Minerals gibbsite

The CAS registry Hsts 5,037 aluminum-containing compounds exclusive of alloys and intermetaUics. Some of these are Hsted in Table 1. Except for nepheline and alunite in the USSR and Poland, bauxite is the raw material for all manufactured aluminum compounds. The term bauxite is used for ores that contain economically recoverable quantities of the aluminum hydroxide mineral gibbsite or the oxide—hydroxide forms boehmite and diaspore. 

The alkali process uses sodium hydroxide and is well known as Bayer s process. It involves relatively simple inorganic and physical chemistry and the entire flowsheet can be divided into caustic digestion, clarification, precipitation and calcination. Although mineral assemblage in bauxites is extensive, processing conditions are primarily influenced by the relative proportions of alumina minerals (gibbsite and boehmite), the iron minerals (goethite and hematite), and the silica minerals (quartz and clays-usually as kaolinite). 

Gibbsite and the "neutral lattice" minerals, 1 1 or 2 1 represent the extremes of chemical variat.on in the clay minerals. Gibbsite is a hydrated form of alumina. Kaolinite and pyrophyllite can be considered to be strictly aluminum-silicates, i.e., no ions other than Al, Si, 0, H are present in appreciable quantities in these minerals. This is not as... 

To illustrate this the solubility constant (Kso) for the dissolution of the mineral gibbsite will be calculated. The dissolution reaction can be written as the dissociation of the mineral, where subscript (s) refers to the solid phase and (aq) to the aqueous state ... 

An octahedral sheet (0) is defined as a two-dimensional array of cations (usually, Al, Mg, Fe) octahedrally coordinated by oxygen and/or OH. The individual octahedra share edges, and the composition of the sheet can vary from M2(0,0H) (all trivalent cations and termed dioctahedral) to M3(0,0H)g (all divalent cations and termed trioctahedral), where M signifies the cation. The octahedral sheet forms the basis of the layer-hydroxide minerals, gibbsite (M = Al) and brucite (M = Mg). 

As an example of the use of 12 to probe a dissolution reaction, consider the soil mineral gibbsite [Al(OH)3] dissolving in an Oxisol ... 

This equilibrium expression represents congruent dissolution of Na-feldspar but the solution quickly becomes saturated with respect to the solid phase Al(OH)3 (gibbsite K p = 10 ) and so the secondary mineral, gibbsite, is formed. 

There are several important hydrated forms of alumina corresponding to the stoichiometries A1(0)0H and Al(OH)3. Addition of ammonia to a boiling solution of an aluminum salt produces a form of A1(0)0H known as boehmite, which may be prepared in other ways also. A second form of A1(0)0H occurs in Nature as the mineral diaspore. The true hydroxide Al(OH)3 is obtained as a crystalline white precipitate when carbon dioxide is passed into alkaline aluminate solutions. It occurs in Nature as the mineral gibbsite. Materials sometimes referred to as /1-aluminas have other ions such as Na+ and Mg2+ present. They possess the idealized composition Na2011Al203. They can act as ion exchangers, have high electrical conductivity, and are potential solid state electrolytes for batteries. 

To determine the possible effect of small particles on the solubility of a similar mineral, gibbsite. Parks (3 ) used the equation of Enustun and Turkevich (15) and Schindler and others (16), shown below ... 

The octahedral sheet is composed of cations, usually aluminium, iron or magnesium, arranged equidistant from six oxygen (or OH) anions (Fig. 4.7b). Aluminium is the common cation and the ideal octahedral sheet has the composition of the aluminium hydroxide mineral, gibbsite (Al(OH)s). Where octahedral sites are filled by trivalent aluminium, only two of every three sites are occupied to... 

Figure 9.16 Schematic log [M" ]/[H ]") versus log[H4Si04] diagram showing the general phase relations among primary silicates such as K-feldspar, the clays smectite, ver-miculite, illite, and kaolinite, and the mineral gibbsite.

Example Problem Ten grams of the mineral gibbsite are mixed into a liter of 10 M oxalic acid. The pH of the suspension is adjusted to 6.0 with NaOH. Calculate the concentrations of free and complexed Al in solution at equilibrium, neglecting corrections for activity coefficients. 

Not much is known for certain about the rate of weathering of secondary minerals. Gibbsite dissolves in acid solutions (pH 2-3) at a rate per unit of surface area that is not greatly different from that of feldspars (A 10 " moles/mVsec). Naturally, because gibbsite crystals tend to be much smaller (micron-sized) than particles of feldspars and other primary minerals, actual dissolution rates of feldspars and gibbsite on a mass basis are very different. Consequently, when H" ions enter acid soils, they are more likely to react with clays and thereby dissolve Al , rather than be consumed by the process of primary mineral (e.g, feldspar) weathering. 

In order to plot the three-mineral (gibbsite-kaolinite-beidellite) system on the three-axis diagram of Figure 6.14, it is necessary to reduce the number of components to three by fixing pMg. This is offset by the assumption in soil systems that acid or base components are added into the system, so that the pH is not fixed internally by mineral equilibria. Consequently, the number of degrees of freedom remains at zero. 

Although aluminium in combinations as alumino-silicate is one of the most abundant constituents of the earth s crust, free alumina is comparatively rare. It is usually found in a hydrated form as bauxite rock, which is actually a mixture of the minerals gibbsite, A1(0H)3, diaspore and boehmite, (both of formula HAIO2). Bauxite is found in Jamaica, British Guiana, Europe and Russia and else-... 

Laterites, the source of Al-ore exists laigly in clay minerals. Gibbsite, bohemite, and diaspore are the Al-hydroxides which provide a major source of A1 and Fe ores. 

Figure 4. Cluster model of the mineral gibbsite, Al(OH)3, om an energy minimization using Gaussian 98 (Frisch et al. 1998). The HF/3-21G basis set reproduces the observed ciystal stmcture (SaaUeld and Wedde 1974). Calculated vibrational frequencies correspond to observed frequencies (Russell et al. 1974) indicating that H-bonding can be modeled accurately. Molecule drawn with the program CiystalMaker 4.0 (Palmer 1998).

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