Gibbsite structure

Gibbsite usually contains several tenths of a percent of alkaU metal ions the technical product, precipitated from a sodium aluminate solution, contains up to 0.3% Na20 wliich carmot be washed out even using dilute HQ. Several authors (1,2) suggest that these alkaU ions are an essential component of gibbsite structure. 

It was easier to prepare clays using cations that formed a brucite structure rather than those forming a gibbsite structure. This is, in part, due to the difficulty of forming octahedrally coordinated Al3+ compounds in the presence of silicon at other than low pH conditions. Henin and co-workers believed that hydroxide sheets are formed on which silica is subsequently fixed. The presence of silica tends to cause the hydroxides to precipitate at lower pH than they normally do. Most of these experiments were carried on at 100°C, although crystalline products were obtained at temperatures of 20°C. The rate of formation varies in the ratio 1 560 between 0°C and 100°C. 

Kaolinite structure is composed of a layer incorporating two networks tetrahedral network made of tetrahedrons [Si04] and octahedral network made of octahedrons [Al(0,0H)d. The latter is similar to network in gibbsite structure, Al(OH)3, with nearly equal lengths of corresponding a and b axes. The networks unite at an insignificant change of interatomic distances. Such structure provides a sharply polar character of the layer. When packed into a structure, the layers are bound to each other by very weak bonds, that is why their stmcture is always not completely ordered. 

Fig. 4. Schematic view of part of an imogolite cylinder formed from twelve gibbsite structural type units, each corresponding to (Al203-Si02 2H202)2) and indicated by dotted lines (Reprinted from [29a], copyright (1986) from John Wiley and Sons)...

Nordstrandite. Tlie x-ray diffraction pattern of an aluininum tiiliydroxide wliich differed from the patterns of gibbsite and bayerite was pubhshed (4) prior to the material, named nordstrandite, being found in nature. Tlie nordstrandite structure is also assumed to consist of double layers of hydroxyl ions and aluininum occupies two-tliirds of the octaliedral interstices. Two double layers are stacked with gibbsite sequence followed by two double layers in bayerite sequence. 

Alumina - Alumina forms a variety of oxides and hydroxides whose structures have been characterized by X-ray diffraction (16). From the catalytic viewpoint y-alumina is the most important. This is a metastable phase that is produced from successive dehydration of aluminum trihydroxide (gibbsite) to aluminum oxide hydroxide (boehmite) to y-alumina, or from dehydration of boehmite formed hydrothermally. y-alumina is converted into a-alumina (corundum) at temperatures around 1000 C. 

When the gibbsite is dehydrated a structural collapse occurs with a large increase in surface area. The boehmite sample has a nominal surface area of 325 m /g. The infrared spectrum of the boehmite shows distinct structure in the OH stretching region, with two peaks located at 3090 and 3320 cm". There are three features at 1648, 1516 and 1392 cm" that are due to adsorbed water and carbonate, which are removed upon heating the boehmite to 350 0 in hydrogen. 

Schematic representation of the kaolinite structure. It reveals the 1 1 structure due to the alternation of silica-type (black) and gibbsite-type layers (white). Furthermore, the edge surface exposes aluminol and siianol groups.

Simple Models. The surface chemical properties of clay minerals may often be interpreted in terms of the surface chemistry of the structural components, that is, sheets of tetrahedral silica, octahedral aluminum oxide (gibbsite) or magnesium hydroxide (brucite). In the discrete site model, the cation exchange framework, held together by lattice or interlayer attraction forces, exposes fixed charges as anionic sites. 

V.C. (1974) Surface structure of gibbsite, goethite and phosphated goethite. Nature 248 220-221... 

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