Bentonite structure

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). 

For use in high resistivity soils, the most common mixture is 75% gypsum, 20% bentonite and 5% sodium sulphate. This has a resistivity of approximately 50 ohm cm when saturated with moisture. It is important to realise that carbonaceous backfills are relevant to impressed current anode systems and must not be used with sacrificial anodes. A carbonaceous backfill is an electronic conductor and noble to both sacrificial anodes and steel. A galvanic cell would therefore be created causing enhanced dissolution of the anode, and eventually corrosion of the structure. 

In addition to the ICDD, publications dealing solely with the powder patterns of drugs appear occasionally [12-15], In 1971, Sadik et al. pointed out that the identification test for kaolin (in NF XIII) was a test for the presence of aluminum, and therefore both kaolin and bentonite gave positive results [16]. Since the two compounds have different crystal structures, their x-ray diffraction patterns are different, and therefore XPD was recommended for identification of these compounds. In the current edition of USP, the identification of bentonite is based on its powder x-ray pattern [3]. 

Benidipine, 5 130, molecular formula and structure, 5 125t Bentazon, 13 323 Bent-core liquid crystals, 15 98 Bentonite, 6 664, 686, 696 9 11, 17. 

As an example, infrared spectroscopy has shown that the lowest stable hydration state for a Li-hectorite has a structure in which the lithium cation is partially keyed into the ditrigonal hole of the hectorite and has 3 water molecules coordinating the exposed part of the cation in a triangular arrangement (17), as proposed in the model of Mamy (J2.) The water molecules exhibit two kinds of motion a slow rotation of the whole hydration sphere about an axis through the triangle of the water molecules, and a faster rotation of each water molecule about its own C axis ( l8). A similar structure for adsorbed water at low water contents has been observed for Cu-hectorite, Ca-bentonite, and Ca-vermiculite (17). 

The ideal layered silicate is the mineral montmorrillonite, which is main component of the clay bentonite (fig 6). Montmorrillonite is a so called 2 1 layered silicate. Each layer consists of two sheets of silica tetrahedrae which are sandwiched by an alumina octahedrae sheet. The layers are only weakly bound, often by hydrogen bonding from water. The structure has been extensively studied by transmission electron diffraction especially by Z5wagin and coworkers [4]. 

Kaolinite is the main constituent in china clay used to make porcelain. The layers are largely held together by van der Waals forces. Bentonite is used in cosmetics, as a filler for soaps, and as a plasticizer, and it is used in drilling-muds as a suspension stabilizer. Bentonite and kaolinite clays are used, after treatment with sulfuric acid to create acidic surface sites, as petroleum cracking catalysts. Asbestos also has a layered structure (Section 12.13). 

Structure Forming Excipient An excipient which participates in the formation of the structural matrix which gives an ointment, cream or gel etc., its semisolid character. Examples are gel fonning polymers, petrolatum, certain colloidal inorganic solids (e.g., bentonite), waxy solids (e.g., cetyl alcohol, stearic acid), and emulsifiers used in creams. 

In 1943, A. C. Byrns et al. (7), of Union Oil of California published the first study showing under semi-industrial conditions the relative activities of Mo03 and CoO and the mechanical mixture of these two oxides, which they compared to C0M0O4 supported on bentonite. These authors demonstrated that a mixture of molybdenum and cobalt in their oxidic state should be chemically associated in order to be very active, while the simple mechanical mixture only showed the additive activities of the individual oxides. However, these authors mainly emphasized the behavior of these catalysts under different industrial conditions and reduced their discussion of catalyst structure and characterization to a few lines of speculation. 

There are more complicated structures intermediate between pyrophyllite and talc with variable substitution of A1J and Mg2. Electroneulrality is maintained by hydrated cations between layers. Thus the montmorillonites arc unusual days forming thixotropic aqueous suspensions that arc used as well-drilling muds and in nondrip puints. They are derived from the formulation AU(OH)jSi40 ,-x-H2o with variable amounts of water, Mg3+ (in place of some Al5 ), and compensaUng cations. M"+ (M = Ca in fuller s earth, which is converted to bentonite, M = Na). Vermiculite likewise has variable amounts of water and cations, (t dehydrates to a talc-like structure with much expansion when heated (see page 750). 

A source of error in chemical analyses of montmorillonites (and in other clays) that is not commonly checked is the presence of amorphous material, particularly Si and Al. Table XXXII lists structural formulas given by Osthaus (1955) for montmorillonites which were purified by size fraction and by extraction with 0.5 N NaOH to remove amorphous Si and Al. In six analyses dissolved silica ranged from 3.6 to 8.4% and alumina from 0.6 to 2.25%. Amorphous silicon dioxide should be expected in most montmorillonites derived from volcanic material. The source glass has more Si than is required for the 2 1 layer and the excess must be leached from the glass. Much of the Si is deposited in the sediments underlying the bentonite bed in the form of chert but it is to be expected that the extraction would not be complete and a portion of the colloidal Si would remain in the bentonite bed. 

Structural formulas for montmorillonites from Cretaceous bentonites of Black Hills region (Wyoming, South Dakota)... 

Weaver (1965) has attempted to calculate the structural formula of the two types of layers in a mixed-layer (75 25) Ordovician K-bentonite by assuming all of the K is in the contracted layers and the expandable layers contain no tetrahedral Al. 

In colloidal suspensions of anisotropic particles, the static structure factor plays a prominent role in particle size analysis. We have used transient electric birefringence (TEB) and electron microscopy, in addition to laser light scattering, to correlate our analysis of particle size distributions of bentonite suspensions. The complementary nature of TEB and photon correlation spectroscopy (PCS) in particle size analysis will be discussed. 

Two structure insensitive reactions have been selected cyclohexene hydrogenation [6] oil surface Pt sites upon silica and but-l-ene isomerisation on acidic sites in bentonite. Both reactions were studied in differential reactors The former was investigated at 273-313K and lOlkPa Samples (5-lGmg) of catalyst were flushed with N2, pre-reduced in H at 423K for lh, flushed with N2 and then the reactant stream (lOlkPa total pressure cyclohexene N2 H2 = 1,7 89.5 10 1 200cm3.min 1 total flow rale) was introduced and analysed... 

When fine inorganic materials (e.g., bentonite, oxides) are added to water containing suspended particles, the dispersion becomes thixotropic and the inorganic materials form a three-dimensional network (i.e., gel) structure in the medium. The gel network has sufficient elastic properties. It entraps the particles and prevents settling and cake formation. The network structure is broken down upon shaking, thus facilitating pouring. However, the gel structure is influenced by pH and electrolyte concentration. 

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