In the absence of oxygen, Fe(II) chloride, sulfate, and carbonate solutions react at pH 6.5 to 7 with aluminum hydroxide suspensions to form new Fe(II)-Al(III) hydroxy anion compounds of the pyroaurite group. The Fe(II)-Al(III) hydroxy-chloride and -sulfate compounds are isostructural with Fe(II)- Fe(III) “green rust” compounds with A1 essentially substituting for Fe(III). Where CO32- is the only anion in the system, an Fe(II)-Al(III) compound isostructural with hydrotalcite is formed. Either in the dried or wet state, these compounds are unstable in air due to oxidation of Fe(II). Oxidation of the dried sample in air yields akaganeite or aluminous ferrihydrite, whereas, if the sample is maintained in a moist condition and oxidized by air under water, lepidocrocite or aluminous goethite is produced along with small amounts of ferrihydrite. On X-ray powder diffraction, the lepidocrocite so formed commonly shows no diagnostic (020) basal reflection, or one with a markedly reduced intensity. The products of oxidation, and the rapidity of their formation, appear to be dependent on the composition of the initial double hydroxy compound and the conditions under which the oxidation is carried out.

The green colored compounds commonly observed in gleyed soils also rapidly become yellow brown on exposure to air, and difficulty arises in the identification of the Fe oxide phases assumed to be present. Similar conditions and reactants to those involved in the formation of the compounds described above are expected in these soils, and it is suggested that Fe(II)-Al(III) members of the pyroaurite group may form in such an environment.

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.

The effects of Na-citrate-dithionite (NaCD), ammonium oxalate in the dark (NH4Ox-D), and photolytic reaction under ultraviolet radiation (NH4Ox-P) on the mineralogy of <2-µm fractions of selected soils from Virginia were investigated. The NH4Ox-D treatment removed the smallest amounts of Al (<0.22%) and Fe (<0.50%) from all soils, indicating low levels of noncrystalline material in these materials. From the six soils examined, NH4Ox-P treatment extracted 5–62% more Fe and 12–300% more Al than the NaCD treatment. The NH4Ox-D and NaCD treatments revealed no X-ray diffraction detectable alterations to mineral phases present in <2-µm fractions of these soils. The NH4Ox-P treatment, on the other hand, produced considerable degradation of hydroxy-interlayered vermiculites in these soils, as evidenced by a shift of the 14-Å X-ray diffraction maxima to lower spacings with heat treatment of the sample. The NH4Ox-P treatment removed variable amounts of hydroxy-Al material from the interlayers of 2:1 layer silicates, depending on their stability and degree of development.

Infrared analysis showed that the bonding habit of oxyanions with freshly precipitated hydrous ferric oxides depends upon the nature of the anion and its hydration level. Monovalent oxyanions adsorb through an electrostatic interaction with the hydrated hydrous oxide surface. All divalent oxyanions, with the exception of tellurate, coordinate directly with surface iron cations. Tellurate, an octahedral anion, apparently penetrates and incorporates in the hydrous oxide structure. The symmetry of the free anion has a significant role in determining the configuration of the resultant complex. For anions of the same charge, those with tetrahedral geometry (in uncoordinated states) show a higher degree of specificity for the surface than the trigonal planer anions. Without exception, each bidentate bridging complex forms by replacement of protonated and unprotonated hydroxyls. With the anion geometry and the charge being equal, the suspension pH determines the adsorption capacity of the hydrous oxide.

The effect of pH on the flocculation-dispersion behavior of noncrystalline aluminum and iron oxides, kaolinite, montmorillonite, and various mixtures of these materials was investigated. The clays were Na- or Ca-saturated and freeze-dried before use. Critical coagulation concentrations (CCC) of all materials and mixtures were found to be pH dependent. The Al oxide was flocculated at pH >9.5 and the iron oxide was flocculated between pH 6.0 and 8.2; i.e., flocculation occurred at pHs near the point of zero charge (PZC). The CCC of both the Na- and Ca-clay systems increased with increasing pH. The effect of pH was greater for the Na-kaolinite (flocculated at pH 5.8 and having a CCC of 55 meq/liter at pH 9.1) than the Na-montmorillonite system (having a CCC of 14 meq/liter at pH 6.4 and a CCC of 28 meq/liter at pH 9.4). A 50/50 mixture of Na-kaolinite and Na-montmorillonite behaved more like montmorillonite (having CCCs of 13 and 33 meq/liter at pH 6.2 and pH 9.0, respectively). The presence of either noncrystalline oxide decreased the CCC over that of the clay(s) alone; the decrease occurred at pHs >7 for Al oxide and at pHs >6.5 for Fe oxide. Aluminum oxide produced a greater decrease in CCC than Fe oxide, especially at pHs >8. The effect of each oxide on CCC was greatest near the PZC, 9.5 and 7.2 for Al and Fe oxide, respectively.

The Bingham yield stress for suspensions of illite-iron oxide complexes is examined as a function of pH and iron oxide content. Addition of iron oxides to illite increased the yield stress over a pH range of 3 to 10. With increasing pH the yield stress decreased for the complexes with iron oxides of 2 and 5% while the complexes with iron oxides of 7, 10, and 20% exhibited the maximum yield stresses at pH values from 6 to 8. The iron oxides which are not associated with the illite enhanced the yield stress more than the iron oxides precipitated on the illite surfaces. The yield stress for the complexes was correlated with their zeta potential, and the higher yield stress is considered due to smaller interparticle repulsion.

Fifteen iron oxide accumulations from the bottoms of two Finnish lakes (“lake ores”) were found to contain as much as 50% Fe. Differential X-ray powder diffraction and selective dissolution by oxalate showed that the samples consisted of poorly crystallized goethite and ferrihydrite. The crust ores of one lake had higher ferrihydrite to goethite ratios than the nodular ores of the other lake. The higher ferrihydrite proportion was attributed to a higher rate of Fe2+ supply from the ground water and/or a higher rate of oxidation as a function of water depth and bottom-sediment permeability.

Values of Al-for-Fe substitution of the goethites determined from unit-cell dimensions agreed with those obtained from chemical extraction if the unit-cell volume rather than the c dimension was used. In very small goethite crystals a slight expansion of the a unit-cell dimension is probaby compensated by a corresponding contraction of the c dimension, so that a contraction of the c dimension need not necessarily be caused by Al substitution.

The goethites of the two lakes differed significantly in their Al-for-Fe substitutions and hence in their unit-cell sizes, OH-bending characteristics, dehydroxylation temperatures, dissolution kinetics, and Möss-bauer parameters. The difference in Al substitution (0 vs. 7 mole %) is attributed to the Al-supplying power of the bottom sediments: the silty-clayey sediments in one lake appear to have supplied Al during goethite formation, whereas the gravelly-sandy sediments in the other lake did not. The compositions of the goethites thus reflect their environments of formation.

Iron salts react readily with calcite in oxidizing calcareous environments to produce solid phase Fe-oxides. These reactions represent important processes in aqueous, geologic, and pedogenic environments. In the present investigation, Fe-oxides were precipitated from Fe(ClO4)2 solutions on undisturbed calcite grains in aqueous suspension. In this way it was possible to investigate the sequence of events in the crystallization process. Following an initial precipitation on the calcite grains, a period of slow growth of lepidocrocite was noted wherein 4-μm euhedral platelets formed with uniform orientation perpendicular to the calcite surface. The slow growth and highly crystalline nature of the Fe-oxide product may be partially due to the diffusion barrier formed by the growing oxide crystal mass which influenced rate of movement of HCO3− to the dissolved Fe phase and Fe ions and H+ towards the calcite surface. Upon continued aging, the supernate became noticeably opaque. As the suspended nuclei settled, new surfaces for crystal growth were provided which resulted in somewhat less crystalline lepidocrocite and goethite.

Changes in hydraulic conductivity and clay dispersivity of clay-sand mixtures (four reference smectites and Fithian illite) as a function of concentration (0.01 M Cl− and distilled water) and sodium adsorption ratio (SAR ≤ 30) of the percolating solution were measured. In addition, the effect of sand percentage, sand particle size, and addition of AlCl3 and FeCl3 on the hydraulic conductivity of the mixtures were measured.

Clay dispersion and migration out of the 3% clay columns was substantial. The clay dispersed only in the distilled water system; dispersion increased with an increase in the percentage of exchangeable Na and was about the same for the Wyoming montmorillonite and Fithian illite. Conversely, the clay swelled in the 0.01 M Cl− solution. The swelling of the montmorillonites increased in the order: Upton, Wyoming = Belle Fourche, South Dakota > Polkville, Mississippi > Otay, California, and was higher than that of the Fithian illite. The swelling and dispersion of the clay accounted for the changes in hydraulic conductivity.

Mixtures treated with FeCl3 and AlCl3 were leached with NaCl-CaCl2 solutions until the pH of the effluent exceeded 6.5. The composition of the exchangeable phase was then determined by the SAR of the leach solutions. At pH > 6.5, the polycations hydrolyzed and were present as the hydroxy-polymer species. The hydraulic conductivity of the mixtures decreased as exchangeable Na increased, but the decrease was less than in untreated mixtures, AlCl3 was more effective in maintaining hydraulic conductivity than FeCl3. In montmorillonite clay with an ESP of 20, less than 5% of a complete Al-interlayer was enough to prevent a reduction in hydraulic conductivity. Packets in the day systems tested explain the high efficiency of the Fe and Al polycations.

Mixtures of magnetite and goethite were formed by the slow oxidation of mixed FeCl2-AlCl3 solutions in an alkaline environment at room temperature. The compositions of the products ranged from almost exclusively magnetite in Al-free systems to goethite only at Al/(Al + Fe) ≈ 0.3. The magnetic phase consisted of a partly oxidized (Fe2+/Fe3+ < 0.5), Al-substituted magnetite. The unit-cell edge length a of the magnetite decreased with increasing Al(Al ≈ 0–0.37 per formula unit, corresponding to 0–14 mole % Al) and decreasing Fe2+ in the structure as described by the empirical relationship α(A) = 8.3455 + 0.0693 Fe2+ - 0.0789 Al. A correlation between the experimentally determined a and that calculated from the unit-cell edge lengths of end-member magnetite, maghemite, and hercynite was highly significant (r = .96) although shifted by about 0.01 Å. Mössbauer spectra showed Al to have entered preferentially the tetrahedral rather than the octahedral sites at low Al substitutions (<0.15 per formula unit), perhaps because of steric reasons. With increasing Al substitution the crystal size of magnetite decreased and structural strain increased, indicating that the structure had a limited capability to incorporate Al under these synthesis conditions. The capacity of the goethite structure to tolerate more Al may explain why goethite replaced magnetite at higher Al concentrations.

The dissolution in 1 M HC1 of Al-, Mn-, and Ni-substituted hematites and the influence of metal substitution on dissolution rate and kinetics of dissolution were investigated. The inhomogeneous dissolution of most of the hematites investigated was well described by the Avrami-Erofe'ev rate equation, kt = √[-ln(l − α)], where k is the dissolution rate in time, t, and α is the Fe dissolved. Dissolution of Al-substituted hematite occurred mostly by edge attack and hole formation normal to (001), with the rate of dissolution, k, directly related to surface area (SA). Dissolution of rhombohedral Mn- and Ni-bearing hematites occurred at domain boundaries, crystal edges, and corners with k unrelated to SA. The morphology of Mn- and Ni-substituted hematites changed during dissolution with clover-leaf-like forms developing as dissolution proceeded, whereas the original plate-like morphology of Al-bearing hematite was generally retained. Acid attack of platy and rhomboidal hematite is influenced by the direct (e.g., metaloxygen bond energy, hematite crystallinity) and indirect (e.g., crystal size and shape) affects associated with incorporation of foreign ions within hematite.

Several Al-substituted goethites were synthesized by hydrolysis of Fe3+ salt solutions. The kinetics of the reductive dissolution of these goethites in dithionite-ethylenediaminetetra acetic acid (D-EDTA) was studied at pH 5.5, at 303, 323 and 333 K. The initial dissolution rate (R) per unit of surface area decreases with Al substitution. In the sample with greater Al content (G″7), the kinetic profiles of the dissolved Fe fraction vs. time gave a small positive intercept. The kinetic profile of R as a function of EDTA initial concentration shows a significant weakening in the presence of Al. The maximum is flatter and wider in Al-substituted goethite than that of pure goethite. In sample G″7, where the Al content is 11.3 mol.% the maximum is obtained when the [D]:[EDTA] initial ratio is ∼4.5 vs. 2 in un-substituted goethite. These results can be attributed to the lesser density of the more active dimeric sites, the presence of more strongly bonded Al-O-Fe with regard to Fe-O and the small value for the ≡ Al-EDTA surface species constant. Activation energy (Ea) increases with Al substitution. Its value is doubled from GO (pure goethite) to G″7 (11.3 mol.% of Al). The frequency factor (A) acts in the opposite sense to Ea, but it is not sufficient to outweigh the effect of Ea.

The form and thickness of the ferrihydrite coating on kaolinite basal surfaces was studied in mixes containing 1–10% ferrihydrite through a combination of TEM, shadowed replicas, selected area electron diffraction, and determinations of surface area and distribution of charge on the surfaces. At pH3 the ferrihydrite was found to attach initially as single particles to certain sites on the kaolinite. These particles grew laterally and vertically until with 4% ferrihydrite they were aggregates of from 8 to 25 layers of particles. With more ferrihydrite the aggregates coalesced. Despite the irregular cover on the kaolinite satisfactory electron diffraction patterns were obtained and as little as 2% ferrihydrite was detected in a mixture at pH3.

The petrology of a massive olivine-sulphide interplanetary dust particle shows melting of Fe,Ni-sulphide plus complete loss of sulphur and subsequent quenching to a mixture of iron-oxides and Fe,Ni-metal. Oxidation of the fayalite component in olivine produced maghemite discs and cellular intergrowths with olivine and rare andradite-rich garnet. Cellular reactions require no long-range solid-state diffusion and are kinetically favourable during pyrometamorphic oxidation. Local melting of the cellular intergrowths resulted in three dimensional symplectic textures. Dynamic pyrometamorphism of this asteroidal particle occurred at ∼1100°C during atmospheric entry flash (5–15 s) heating.