Clays minerals

By comparison with many other silicate minerals, isotope studies of natural clays are complicated by a number of special problems related to their small particle size and, hence, much larger specific surface area and the presence of interlayer water in certain clays. Surfaces of clays are characterized by 1 or 2 layers of adsorbed water. Savin and Epstein (1970a) demonstrated that adsorbed and interlayer water can exchange its isotopes with atmospheric water vapor in hours. Complete removal of interlayer water for analysis with the total absence of isotopic exchange between it and the hydroxyl group, may not be possible in all instances (Lawrence and Taylor 1971). 

One portion of the oxygen in clay minerals occurs as the hydroxyl ion. Hamza and Epstein (1980), Bechtel and Hoemes (1990) and Girard and Savin (1996) have attempted to separate the hydroxyl and nonhydoxyl bonded oxygen for separate isotope analysis. Techniques include thermal dehydroxylation and incomplete flu-orination, both of which indicate that hydroxyl oxygen is considerably depleted in 0 relative to nonhydroxyl oxygen. 

The primary rock-forming silicate minerals in terrestrial rocks can react with liqnid water to form various two-layer and three-layer clays (Faure 1998). For example, Na-plagioclase (i.e., albite) is transformed into kaoUnte by the reaction  

The first evidence for the conversion of silicate minerals of stony meteorites in Antarctica was reported by Gooding (1986a) who detected the presence of clay mineraloids, gypsum, K-Fe sulfates (jarosite ), and rust on the surfaces and in cracks of achondrites and chondrites from Elephant Moraine (EET) and the Allan Hills. These weathering products had formed primarily from glass and plagioclase in the fusion crust and in cracks in the interiors of the meteorite specimens. 

The formation of clay minerals in stony meteorites exposed to solar radiation on the ice fields of Antarctica is attributable to the episodic melting of ice and snow in contact with meteorite specimens that are warmed by sunlight. However, meteorites that were still embedded in the ice when they were collected also contained aluminosilicate weathering products (Gooding 1986b Gow and Cassidy 1989 Gow 1990). Harvey and Score (1991) suggested that meteorites that were weathered while they were stiU embedded in the ice could be 

The evidence that stony meteorites collected in Antarctica are weathered implies that certain chemical elements are mobilized within the affected meteorite specimens. In addition, glacial meltwater and atmospheric carbon dioxide invade the affected meteorite specimens together with halogens, sulfur-bearing componnds, and organic molecules. Therefore, meteorites that fell on the East Antarctic ice sheet are altered mineralogically as well as chemically and, for that reason, their trace-element concentrations may differ from those of non-Antarctic meteorite falls. 

The concentrations of potentially mobile trace elements in H5 and L6 chondrites of Antarctic and non-Antarctic origin (falls only) reported by Lipschntz (1989) permit the conclusion with 90% or higher confidence that the trace-element concentrations of Antarctic meteorites actually do differ from those of the non-Antarctic falls. However, the cause for this difference is still in doubt. On the one hand, the Antarctic meteorites may have originated from a different set of 

The term primary clay refers to those clays that are found in the location in which they were geologically formed. They are found in deposits that contain substantial quantities of other minerals. The kaolin is processed in an aqueous slurry and must be separated 

Secondary clays are those clays, which have been transported away from their primary source to another location, where they have been deposited as a sediment. The transportation process was usually effective at both mineral separation and at particle size selection, in that only the finer particles remained in suspension long enough to reach the final deposit. Important secondary deposits are foimd in the kaolin belt of south-eastern United States (Alabama, through Georgia into South Carolina) and southwestern England. These clays are very fine with mean particle sizes from 0.1 to 2.0 pm. These are all semi-reinforcing fillers. 

When kaolin is heated, it undergoes chemical and physical changes. Between approximately 450 °C and 700 °C, dehydroxylation occurs (often described as loss of water of crystallisation) to form a product called metakaolin. This is accompanied by approximately 13% loss of mass as water. At higher temperatures, between 950 °C and 1030 °C, metakaolin undergoes an internal rearrangement to form an amorphous product described as a defect spinel. For rubber applications, products are manufactured at temperatures just above these transitions. Metakaolin is produced primarily for use in flexible polyvinyl chloride wire and cable insulations, but is also used in EPDM cable insulations. 

The defect spinel form is used extensively in rubber applications. The primary application is in low- and medium-voltage power-cable insulations where it provides both excellent insulation stability under wet conditions and useful extrusion performance. Pharmaceutical closures form another important application due to the need for a chemically inert, white, filler. 

It is also used in a variety of extruded products, especially those vulcanised under low-pressure conditions because of it s very low moisture content. 

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). 

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 

Kinetic data for dehydroxylation of representative clay minerals (See also ref. 36.) 

Mineral Water vapour pressure, PH2o (Torr) Activation energy, E (kJ mole-1) Preexponential factor, logioA (molecules m-2 s-1) Temperature range (K) Ref. 

Kodama and Brydon [631] identify the dehydroxylation of microcrystalline mica as a diffusion-controlled reaction. It is suggested that the large difference between the value of E (222 kJ mole-1) and the enthalpy of reaction (43 kJ mole-1) could arise from the production of an amorphous transition layer during reaction (though none was detected) or an energy barrier to the interaction of hydroxyl groups. Water vapour reduced the rate of water release from montmorillonite and from illite and 

The catalytic effects of clays on hydrolysis processes is generally associated with the acidic pH values measured at clay mineral surfaces. Numerous studies have demonstrated that the surface pH of clay minerals can be as much as 2 to 3 units lower than the bulk solution (Mortland, 1970 Bailey et al., 1968 Frenkel, 1974 Karickhoff and Bailey, 1976). The Bronsted acidity of clays arises primarily from the dissociation of water coordinated to exchangeable cations (2.115). 

El-Amamy and Mill (1984) measured the effect of the surface acidity of mont-morillonite and kaolinite on the hydrolysis rate constants for a number of chemicals containing hydrolyzable functional groups that exhibit acid-catalyzed, base-catalyzed, and neutral hydrolysis. The chemicals that were studied included ethyl acetate, cyclohexene oxide, isopropyl bromide, l-(4-methoxyphenyl)-2,3-epoxypropane, and N-methyl-p-tolyl carbamate (MTC). Aqueous suspensions of 

It is apparent that at low moisture content ( 10% for the Na-saturated clay mineral and 5% for the Ca-, or Mg-saturated clay mineral), where water is not available for hydrolysis, hydrolysis does not occur. This low moisture content corresponds with the saturation of the cation s first hydration shell. As the moisture content is increased to the upper limit of bound water (50% moisture content), a significant enhancement of the hydrolysis of the epoxide is observed. When the moisture content exceeds the upper limit of bound water ( 50%), the rate constant for the hydrolysis of the epoxide was reduced by a factor of 4. It was concluded that water in excess of sorbed water diminishes the catalytic activity of clay surfaces by reducing the concentration gradient across the double layer, effectively raising the surface pH closer to that of the bulk water. In similar studies with MTC, the addition of water to oven-dried Na-montmorillonite and Na-kaolinite retarded the hydrolysis rate of the carbamate. This observation is consistent with the fact that MTC exhibits only neutral base-catalyzed hydrolysis. 

Moisture content was also found to have a significant effect on the hydrolysis kinetics of parathion and methyl parathion on kaolinite. For example, the hydrolysis rate constant for parathion and methyl parathion on kaolinite increased by 2 orders of magnitude when the moisture content was increased to the limit of sorbed water (11% moisture content) (Saltzman et al. 1976). At moisture contents above the limit of sorbed water, however, a significant decrease in the hydrolysis rate constant was observed. Mingelgrin et al. (1977) concluded that the hydrolysis of parathion. 

Hydrolysis mechanisms in clay-water systems also can be dependent on the nature of the exchangeable cation. For example, Pusino et al. (1988) studied the catalytic hydrolysis of quinalphos on homoionic bentonite clays. On the Na- and K-clays, deethylation occurred, resulting in the formation of O-ethyl O-quinoxalin-2-yl thiophosphoric acid, whereas 2-hydroxyquinoxaline is the main reaction product on the Cu-, Fe- and Al-clays. 

The reactions to illustrate weathering of complex silicates during acid hydrolysis (eqns. 4.13 4.14) predict that clay minerals will be an important solid product and this is confirmed by looking at soils. Clay minerals are important constituents in most soils. These sheet silicates that are less than 2 pm (Section 4.2.3) are constructed of layers of atoms in tetrahedral and octahedral coordination, known as tetrahedral and octahedral sheets. 

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 

Layer type Group Common minerals Octahedral character Interlayer material 

2 1 Smectite Montmorillonite Dioctahedral Hydrated exchangeable cations 

The architecture of a silicate layer results from SiO coordination in which each SiOj unit shares oxygen atoms with three neighboring SiO groups, thus forming 

As a function of their structural properties, clays interact differently with organic and inorganic contaminants. Two major groups of clay minerals are selected for discussion here (a) kaolinite, with a 1 1 layered structured aluminosilicate and a surface area ranging from 6 to 39 m g (Schofield and Samson 1954) and (b) smectites with a 2 1 silicate layer and a total surface area of about 800m g (Borchardt 1989). 

Kaolinite crystals in the subsurface are submicron sized and exhibit a platelike morphology. They usually are found mixed with other layered structured minerals. In a comprehensive review, Dixon (1989) summarizes the structural properties of kaohnite. This mineral is composed of tetrahedral and octahedral sheets constituting a 0.7 mn layer in a triclinic unit cell. Two thirds of the octahedral positions are occupied by Al the tetrahedral positions are occupied by Si and Al, which are 

Smectites are clay minerals with an expanding nature, a negative charge, and a large total surface area. These properties are of major importance in controlling the fate of chemicals in the subsurface, by affecting their retention, transport, and persistence. 

Al- and H-saturated montmorillonite results in fading out of the 17(X)cm band as a result of proton migration (Yariv and Heller-KaUai 1973). 

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Some clay minerals may absorb some of the water contained in the drilling mud. This will cause the clays to swe//and eventually reduce the borehole size to the point where the drill pipe becomes stuck. Prevention mud additives which prevent clay swelling e.g. potassium salt. 

T. J. Pinnavaia, Advanced Techniques for Clay Mineral Atuxlysis, J. J. Fripiat, ed., Elsevier, New York, 1981. 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

R. M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves , p. 174, Academic Press, London and New York (1978). 

A lamellar solid of especial interest is montmorillonite, a clay mineral. 

Using X-ray diffraction, Karstang and Kvalhein reported a new method for determining the weight percent of kalonite in complex clay minerals To test the method, nine samples containing known amounts of kalonite were prepared and analyzed. The results (as %w/w kalonite) are shown. 

R. M. Barter, Zeolites and Clay Minerals asMdsorhents and Catalysts Academic Press, London, 1978, p. 164,174, 185. 

Certain grades of siUca gel or selected clay minerals are often used. The buffeting material is preconditioned under the selected relative humidity and, after equihbration, installed in the case. This method of microclimate control has proven to be very efficient, not only in exhibition cases and storage spaces, but also in packing crates used for the transportation of sensitive objects. 

Fireclay. Fireclays consist mainly of the mineral kaolinite [1318-74-7] 2 3 small amounts of other clay minerals, quart2ite,... 

Fireclay Refractories. These products are made from clay minerals containing ca 17—45% AI2O2. Pure kaolin has the highest alumina content. 

Effect on Oxide—Water Interfaces. The adsorption (qv) of ions at clay mineral and rock surfaces is an important step in natural and industrial processes. SiUcates are adsorbed on oxides to a far greater extent than would be predicted from their concentrations (66). This adsorption maximum at a given pH value is independent of ionic strength, and maximum adsorption occurs at a pH value near the piC of orthosiUcate. The pH values of maximum adsorption of weak acid anions and the piC values of their conjugate acids are correlated. This indicates that the presence of both the acid and its conjugate base is required for adsorption. The adsorption of sihcate species is far greater at lower pH than simple acid—base equihbria would predict. 

The disadvantage of this procedure is that the minerals maybe physically or chemically altered during burning. Eor example, the refractive index of clay minerals is changed the color, birefringence, and pleochroism of micas is altered carbonates are destroyed and the iron sulfides are oxidized to iron oxides. 

Binders. To create needed physical strength in catalysts, materials called binders are added (51) they bond the catalyst. A common binder material is a clay mineral such as kaolinite. The clay is added to the mixture of microparticles as they are formed into the desired particle shape, for example, by extmsion. Then the support is heated to remove water and possibly burnout material and then subjected to a high temperature, possibly 1500°C, to cause vitrification of the clay this is a conversion of the clay into a glasslike form that spreads over the microparticles of the support and binds them together. 

Calcium siHcate hydrate is not only variable ia composition, but is very poody crystallised, and is generally referred to as calcium siHcate hydrate gel or tobermorite gel because of the coUoidal sizes (<0.1 fiva) of the gel particles. The calcium siHcate hydrates ate layer minerals having many similarities to the limited swelling clay minerals found ia nature. The layers are bonded together by excess lime and iatedayer water to form iadividual gel particles only 2—3 layers thick. Surface forces, and excess lime on the particle surfaces, tend to bond these particles together iato aggregations or stacks of the iadividual particles to form the porous gel stmcture. 

Because clays (rocks) usually contain more than one mineral and the various clay minerals differ in chemical and physical properties, the term clay may signify entirely different things to different clay users. Whereas the geologist views clay as a raw material for shale, the pedologist as a dynamic system to support plant life, and the ceramist as a body to be processed in preparation for vitrification, the chemist and technologist view clay as a catalyst, adsorbent, filler, coater, or source of aluminum or lithium compounds, etc. 

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. 

The development of apparatus and techniques, such as x-ray diffraction, contributed gready to research on clay minerals. Crystalline clay minerals are identified and classified (36) primarily on the basis of crystal stmcture and the amount and locations of charge (deficit or excess) with respect to the basic lattice. Amorphous (to x-ray) clay minerals are poody organized analogues of crystalline counterparts. 

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