adsorption per unit of surface area however, followed the order: ferrihydrite > hematite > noncrystalline Fe oxide ≫ goethite. The hematite adsorption reaction appeared to be driven by entropy, inasmuch as the reaction was endothermic. The difference among the oxides in surface reactivity toward p-hydroxybenzoate is hypothesized to have been caused by differences in both quantity and structural arrangement of reactive sites on the oxide surface.

Mössbauer spectra of 15 hematites with Al substitutions between 0 and 10 mole % were taken at room temperature. X-ray powder diffraction indicated dimensions of these hematites in the c-direction to range upwards from 27 nm to crystals large enough to show no line broadening. The Mössbauer spectra showed that magnetic hyperfine fields decreased both with increasing Al-for-Fe substitution and with decreasing crystal size. These relationships indicate that hyperfine field variations cannot, as has been done in the past, be unequivocally related to Al substitution alone. Hyperfine field reductions were paralleled by Mössbauer line broadening due to hyperfine field distributions. Only the hematites heated to 1000°C showed a significant variation of quadrupole splittings with Al substitution. No dependence of quadrupole splitting on crystal size was observed, indicating no detectable distortion of coordination polyhedra in the particle size range studied.

The mineralogy of the clay fraction was studied for soils and saprolite on two Blue Ridge Front mountain slopes. The clay fraction contained the weathering products of primary minerals in the mica gneiss and schist parent rocks. Gibbsite is most abundant in the saprolite and residual soil horizons, where only chemical weathering has been operable. In colluvial soil horizons, where both physical and chemical weathering have occurred, the clay fraction consists largely of comminuted primary phyllosilicates —muscovite, chlorite, and possibly biotite—and their weathering products: vermiculite, interstratified biotite/vermiculite (B/V), and kaolinite. The clay-size chlorite contains Fe2+ as indicated by Mössbauer spectroscopy, and is more resistant to weathering than biotite. The vermiculite and B/V, both weathering products of biotite, contain Fe3+. Vermiculite in colluvial soils and, especially, surface horizons is weakly hydroxy-interlayered. The kaolinite in the clay fraction resulted at least partly from the comminution of kaolinized biotite in coarser fractions.

The hematite content ranged from 0 to 8% of the clay fraction and strongly correlates (r =.95) with dry clay redness, as measured by the redness rating: RR = (10 - YR hue) × (chroma) ÷ (value). The hematite is largely a product of the weathering of almandine; thus, the soil redness is dependent on the amount of almandine in the parent materials and its degree of weathering in the soils. Goethite (13–22% of the clay fraction) imparts a yellow-brown hue to soils derived from almandine-free parent rocks. The release of Fe in relatively low concentrations during the weathering of Fe-bearing primary minerals, particularly biotite, appears to have promoted the formation of goethite.

An X-ray thermodifferential powder diffraction method for the quantitative determination of goethite and hematite in lateritic bauxites has been developed and consists of measuring the integrated intensities of the 012 line of hematite before and after heating the sample at 900°C and of correcting the obtained values by the X-ray mass absorption coefficient of either the untreated or heated matrix. From the corrected line intensities and the chemical analyses, the amounts of iron to be allocated to goethite and hematite in the untreated samples can be estimated. The actual content of goethite and hematite in a sample is calculated by taking into account the degree of Al substitution in each of these minerals. The method was tested on artificial mixtures of goethite and hematite and subsequently used to analyze 98 auger drill samples from lateritic bauxites of Guinea Bissau. The estimated precision of the determination of goethite and hematite content was ±2% (absolute). The method can not be applied to samples containing < 10% Fe2O3 (on a whole weight basis) unless preconcentration is carried out.

As part of the characterization of a Tunisian red soil profile, six samples, taken at different depths, were investigated by Mössbauer spectroscopy at room temperature and at 80 K to obtain information about the various types of Fe oxides present. By considering magnetic hyperfine field distributions, the spectra of goethite and hematite were well resolved. Chemical analyses of the samples revealed a partial substitution of Fe by Al and Mn. The spectral behavior of the goethite was predominantly influenced by crystallinity and amount of Al substitution which resulted in a reduction of the magnetic hyperfine field. The effect of Mn substitution was much more pronounced in the hematite spectrum as a consequence of a stronger suppression of the Morin transition by Mn than by Al.

Ferruginous clay partings in limestones of the marine, largely evaporitic, Upper Triassic Mohila Formation (Makhtesh Ramon, Israel) contain hexagonal plates and cube-like bodies up to one millimeter across. Analyses by electron microscopy, infrared and Mössbauer spectroscopy, and X-ray powder diffraction indicate that the matrix contains Fe-rich anhedral kaolinite, up to 100 μm in size; the hexagonal plates are composed of euhedral, Fe-free kaolinite covered with well-developed acicular goethite and platy hematite (0.5 to 2 μm in size), and the cubes consist of fine-grained goethite with minor amounts of kaolinite. The anhedral kaolinite appears to be detrital, the hexagonal plates to be authigenic, and the cubes to be pseudomorphs after pyrite. Oxidation appears to have altered Fe-rich kaolinite and pyrite to Fe-free kaolinite, goethite, and hematite and was accompanied by recrystallization and pseudomorphic replacement. The alteration process was slow and was probably induced by a small increase in pH and in the Al/Fe ratio, resulting from oxidation of reduced components (pyrite, ferrous carbonate, organic matter) in a semiclosed, sediment-mud system. Overlying kaolinitic flint clay deposits may be the final product of a similar process.

Synthetic Al-hematites prepared from ferrihydrites, at low (∼ 100°C) and high (400° and 800°C) temperatures were studied for their morphological, crystallochemical, and infrared (IR) characteristics. Low-temperature Al-hematites had a platy morphology (the plate thickness was inversely related to amount of Al substitution), and the high temperature Al-hematites showed a poorly defined morphology due to interparticle sintering. In the low-temperature Al-hematites shifts in the IR mode frequencies were noted and could be partly explained by a shape factor that was deduced from particle morphology. The intrinsic effect of Al substitution, however, was to produce shifts of as much as 10–15 cm−1 for the highest Al substitution (∼ 16%). Similar shifts were observed for the high-temperature hematites in which morphology was not appreciably affected by Al substitution.

The interaction of Mn and akaganéite in neutral to alkaline media has been investigated using X-ray powder diffraction and transmission electron microscopy. Akaganéite transformed into goethite and/or hematite, whereas Mn precipitated as hausmannite and birnessite at pH > 12 and as manganite at pH 7.5–8.5. Mn influenced the kinetics of the transformation of akaganéite: the rate-determining step, i.e., the dissolution of akaganéite, was retarded by adsorbed Mn species. Hematite formation was not suppressed. By catalyzing the air oxidation of adsorbed Mn(II), akaganéite promoted the formation of birnessite. Akaganéite did not retard recrystallization of the Mn phases. The incorporation of Mn in the structure of goethite formed in this system was negligible, and jacobsite (MnFe2O4) did not form. The formation of mixed Mn-Fe phases appeared to require a ratio of Mn2+: Fetotal > 0.02; this ratio was not achieved due to the oxidation of Mn2+ at the akaganéite surface.

Ferrihydrite prepared in different manners was kept under relative humidities ranging from 75 to 100% and at temperatures of 45° and 55°C for 180 days. Ferrihydrite transformed to hematite and goethite at relative humidities close to 100%, but at lower relative humidities the transformation was less pronounced and hematite was highly favored over goethite. Increasing temperature also favored hematite over goethite, and Al substitution completely prevented goethite formation. These results suggest that hematite can form in relatively dry, warm soils or sediments, although more slowly than in moister environments.

The conversion of akaganéite to goethite and/or hematite in alkaline media has been followed by X-ray powder diffraction and transmission electron microscopy (TEM). The rate of transformation fell and the amount of hematite in the product increased as the [OH−] decreased to < 1 M. Kinetic studies and TEM indicated that the transformation involved dissolution of akaganéite followed by reprecipitation of goethite and/or hematite. The rate-determining step was the dissolution of akaganéite.

Silicate species retarded the formation of goethite + hematite principally by inhibiting dissolution of akaganéite; to a lesser extent, they interfered with the nucleation of goethite. Silicate modified the morphology of goethite, but not hematite.

Comparison of the transformation behavior of akaganéite with that previously observed for ferrihydrite indicated that the composition of the reaction product depended strongly on the transformation conditions, i.e., pH and the presence of foreign species. The nature of the solid precursor was important insofar as its degree of crystallinity governed the dissolution kinetics and its surface properties influenced interaction with any foreign species in the system.

The kinetics of hydroquinone oxidation by aqueous suspensions of pure hematite and goethite-ferrihydrite mixtures at pH 6.0, 7.4, and 9 was studied using an on-line analysis system. The electron transfer between hydroquinone and the Fe oxides was monitored by UV-visible and electron spin resonance spectroscopy. The adsorption of organics on the Fe oxide surface was detected by Fourier-transform infrared spectroscopy. For different Fe oxides, a higher surface area was correlated with a greater oxidizing ability and greater adsorption of organics, suggesting that the oxidation reaction was a surface process. A reversal of the initially rapid redox reaction was found in this system, suggesting a delayed release of Fe2+ into solution as the reduction of the Fe oxide proceeded. Redox potential calculations confirmed the thermodynamic favorability of the reaction reversal. A distribution of the reduced state over neighboring Fe atoms on the oxide surface probably was responsible for the initial suppression of Fe2+ release into the aqueous phase. Based upon these observations and detection of the semiquinone radical as an intermediate of hydroquinone oxidation, an inner-sphere one-electron transfer mechanism for the oxidation of hydroquinone at the oxide surface is proposed.

Pisolites from an iron crust in western Senegal were studied by conventional and high-resolution electron microscopy to determine their internal structure and the genetic processes that led to their formation. Each pisolite consisted of a concentric structure of hematite rimmed by goethite. Two types of goethite were distinguished: (1) large (≃ 0.6 μm long and 0.06 μm wide), euhedral laths arranged in fibrous aggregates of slightly misoriented grains devoid of internal defects as shown by their two-dimensional lattice images, and (2) a matrix of smaller (≃400 Å), anhedral grains surrounded by the larger laths. Based upon the crystal habit and the presence or absence of internal alveoles, the large goethite laths probably grew at the expense of the matrix goethite. Poorly crystalline kaolinite, presumably formed from well-crystalline precursor kaolinite, and clusters of partially dissolved quartz grains were also imaged. In addition, two uncommon phases were found—maghemite in topotactic relationship with hematite and a layered, Fe-rich, mica-like mineral with a 2M superstructure. Unlike kaolinite, this latter phase was likely in equilibrium with iron oxyhydroxides. Substituted Al probably was released during goethite recrystallization, and mass transfers probably took place through the heterogeneous porosity (i.e., large voids and cracks coupled with fine pores) revealed by transmission electron microscopy.