The Clay Minerals

In the introduction it was pointed out that clays are seldom pure in addition to the clay mineral, which is the essential and preponderant substance, and so is responsible for the characteristic properties, a number of adventitious minerals such as quartz, mica, and iron oxide aie present. 

In order to gain a clear idea of the clays from which ceramics are manufactured, it is necessary to study the structures and properties of their clay minerals, but it must be remembered that the properties of any clay in the bulk will also depend on the nature and proportion of the impurities. 

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J) The extreme fineness of iadividual clay particles, which may be of colloidal size ia at least one dimension. Clay minerals are usually platy ia shape, and less often lathlike and tubular or scroU shaped (13). Because of this fineness clays exhibit the surface chemical properties of coUoids (qv) (14). Some clays possess relatively open crystal lattices and show internal surface colloidal effects. Other minerals and rock particles, which are not hydrous aluminosihcates but which also show colloidal dimensions and characteristics, may occur intimately intermixed with the clay minerals and play an essential role. 

W. D. KeUer, "Processes of Origin of the Clay Minerals," Proceedings of the Soil Clay Mineral Institute, Virginia Polytechnic Institute, Blacksburg, Va., 1962. 

In ceramics, plasticity is usually evaluated by means of the water of plasticity. Values for the common clay minerals are given in Table 1. Each clay mineral can be expected to show a range of values because particle size, exchangeable ion composition, and crystallinity of the clay mineral also exert an influence. Nonclay mineral components, soluble salts, organic compounds, and texture can also affect the water of plasticity. 

MiscelDneous. Other important properties are resistance to thermal shock, attack by slag, and, in the case of refractories (qv), thermal expansion. For whiteware, translucency, acceptance of glazes, etc, may be extremely important. These properties depend on the clay mineral composition, the method of manufacture and impurity content. 

Clays composed of mixtures of clay minerals having from 20—50% of unsorted fine-grain nonclay materials are most satisfactory. Large amounts of iron, alkaHes, and alkaline earths, either in the clay minerals or as other constituents, cause too much shrinkage and greatiy reduce the vitrification range thus, a clay with a substantial amount of calcareous material is not desirable. Face bricks, which are of superior quaHty, are made from similar materials but it is even more desirable to avoid these detrimental components (see Building materials, survey). 

Silicates with layer. structures include some of the most familiar and important minerals known to man, partieularly the clay minerals [such as kaolinite (china clay), montmorillonite (bentonite, fuller s earth), and vermiculite], the micas (e.g. muscovite, phlogopite, and biotite), and others such as chrysotile (white asbestos). 

The technological importance of the clay minerals is outlined in the Panel on p. 356. 

In the wet process the clay minerals are crushed and slurried with water to allow pebbles and other rock particles to settle out. The limestone is also crushed and slurried. Both materials are stored in separate bins and analyzed. Once the desired ultimate composition is determined, the slurry blend is ground and then partially dried out. 

After the blends have been prepared (either in the dry or wet process), these materials are fed at a uniform rate into a long rotary kiln. The materials are gradually heated to a liquid state. At temperatures up to about I,600°F the free water evaporates, the clay minerals dehydroxylate and crystallize, and CaCO, decomposes. At temperatures above 1,600°F the CaCO, and CaO react with aluminosilicates and the materials become liquids. Heating is continued to as high as 2,800°F. 

Dehydroxylation of the clay mineral kaolinite [71,626—629] is predominantly deceleratory and sensitive to PH2o (Table 11). Sharp and co-workers [71,627] conclude that water evolution is diffusion controlled and that an earlier reported obedience to the first-order equation is incorrect. A particularly critical comparison of a—time data is required to distinguish between these possibilities. Anthony and Garn [629] detected a short initial acceleratory stage in the reaction and concluded that at low Ph2o there is random nucelation, which accounts for the reported... 

The clay mineral bentonite (sodium montmorillonite) has an excellent ion exchange and adsorption capacity. Films can be applied to electrode surfaces from colloidal clay solutions by simple dip or spin coating that become electroactive after incorporation of electroactive cations or metal particles 136-143)... 

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

Bentonite is the name for a hydrous aluminum silicate comprised principally of the clay mineral montmorillonite, notable for its ability to swell in water and to form a very low-permeability seal." It is available as powder, granule (chip), or pellets. Powder and granule sizes are produced by processing after mining. Bentonite powder... 

High adsorption loss observed in the present work in both dynamic and static tests indicates a possibility of multilayer adsorption. However, the long times required to achieve adsorption equilibrium may indicate interlayer adsorption in the clay minerals. 

The clay mineral montmorillonite, which is often used in different prebiotic syntheses, is probably now the most important mineral for experiments on prebiotic chemistry. It has shown its abilities in the area of simulation experiments on the formation of primitive cellular compartments montmorillonite accelerates the spontaneous conversion of fatty acid micelles to vesicles. Clay particles are often incorporated into the vesicle, just as is RNA, which is adsorbed at such clay particles. If the vesicles have been formed, they can continue to grow if fatty acids are fed to them via micelles. If the vesicles are pressed through 100 nm pore filters, they divide without dilution of their contents. 

Chromium has a similar electron configuration to Cu, because both have an outer electronic orbit of 4s. Since Cr3+, the most stable form, has a similar ionic radius (0.64 A0) to Mg (0.65 A0), it is possible that Cr3+ could readily substitute for Mg in silicates. Chromium has a lower electronegativity (1.6) than Cu2+ (2.0) and Ni (1.8). It is assumed that when substitution in an ionic crystal is possible, the element having a lower electronegativity will be preferred because of its ability to form a more ionic bond (McBride, 1981). Since chromium has an ionic radius similar to trivalent Fe (0.65°A), it can also substitute for Fe3+ in iron oxides. This may explain the observations (Han and Banin, 1997, 1999 Han et al., 2001a, c) that the native Cr in arid soils is mostly and strongly bound in the clay mineral structure and iron oxides compared to other heavy metals studied. On the other hand, humic acids have a high affinity with Cr (III) similar to Cu (Adriano, 1986). The chromium in most soils probably occurs as Cr (III) (Adriano, 1986). The chromium (III) in soils, especially when bound to... 

Perez-Castells, R., Alvarez, A., Gavilanes, J., Lizarbe, M.A., Martinez del Pozo, A., Olmo, N. and Santaren, J. (1987) (eds L.G. Schultz H.vanOlphen, F.A. Mumpton ), Proceedings of the International Clay Conference, Denver, 1985, The Clay Mineral Society, Bloomington, 359—362. 

The entrapment of various enzymes and proteins by clay minerals proceeds by weak interactions including electrostatic interactions, hydrogen and van der Waals bonding. Additivity of these various attractive forces renders the adsorption irreversible in some cases, but usually a leaching of enzyme is observed under working conditions. In order to fix the enzyme irreversibly at the surface of the clay layers different processes have been tried. In order to fix invertase on bentonite, Monsan and Durand [90] previously treated the clay mineral with a coupling agent,... 

Manning BA, Goldberg S (1997a) Adsorption and stability of arsenic (III) at the clay mineral-water interface. Environ Sci Technol 31 2005-2011 Manning BA, Goldberg S (1997b) Arsenic(III) and arsenic(V) adsorption on three California soils. Soil Sci 162 886-895... 

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