The Origin of High-Alumina Clay Minerals—A Review

118

Abstract-An attempt has been made to assemble the best thermodynamic information currently available for soil minerals in the AI.203-SiO2-H._,O system at 25~ and 1 atm. Montmorillonite is included by considering its aluminum silicate phase. Diagrams are presented so that the stability of the minerals can be visualized in relation to the ionic environment. Although the AI20:~-SiO2-H20 system is a very simple one compared to soils and sediments, the stability diagrams depict a mineral stability sequence and mineral pair associations that are in good agreement with natural relations.According to the stability diagram, mineral pairs that can form in intimate association are gibbsitekaolinite, kaolinite-montmorillonite, and montmorillonite-amorphous silica. Forbidden pairs are amorphous silica-kaolinite, amorphous silica-gibbsite, and montmorillonite-gibbsite. The formation of intimate mixtures of three or more of these minerals is also forbidden. The stability diagrams predict ion activity relationships that are in reasonable agreement with those obtained from soils and sediments.Amorphous silica probably limits high silica levels, with montmorillonite also forming at high silica levels. Kaolinite forms at intermediate and gibbsite at low silica levels. These minerals in turn probably control the activity of aluminum ions at a level appropriate to the pH. The formation of gibbsite, kaolinite, montmorillonite and amorphous silica appears to be controlled by a combination of kinetics and equilibria. That is, the kinetic dissolution of unstable silicates appears to control the H4SiO4 level. The new mineral(s) most stable at that H4SiO4 level appear to precipitate in response to solution equilibria.

Section: Aluminum Hydroxide and Oxyhydroxidessupporting

confidence: 59%

Section: Aluminum Hydroxide and Oxyhydroxidesmentioning

confidence: 99%

Soil Minerals in the Al₂O₃-SiO₂-H₂O System and a Theory of Their Formation

Kittrick

1969

118

“…On the other hand, Keller (1964) reported that gibbsite and boehmite coexist in some bauxites, suggesting that they may have formed under similar conditions.…”

Section: Introductionmentioning

confidence: 99%

Influence of Inorganic and Organic Ligands on the Formation of Aluminum Hydroxides and Oxyhydroxides

Violante

1985

107

Abstract--Hydroxide and oxyhydroxide products of aluminum were formed at room temperature at an initial AI concentration of 2 x 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 AI(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/A1 molar ratio at which the production oral hydroxides or oxyhydroxides was inhibited ranged from 0.02 to 15. Above critical ligand/A1 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 AI(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/A1 molar ratios for the formation ofpseudoboehmite 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 weathering of aluminosilicate minerals, primarily plagioclase, biotite, and almandine (Graham et aL, 1989), released Si and AI into solution in the soils and saprolite. Si was depleted in these materials because it is relatively soluble under all common soil pH conditions, but AI has a very low solubility between pH 5 and 8 (Keller, 1964). Because the pH in the C and Cr horizons of these soils was typically between 5 and 6 (Graham, 1986), AI precipitation as gibbsite was favored in those horizons.…”

Section: Oxides and Oxyhydroxidesmentioning

confidence: 99%

Weathering of Iron-Bearing Minerals in Soils and Saprolite on the North Carolina Blue Ridge Front: II. Clay Mineralogy

Graham

1989

Abstract--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 Fe 2 § as indicated by MSssbauer spectroscopy, and is more resistant to weathering than biotite. The vermiculite and B/V, both weathering products of biotite, contain Fe 3 § 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) x (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.