Thirty two boehmites, synthesized at temperatures ranging from room temperature to 300°C, were examined by scanning electron microscopy, transmission electron microscopy, electron diffraction, X-ray powder diffraction, differential thermal analysis, and infrared spectroscopy. The results show that boehmite exhibits a continuous gradation in crystallite size ranging from single octahedral layers or a few unit cells to about 65 unit cells in the y-direction. This conclusion suggests that the term pseudoboehmite is inappropriate for finely crystalline boehmite. Finely crystalline boehmite contains more sorbed water than coarsely crystalline boehmite; this water is commonly intercalated between octahedral layers, usually randomly but sometimes regularly. The regularly interstratified boehmite gives rise to a diffuse “long spacing” X-ray diffraction reflection. Calculated 020 X-ray diffraction peaks approximate closely those observed experimentally when a range of crystallite sizes is taken into account.

The crystal structure of natural boehmite, AlOOH, from Tveidalen, Langesundsfjorden, Norway, with a = 3.693(1), b = 12.221(2) and c = 2.865(1) Å, was refined from single crystal X-ray diffraction data (MoKα maximum 2θ = 120°) to an Rw value of 0.032 in space groups Amam and A21am. In both cases the proton was found to occupy positions of the type (x, 0.038, 0), where x = 0.104 and 0.396, about 0.75 Å from the donor oxygen atom. This arrangement corresponds to the presence of chains of asymmetric hydrogen bonds with random polarity parallel to the a axis, and indicates that the effective space group is Amam. No significant electron density was observed either at the center (0, 0, 0) or at the position (1/4, y, 0), corresponding to symmetric and bifurcated hydrogen bonds, respectively, between the layers of octahedra.

Calculations of the total energy of a representative portion of the boehmite structure as a function of hydroxyl orientation, using semiempirical molecular orbitals of the Hückel type, are in agreement with the observed electron density distribution and suggest a preference for (1) sp3, rather than sp2, hybridization of the donor oxygen atom orbitals, and (2) asymmetric, rather than symmetric hydrogen bonds. Disorder of the hydrogen atom positions within individual hydrogen bond chains is shown to be much less likely than disorder between adjacent fully coherent chains.

The crystal structure of a synthetic boehmite sample has been refined to an R of 0.047 in the space group Amam from X-ray powder diffraction data. Inclusion of hydrogen atoms and/or refinement in the space group A21am gave poorer results. Cell dimensions were determined as a = 3.6936 (± 0.0003), b = 12.214 (± 0.001), c = 2.8679 (± 0.0003) Å. All Al-O(OH) distances lie between 1.88 and 1.91 Å. Shared octahedral edges are 2.51–2.52 Å, and unshared octahedral edges are 2.86–2.87 Å, in excellent agreement with those for layered silicates. The O-H … O distance between contiguous octahedral sheets is 2.71 Å. The computed X-ray pattern matches closely with the experimental pattern, indicating the degree to which the crystal structure has been determined.

Sixty-five bauxite samples of different ages and origins were studied by scanning electron microscopy. Only broken surfaces of the specimens were investigated. Size and form of individual crystals and of grain aggregates were studied as were different types of microtextures and space-fillers.

Grain size varies from 0.05 μm to 1 mm. Smallest is the grain size of young karstic bauxite deposits that is explained by a physicochemical retardation effect of the carbonate environment. Significant differences were found by comparing the space-filling of karstic and lateritic bauxite deposits. High-level and low-level lateritic deposits show differences as well.

A combined use of macroscopic observations, petrographie microscopy, electron microprobe, SEM, and TЕM furnishes the best clues for any genetic interpretation of bauxites. SEM studies are useful in solving technological problems of bauxite processing.

Sedimentologic zones that are differentiated by changes in lithology, mineralogy, chemical composition, and crystal morphology observable in scanning electron micrographs occur in Missouri high-alumina clay deposits. These properties and changes suggest that the high-alumina materials originated from Pennsylvanian-age, paludal sediments deposited in depressions underlying Paleozoic carbonate rocks. Alumina was relatively enriched in zones of the deposits by leaching of silica and of alkali and alkaline earth metals from the sediments. The most intense leaching occurred on the highest parts of the Ozark Dome.

Diaspore is the predominant high-alumina mineral. Boehmite, although far less abundant than diapsore, may have paragenetically preceded diaspore in some deposits. Chlorite, presumably an Al-rich variety because the content of MgO is typically <0.5%, is also present. Li, which is sporadically present, is inferred to have accumulated in the chlorite which may be a proto-variety of cookeite. Because Li+ and Al3+ are similar in size, Li is inferred to have accompanied Al as a resistate element in contrast to K and Na which were leached from parent phyllosilicates.

Hydroxide and oxyhydroxide products of aluminum were formed at room temperature at an initial Al concentration of 2 × 10-3 M, pH 8.2, and at varying concentrations of organic and inorganic ligands commonly found in nature. The effectiveness of the ligands in promoting the formation of noncrystalline products over crystalline Al(OH)3 polymorphs was found to be in the following order: phthalate ≅ succinate < glutamate < aspartate < oxalate < silicate ≅ fluoride < phosphate < salicylate ≅ malate < tannate < citrate < tartrate. The lowest ligand/Al molar ratio at which the production of Al hydroxides or oxyhydroxides was inhibited ranged from 0.02 to 15. Above critical ligand/Al ratios, crystalline products were inhibited and ligands coprecipitated with noncrystalline products which remained unchanged for at least 5 months. Polydentate and large ligands generally were more inhibitive than those with fewer functional groups or of smaller size.

The perturbing ligands promoted and stabilized the formation of pseudoboehmite over crystalline Al(OH)3 polymorphs in the following sequence: chloride < sulfate < phthalate ≅ succinate < glutamate < silicate < aspartate < phosphate < salicylate ≅ malate < tannate < citrate < tartrate. The optimal range of the ligand/Al molar ratios for the formation of pseudoboehmite varied, for example, from 0.005–0.015 for tartrate to 600–1000 for chloride. Pseudoboehmite was not formed in the presence of fluoride.

The adsorption of Cu2+ on microcrystalline gibbsite and boehmite from aqueous solutions having 2/1 and 5/1 glycine/Cu ratios has been studied using electron spin resonance (ESR). The presence of glycine inhibited metal adsorption on gibbsite above pH 5, apparently by reducing Cu hydrolysis. The preferred adsorbed species on gibbsite and boehmite, based upon the ESR parameters, were probably Cu(gly)+ and Cu(gly)20, respectively. In both experiments, rigidly-bound ternary complexes formed with Cu2+ simultaneously bonding to the surface and one or more ligands. A large excess of glycine destabilized the ternary complex and caused the desorption of Cu2+. The preferred orientation of the Cu complex on gibbsite suggests that the adsorption occurred at crystal steps and that the glycine molecule hydrogen bonded to hydroxyls of the (001) surface.

The solubilities of HCl-treated samples of corundum, gibbsite, boehmite, and diaspore in aqueous solutions at 298 K and one atmosphere pressure were determined from undersaturated and supersaturated initial conditions. Solution characteristics at apparent equilibrium were measured and used to calculate equilibrium constants (Kr) for mineral dissolution reactions:

$$\begin{array}{l} A{l_2}{O_3}\left( {corundum} \right) + 3{H^ + } = A{l^3} + 1.5\,{H_2}O,\\ Al{\left( {OH} \right)_3}\left( {gibbsite} \right) + 3{H^ + } = A{l^3} + 3\,{H_2}O,\\ and\\ AlOOH\left( {boehmite\,or\,diaspore} \right) + 3{H^ + } = A{l^3} + 2{H_2}O, \end{array}$$

The nature of organo-phosphate and carboxylate bonding in aqueous suspensions of noncrystalline alumina, boehmite, and gibbsite has been studied using the anionic nitroxide spin probes 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl and 4-hydroxy-2,2,6,6-tetramethyl-piperidinooxyl dihydrogen phosphate. Analysis of the electron spin resonance (ESR) spectra of these molecules revealed that adsorption of both molecules occurred rapidly on the high surface area alumina and boehmite, whereas only the organophosphate adsorbed on gibbsite. A loss in rotational motion of the molecules accompanied adsorption, with the greater degree of motional restriction observed for the carboxylate attributed to steric restrictions to rotation about the surface-bound C-COO− bond axis. The ease of displacement of adsorbed carboxylate from surface binding sites by weakly adsorbing anions (Cl−, ClO4−) compared to organophosphate suggests that the carboxylate adsorbed by ligand exchange of a single surface OH, whereas the organophosphate probably formed a bidentate bond. The carboxylate bound on noncrystalline alumina showed evidence of nonspecific electrostatic adsorption in addition to ligand exchange of surface OH. This weak bonding was indicated by a moderate loss in rotational motion and ease of exchangeability.

During the epigenetic alteration of boehmitic karstic bauxite in the Vlasenica region of Yugoslavia, which was caused by siliceous water descending through the deposit by means of cracks, fissures, etc., oolites were less altered compared with the coexisting matrix material, which was intensely kaolinized. The following zonal alteration pattern of the oolites was identified: a diaspore enrichment zone, followed by a diaspore-boehmite enrichment zone, and then the original bauxite. Considering this alteration pattern and that determined previously for the matrix material, the following overall pattern of alteration was established: a kaolinite-diaspore zone, followed by a boehmite-diaspore zone, and then the original bauxite. The alteration solutions reacted with the bauxite to form solution-matrix and solution-oolite subsystems, which were characterized by different types of diffusion and geochemistry.

In the kaolinite-diaspore zone Si-metasomatism kaolinized the matrix, and excess Al partly migrated into the oolites, forming diaspore, and outward into the bauxite. The latter Al-remobilization resulted in a transition zone, in which new boehmite was formed in both the matrix and the oolites, and diaspore only in the oolites. Thermodynamic models of oolites and whole bauxite alterations were established, on the basis of stability diagrams calculated for the mineral assemblages of both the oolites and the matrix, in the alteration zones and in the deposit as a whole. Based on these models, the genesis of the diaspore and the relationship of diaspore-boehmite-kaolinite assemblages in natural systems at low temperature and low pressure can be described.

The nature of Cu2+ adsorption by boehmite, gibbsite, and noncrystalline alumina was studied over a range of equilibrium pH (4.5–7.5) and Cu2+ concentration (10−3–10−8 M) by electron spin resonance (ESR). Available chemisorption sites at pH 4.5 were the most numerous for noncrystalline alumina (~1 mmole/100 g), less for boehmite, and least for gibbsite as indicated by the relative strength of the rigid-limit ESR signal attributed to Cu2+ adsorbed at discrete sites. The chemisorption process involved immobilization of Cu2+ by displacement of one or more H2O ligands by hydroxyl or surface oxygen ions, with the formation of at least one Cu-O-Al bond. As the pH was raised from 4.5 to 6.0, essentially all of the solution Cu2+ appeared to be adsorbed by the solids. However, the noncrystalline alumina and boehmite chemisorbed much of the total adsorbed Cu2+ (10 mmole/100 g), whereas precipitation or nucleation of Cu(OH)2 in the gibbsite system was indicated. Precipitated Cu2+ was more readily redissolved by exposure to NH3 vapor than chemisorbed Cu2+.

The crystal structure of a deuterated synthetic boehmite powder, γ-AIOOD, has been determined by time-of-flight neutron diffraction and Rietveld profile refinement in space group Cmcm to a weighted profile R of 3.71%. Cell dimensions are a = 2.868(1), b = 12.2336(4), and C = 3.6923(1) Å. Alternate space groups for the boehmite structure suggested in the recent literature were found to be unacceptable. Atom positions, bond distances, and angles, with the exception of those involving hydrogen, were nearly identical to those determined by R. J. Hill in 1981 who studied a single crystal of boehmite by X-ray diffraction. All atoms were refined with anisotropic thermal parameters. The b value is 0.013 Å larger, and the thermal ellipsoids of oxygen are slightly more anisotropic than those reported by Hill and may reflect the different samples studied.

The kaolinization of bauxite has generally been thought to be a simple process of epigenetic resilification. A study of the karstic boehmitic bauxites in the Vlasenica region of Yugoslavia, however, shows that kaohnization took place in a rather complex manner, in which the alteration was caused by the percolation of siliceous water descending through the deposit by means of cracks, fissures, etc. The matrix of one such Vlasenica deposit was found to be more highly kaolinized than the oolitic fraction. Based on a mineralogical and geochemical examination of matrix material, the following pattern of zoned alteration was identified: kaolinitic zone-boehmite enrichment zone-original bauxite. In the kaolinitic zone, well-crystallized kaolinite, formed by the reaction of dissolved silica with boehmite, has replaced all other minerals in the matrix. This Si metasomatism was accompanied by an outward migration of Al and resulted in the formation of a transition zone in which new boehmite partly replaces both kaolinite and hematite (Al remobilization). A thermodynamic model of the process has been established on the basis of stability diagrams calculated for the mineral assemblages in the alteration zones and in the deposit as a whole.

The effects of humic acid (HA) on the crystallization of precipitation products of Al under mildly acidic to alkaline conditions were investigated. The extent of Al precipitation substantially decreased with increasing HA concentration (0 to 75 µg/ml) in the pH range 6 to 10 during the 80-day aging period. The X-ray powder diffraction (XRD) data show that, at pH 6.0, the amount of pseudoboehmite formed in the systems decreased with the increasing concentration of HA present. The proportion of Al hydroxide polymorphs (gibbsite and bayerite) was greatly influenced by the amounts of HA present in the systems. At pH 6.0 and HA concentration of 37.5 µg/ml, all the precipitation products of Al were noncrystalline. At pH 8.0 and a HA concentration of 12.5 µg/ml, the formation of gibbsite and bayerite was completely inhibited and only pseudoboehmite was evident in the XRD patterns. Further increase in HA concentration (25–75 µg/ml HA) at pH 8.0 resulted in no precipitation of Al, and only a broad peak at ∼3.3 Å, characteristic of HA, was observed. The XRD patterns of the precipitates of Al formed in the absence of HA at pH 10.0 showed the characteristic peaks of bayerite. At pH 10.0, the precipitation products of Al formed even at the HA concentration of 2.5 µg/ml yielded no XRD peaks. Infrared absorption spectra and transmission electron micrographs of the precipitation products of Al formed in the systems studied substantiate the findings obtained by XRD. The data obtained in this study indicate that HA affects the formation of Al hydroxide polymorphs, pseudoboehmites and short-range ordered mineral colloids.

Gibbsite samples of various particle sizes (0.2–80 μm) were heated at 250°C in a series of straight-chain primary alcohols under the autogenous vapor pressure of the alcohol (alcohothermal treatment of gibbsite). The treatment in ethanol yielded pure boehmite, the morphology of which was similar to that of the boehmite obtained by hydrothermal treatment of gibbsite. In middle-range alcohols, the boehmite yields were low (50% if 80 μm gibbsite was used), and the products were contaminated by a poorly crystallized phase, having a χ-alumina-like structure. The products preserved the morphology of the originating gibbsite, this feature being similar to the thermal dehydration of gibbsite. Complete conversion to boehmite was also attained in mineral oil (a hydrocarbon mixture, which was used as a limit of higher alcohol. The morphology of the boehmite formed in this medium was identical to that of the product prepared by thermal dehydration of gibbsite in a sealed bomb without a medium. If fine particle-size gibbsite was used, the yield of boehmite decreased and the yield of the poorly crystallized phase increased in all the media.

The reaction mechanisms may be discussed in terms of the reported mechanisms for the thermal and hydrothermal formations of boehmite from gibbsite. Thus, in lower alcohols boehmite formed by a dissolution-recrystallization mechanism, whereas in middle-range or higher alcohols it formed by intra-particle hydrothermal reaction mechanism proposed by de Boer and coworkers for the thermal dehydration of gibbsite. The difference in behavior in middle-range and higher alcohols can be explained in terms of the solubility of water in the medium: In the middle-range alcohols, water molecules formed by partial dehydration of gibbsite were removed from the gibbsite particles into the medium so that dehydration proceeded in a manner similiar to that of thermal dehydration, whereas in the higher alcohols, the low solubility of water in the medium allowed the water molecules to remain on the surface of the particles, thereby promoting the complete hydrothermal formation of boehmite.

The reaction of gibbsite in various organic solvents at 250°–300°C under spontaneous vapor pressure of the solvents was examined. Glycols and aminoalcohols afforded the organic derivatives of boehmite in which one of the oxygen atoms of the glycol molecule or the alcoholic oxygen atom of aminoalcohol was incorporated into the boehmite layers. By increasing the molecular size of the solvent, the yield of the boehmite derivative decreased, and, at the same time, the basal spacing of the boehmite derivative increased. The product had a honeycomb texture on the surface of the particle, which suggests a dissolution-recrystallization mechanism for the formation of the boehmite derivatives. A hydroxyl group and a functional group, such as hydroxyl, methoxyl, or amino group having the ability to donate its lone pair electrons, were apparently necessary for the organic solvent molecules to form the boehmite derivative by this mechanism.

Fifty-two Bidi-Koum bauxites from Guinea, Africa, of diversified chemical composition were characterized for their mineral composition. First, 14 element oxide concentrations were determined by X-ray fluorescence (XRF) using a fusion sample preparation technique. Loss of Mass (LOM) and organic carbon (OC) concentrations were also determined. The initial X-ray diffraction (XRD) phase quantification was carried out employing XDB software. This software allows for full interpretation of a sample diffractogram and helps generate initial concentrations of identified minerals based on a standardless approach. In the stage that followed, the mass balance procedure on the XDB software helped refine the final phase composition. Then, gibbsitic Al2O3 concentrations obtained by wet chemistry for all samples and kaolinitic SiO2 concentrations obtained for selected samples were compared with the concentrations obtained using the XDB software. Phases that were quantified are: gibbsite, boehmite, kaolinite, wavellite, goethite, hematite, quartz, anatase, rutile and illite. Phase concentrations were obtained for illite from K2O and for wavellite from P2O5 concentrations. The alumina substitution in the goethite lattice was also estimated.

Experimental alteration of obsidian with HCl solution was performed to elucidate dissolution mechanism and formation process of clay minerals in acid solution. Reactions were carried out using 0.1, 0.5, and 4.0 g of obsidian to 100 ml of 0.01 N HCl solution at 150° and 200°C for 1 to 60 days. The reaction products were examined by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive X-ray analysis. The surface composition of obsidian before and after alteration was investigated by X-ray photoelectron spectroscopy (XPS). TEM showed that boehmite precipitated at early stage and spherical kaolinite appeared subsequently by 200°C reactions. However, spherical halloysite occurred predominantly with small amounts of allophane, boehmite, and kaolinite by 150°C reaction in which formation process of the halloysite from allophane passing through an intermediate phase of small size rounded aggregate that consists of fine particles of allophane was observed. A boehmite exhibiting hexagonal platy habit with higher degree of crystallinity was formed by 200°C reaction as a stable phase in solution containing lower Si concentration at which the solution composition coincides with the stability field of boehmite on activity diagram for the system Na2O-Al2O3-SiO2-H2O. The fibrous boehmite having lower crystallinity appeared with increasing Si concentration, considered as a metastable phase in the stability field of kaolinite. XPS indicated that dissolution of obsidian in acid solution proceeded initially by cation exchange between Na ions and hydronium ions in solution and subsequently by preferential release of Al ions relative to Si from the Na depleted surface.