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 influence of the activity of water on the phase composition of aluminum hydroxides obtained by reaction of amalgamated aluminum with water has been studied. The reaction was carried out in solutions of water and dioxane, and in sodium chloride solutions of different concentrations; amorphous aluminum hydroxide was precipitated initially. The aging of this primary product to bayerite, pseudoboehmite, or an amorphous gel was controlled by the water content of the system. The pseudoboehmite has shown a significant reactivity when used as a source of alumina in the hydrothermal syntheses of kaolinite at relatively low temperature and pressure.

Chelating organic acids hampered the hydrolytic reactions of Al and affected the nature of the crystalline aluminum hydroxides. Chemical composition, structure, size, nature of functional groups, and concentration of each organic anion, as well as the pH of the system, controlled the rate of Al(OH)3 crystallization. The order of effectiveness of the various acids was: glutaric < succinic = phthalic < glycine < malonic < glutamic < aspartic < oxalic < salicylic = malic < citric < tartaric. An increase in the stability of complexes formed between the organic ligands and Al decreased the rate of crystallization and changed the final aluminous products from bayerite to nordstrandite and/or gibbsite and then to pseudoboehmite and/or amorphous material. In the presence of anions with a great affinity for Al, particularly at pH equal to or less than 9.0, the reaction products were commonly poorly crystalline or structurally distorted. In the range of pH 8.0 to 10.0 moderately or strongly chelating anions acted to retard or prevent olation and facilitated the formation of stable pseudoboehmite or X-ray-amorphous products. The stronger the chelating power or the higher the concentration of organic anions, the easier was the formation of pseudoboehmite or amorphous material.

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 citrate, at 10 4−10−3 M initial concentration, by pseudoboehmite suspended in 0.02 M NaC1O4 was investigated at varying pH. Citrate shows a strong affinity for the pseudoboehmite surface, as seen in the adsorption isotherm at pH 6. Adsorption envelopes of adsorbed citrate vs. pH for a given initial citrate concentration are characteristic for the adsorption of a polyprotic acid by a variable-charge mineral. The envelope data were fit well by the Constant Capacitance Model assuming a ligand-exchange adsorption mechanism, three monodentate surface species of citrate, and a reactive surface OH density of 0.4 mol kg−1. Aqueous speciation calculations suggest that solubility equilibrium with pseudoboehmite was attained at pH > 9 and that particulate or polymeric Al may have existed at 6 < pH < 9. Dissolved Al appeared to reduce the adsorption of citrate at pH < 5.5 via solution complexation reactions.

Reactions between hydroxy-Al ions and orthosilicic acid as influenced by citric acid were studied at an initial Si concentration of 1.6 × 10−3 M, Si/Al molar ratios of 0.5 and 1.0, OH/Al molar ratios of 1.0-3.0, and citric acid/Al molar ratios of 0-0.3. In the absence of citric acid and at OH/A1 ratios of 1.0-2.8, imogolite (>0.01 μm) was dominant in the precipitates. At citric acid/Al ratios of 0.01-0.1, imogolite and/or pseudoboehmite were dominant in the precipitates at OH/Al ratios of 1.0 and 2.0, and imogolite and/or ill-defined aluminosilicate complexes at OH/Al ratio of 2.8. Instead of allophane or “proto-imogolite” allophane being the predominant species in the precipitates, the formation of ill-defined aluminosilicate complexes at OH/Al ratio of 3.0 was steadily promoted by increasing the solution citric acid/Al ratios from 0 to 0.3. The freeze-dried soluble products (<0.01 μ) ranged from silica gel to “proto-imogolite,” depending upon the basicity of Al and the level of citric acid of the parent solution. The amount of “proto-imogolite” increased with increasing citric acid/Al ratios from 0 to 0.1 in solution. Complexing low molecular weight organic acids, such as citric acid, impeded the formation of the short-range ordered aluminosilicates, allophanes and imogolite.

Quaternary debris flows derived from ophiolite melange in northern Southland, New Zealand, are locally cemented and veined by cross-fiber and fine-grained chrysotile. Chrysotile was identified by optical, X-ray powder diffraction, electron microprobe, differential thermal (DTA), and infrared analysis methods. Microprobe and DTA also suggested the presence of as much as 6% of sub-microscopic, noncrystalline alumina, probably in the form of pseudoboehmite. Cavity fillings and irregular veins containing chrysotile are as thick as 40 cm and commonly contain lizardite clasts. Calcite commonly accompanies chrysotile as a vein mineral. Magnetite and pyrite are found near and adjacent to chrysotile veins. These accessory minerals imply that the chrysotile formed under alkaline, reducing conditions. Surface water seepages have pH = 9. Remnant debris-flow topography, Holocene radiocarbon dates, and the absence of hot spring activity confirms that chrysotile can form under near-surface, low-temperature conditions. Growth of chrysotile fibers on a lizardite substrate suggests that the chrysotile formed by solution of detrital lizardite and subsequent precipitation in cavities within the debris flow.

The influence of different ions in the formation of Al(OH)3 polymorphs has been studied experimentally by promoting stoichiometric reactions between the aluminum salts (AlCl3, Al(NO3)3, Al2(SO4)3) and bases (NaOH, KOH, NH4OH). In all cases the polymorphs obtained were a mixture of gibbsite, bayerite and nordstrandite or pseudoboehmite with the exception of the reaction between KOH, or NH4OH and Al2(SO4)3 which produced amorphous gels. Ageing of these gels at ambient temperature and pressure for 180 days or at 60°C for 20 days resulted in crystalline structure. Specifically, pseudoboehmite was crystallized from the reaction between Al2(SO4)3 and NH4OH. Significantly, of all ions present in solution in the present experiment, only the sulphate ones were observed to have a marked influence in the precipitation of Al oxyhydroxides.

Substantial studies have been carried out to investigate the mechanism of the formation of Al(OH)3 polymorphs. The influence of the nature of Al precipitation products on the formation of Al(OH)3 polymorphs still remains obscure. In this study, X-ray diffraction, infrared and thermal analyses, and electron microscopic observations of the Al precipitates formed at the initial pH 8.2 and at a citric acid/Al molar ratio of 0.01 and aged for 3 hr to 60 days revealed that the transformation from the initially formed noncrystalline materials to pseudoboehmite occurred through the formation of intermediate materials with various degrees of ordering and sizes of particles that apparently had a wide range of solubility. By increasing the pH of the suspension of precipitation products of Al to 10.0 after 3 hr and 3, 11, 31, and 60 days or longer, the crystalline precipitation products were hayerite, nordstrandite and bayerite, nordstrandite and pseudoboehmite, pseudoboehmite and gibbsite, and pseudoboehmite, respectively. This work shows evidence that, as the nature of the starting Al precipitates changed, the rate of their dissolution apparently changed, and various Al(OH)3 polymorphs consequently formed. Therefore, the data substantiate the hypothesis that the mechanism of the formation of an Al(OH)3 polymorph is determined by the rate of its nucleation, which is, in turn, influenced by the rate of dissolution of the noncrystalline or poorly ordered Al-oxides initially formed.

More than 40 samples of Al precipitation products formed in the presence of organic (aspartic, oxalic, citric, tartaric, malic, salicylic, and tannic acid, and acetylacetone) and inorganic (chloride, sulfate and phosphate) ligands, which were short-range-ordered materials after 2–5 months of aging, were analyzed by X-ray diffraction (XRD), infrared spectroscopy (IR), and transmission electron microscopic (TEM) examination after prolonged periods of aging in the mother liquids (from 7–15 years). Noncrystalline materials were found after 7–10 years of aging in samples formed in the presence of citrate, tartrate, and tannate at pH ≤ 8.0 and at ligand/Al molar ratios (R) ranging from 0.05–0.1; they were found as well in the presence of phosphate and malate at ligand/Al molar ratios from 0.1–0.5. Poorly crystalline Al-oxyhydroxides (pseudoboehmite) without Al(OH)3 polymorphs were found in solutions with a wide range of pH (from 6.0–11.0) in the presence of tartrate, citrate, tannate, malate, salicylate, sulfate, and phosphate after 7–15 years. The crystallinity of these samples was indeed very poor. On the contrary, gibbsite formation was observed in samples formed at pH 7.0 or 8.0 in the presence of oxalate (R = 0.1 or 1.0), aspartate (R = 0.1 and 0.5), malate (R = 0.03 or 0.17), salicylate (R = 0.05), and in samples containing very high concentrations of chloride (R = 700 or 1000). Finally, the formation of gibbsite was promoted in the presence of montmorillonite, but some samples at pH ≤ 6.0 in the presence of citrate (R = 0.1), tartrate (R = 0.1), or tannate (R = 0.02 or 0.1) showed after 6 years of aging interstratification of OH-Al species in the interlayers of the clay mineral and complete lack of Al(OH)3 polymorphs.

Citrate is distributed widely in the Earth’s surface environments as a biological product released by microbes and plants. Citrate is also often used as a chelating agent for the selective dissolution of iron coatings and free iron oxides in soils. Adsorption experiments of Cs+ and IO3− before and after the complexation of citrate with the pseudoboehmite surface were conducted to evaluate the effects of citrate on the adsorption of these ions on the surface of pseudoboehmite. Additional adsorption experiments of Cs+ and IO3− after the decomposition of citrate adsorbed on the pseudoboehmite surface were also performed to confirm the recovery of the original surface properties. Citrate decomposition was carried out by means of 10% H2O2 treatments at 75°C and pH 5, 7, and 9. The results indicated that citrate complexation decreased remarkably the adsorption of both Cs+ and IO3− in the pH range 3–10, which was due to a decrease in the number of active charged sites available for adsorption of these ions. Decomposition of citrate adsorbed on the pseudoboehmite surface was found to be complete after three rounds of treatment with 10% H2O2 at 75°C and pH > 7. After the decomposition of citrate adsorbed on the pseudoboehmite surface, the adsorption of both Cs+ and IO3− was restored completely to the initial amounts before citrate complexation, and the inhibition effect of citrate on the adsorption of these ions disappeared under all pH conditions.