CLAY AND MICA

Aluminum [7429-90-5] Al, atomic number 13, atomic weight 26.981, is, at 8.8 wt %, the third most abundant element in the earth s cmst. It is usually found in siUcate minerals such as feldspar [68476-25-5] clays, and mica [12001 -26-2]. Aluminum also occurs in hydroxide, oxide—hydroxide, fluoride, sulfate, or phosphate compounds in a large variety of minerals and ores. 

If three oxygens of each tetrahedron are shared, sheet struetures form (Fig. 16.4d). This is the basis of clays and micas. The additional M attaches itself preferentially to one side of the sheet - the side with the spare oxygens on it. Then the sheet is polarised it has a net positive charge on one surface and a negative charge on the other. This interacts strongly with water, attracting a layer of water between the sheets. This is what makes clays plastic the sheets of silicate slide over each other readily, lubricated... 

Many studies have been made of the rates of water evolution from layer-type silicate minerals which contain structural hydroxyl groups (clays and micas). Variations in composition of mineral specimens from different sources hinders comparison of the results of different workers. Furthermore, the small crystallite sizes and poor crystallinity that are features of clays limit and sometimes prevent the collection of ancillary observations (e.g. microscopic examination and diffraction measurements). 

In addition to the chemical groups of the Dana system, there are also structural groups of minerals. These describe two or more mineral species that have the same or similar crystalline structure, while differing chemically. Not all species are placed within structural groups. Well-known groups include garnets, tourmalines, feldspars, clays, and micas. 

Dolomite, quartz, and analcite were the major minerals found in our shale sample. Lesser amounts of calcite, albite, K-feldspars, K-clays, and mica-clays were observed as well as trace levels of siderite and either pyrite or marcasite. This mineralological composition is similar to that reported elsewhere for the Green River shales (3,4). 

The clays and micas are compositionally complex hydrous aluminum silicates. Structurally they are called phyllosilicates, from the Greek word phyllon, or leaf. The phyllosilicates are continuous sheet... 

Accurate and meaningful thermodynamic data are available for alt the solids and aqueous species being considered. This assumption may be questionable given the highly complex and variable compositions, thus stabilities, of the clays and micas in particular. Naturally occurring clays and micas rarely have the same composition as the specific clays and micas for which experimentally determined thermodynamic data have been obtained. 

Know the basic structures of the major clay and mica mineral groups and the definitions of octahedral, dioctahedral, gibbsite, brucite, and tetrahedral layers. 

Pyrite averages 0.2 vol%, and only in a few samples forms up to 1.3 vol%. It shows two occurrence habits (i) fine crystals (< 2. im) or framboids scattered in kaolinized or chloritized detrital clays and micas, or engulfed by coarse carbonate cements (Fig. 15E) and (ii) coarsely crystalline (up to 200 pm across), intergranular replacive cement. 

Deposited cesium penetrates slowly from the soil surface into deeper soil layers [3] depending strongly on the soil type [2]. Sorption processes can further retard the Cs migration rate. The relative abundance of clay and mica minerals, particularly illite, results in the rapid and nearly irreversible cesium immobilization in the topsoil layer [4]. Meanwhile, cesium, as well as the other radionuclides that behave like cations, can be moved upward by plant uptake. This process depends on various factors plant... 

To demonstrate nonuniqueness, we pose here three problems in geochemical modeling that each have two physically realistic solutions. In the first example, based on data from an aluminum solubility experiment, we assume equilibrium with an alumina mineral to fix the pH of a fluid of otherwise known composition. Setting pH by mineral equilibrium is a widespread practice in modeling the chemistry of deep groundwaters, and of fluids sampled from hydrothermal experiments, because it is difficult to directly measure the in situ pH of hot fluids. In this case, however, there are two possible solutions because many aluminous minerals, including hydroxides, clays, and micas, are amphoteric and hence equally soluble at low and high pH. 

Figure 6.13 Structures of some clay and mica related minerals, formed from composite silicate-hydroxide layers...

Photograph 10-21 Thin section of high-silica shale particles in white-cement raw feed. Grain at left has well-oriented sericite mica flakes grain on right shows very finely microcrystalline assemblage of quartz, clay, and mica. (S A6889)... 

If the linking of silicate chains continues in two dimensions, sheets of SiO tetrahedral units result (Table 18.4). Various clays and mica have this sheetUke structure. Clays, which are essential components of soils, are aluminosilicates— some Si + ions are replaced by Al ions plus other cations that take up the additional positive charge. Feldspar, a component of many rocks and a network silicate, is weathered in the following reaction to form clay. 

By a condensation reaction involving three of the four hydroxyl groups of each silicic acid molecule, a condensed silicic acid can be made, with composition (H2Si205)x, which has the form of an infinite layer, as shown in Figure 18-6. The mineral hydrargillite, Al(OH).3, has a similar layer structure, which involves AlO, octahedra (Figure 18-7). More complex layers, involving both tetrahedra and octahedra, are present in other layer minerals, such as talc, kaolinite (clay), and mica. 

Casting Spot. A fault that sometimes appears on cast pottery as a vitrified and often discoloured spot on the bottom of the ware or as a semi-elliptical mark on the side. It occurs where the stream of slip first strikes the plaster mould and is attributable to local orientation of platy particles of clay and mica in the body. The fault can be largely eliminated by adjusting the degree of deflocculation of the slip so that it has a fairly low fluidity. The fault is also sometimes called... 

Platy nano-particles, such as nano-clays and micas, have some potentially useful flame retardant effects, and are currently receiving a lot of attention for this application. This topic has recently been reviewed [34, 35]. The other forms of nano-particle do not seem to have the same effectiveness. This subject, which was briefly treated in Chapter 6, is discussed in more detail here, but the earlier chapter should be referred to for details of fire retardant tests, especially the cone calorimeter which is widely used in studies involving nano-plate fillers. 

Polymer matrices are also commonly reinforced with mineral fillers or fibers, such as calcium carbonate, talc, wollastonite, clay and mica [306]. SE images of fracture surfaces show the wetting behavior or adhesion of the filler by the polymer matrix. Figure 4.34 shows secondary electron image (A) and backscattered electron image (B) micrographs of a mineral filler in a matrix of a commercial polymer. SEI does not reveal the nature of the filler in the matrix whereas BEI does reveal the mineral filler due to atomic number contrast. BE imaging is important in the... 

Silanes are particularly useful for coating silica, silicates such as calcined clay and mica, oxides and hydroxides. They are not reactive with carbonates, so they cannot readily be used with these materials. 

Platy fillers such as talc, clay, and mica increase the polymer modulus more strongly than do isotropic fillers, such as, calcium carbonate, dolomite, silica, and fly ash. In general, the higher the aspect ratio of the filler, the greater the increase is in modulus. 

Personal experience of rigid particles confirms that moderately irregular particles like milled silica or alumina have no measurable shape effect, and that only grossly irregular shapes do, such as the flaky particles of some clays and mica. These have been found to give an apparent volume response double or even treble the physical volume, i.e. the diameter calibration factor could differ by up to 66% from the calibration factor derived from spherical latex particles. 

Polymer matrices are also commonly reinforced with mineral fillers or fibers, such as calcium carbonate, talc, wollastonite, clay, and mica [506], and more recently fine additives are used to manufacture nanocomposites. Scanning electron microscopy images of fracture surfaces... 

Cleaning Up Spills of Flammable or Combustible Liquids. It is customary to clean up small spills of many liquid materials with sand or other noncombustible absorbent material. Some absorbing agents, such as untreated clays and micas, will cause an exothermic reaction with some liquids, especially monomers, which might ignite the liquid if it is flammable or combustible. Before any material is provided to be used to soak up spills of oxidizable material, tests should be made to determine if the material can cause fires with potential spills. 

---