Talc structure

Silicate lattices. The red circles represent oxygen atoms. The black dot in die center of die red circle represents the Si atom, which is at the center of a tetrahedron. (Left) Diopside has a one-dimensional infinite chain. (Right) A portion of the talc structure, which is composed of infinite sheets. 

Its structure is three-layered, i.e., it is formed from two hexagonal silica layers and one central brucite layer between them. There are four types of chemical bonds in talc structure ionic Mg-0 bonds, covalent, partially ionic Si-0 and H-0 bonds hydrogen bonds between OH and O-Si, and van der Waals bonds between the layers. Van der Waals bonds are the weakest among all bonds. Therefore, early stages of grinding are accompanied by mechanochemical dehydration which can be represented by the equation ... 

Robie and Waldbaum, 1968) and H2O = 3.0— partial entropy of water in the talc structure, calculated from 2% Mg3Si40,o(OH)2. 

The most significant representative, talc, is hydrated magnesium silicate having the formula Mg3(Si205)2 (OH)2, i.e. molecular composition 3 MgO. 4 Si02. H2O, It has a layer structure similar to that of clay minerals, which is the cause of ready cleavability and of the macroscopically laminar character of some varieties. Its density lies in the range 2.6 —2.8 g/cm. The talc structure does not provide conditions for major substitution by other elements. 

Slices of the mica (talc) structure cut perpendicularly to the layer are present in... 

Geologically, talc is typically formed by the alteration of a dolomite or serpentin-ite host rock. The talc formed from a dolomite host is typical of the type found in Montana, France, and China. These massive deposits are characterized by a microcrystalline talc structure, with talc concentrations in the deposit ranging from 93% to 99% talc by weight. Talc from these deposits can be sorted manually, optically, or mechanically to enhance color and talc content. 

The uncertainty on the number of Si atoms is larger than that on the number of Ni atoms. The value of the difference A N i equals 0.7 for talc and nepouite synthesized at 2S°C and seems too low (as compared to the theoretical value A Nsi = 2), to permit the discrimination of of a phyllosilicate of talc structure from a phyllosilicate of nepouite structure. 

Haubruge HG, Daussin R, Jonas AM, Legras R, Wittmann JC, Lotz B. Epitaxial nucleation of poly (ethylene terephthalate) by talc, Structure at the lattice and lamellar scales. Macromolecules 2003 36 4452 456. 

Talc - If a sheet of silica rings is attached to the magnesia side of chrysotile, the bending tendencies on either side of the octahedral layer negate each other. The mineral structure remains planar, and the laminar trioctahedral analogue of pyrophyllite results. This is the talc structure shown in Figure 11. 

Vermiculite - The basic talc structure also typifies vermieulite, as illustrated in Figure 16. Vermiculite differs from talc primarily in its substitution of Al for tetrahedral Si" and the presence of two oriented layers of water between individual laminae. Limited substitution of octahedral by Fe and Al also occurs. The charge imbalance arising primarily from tetrahedral substiutions is compensated by cations, usually Mg, between interlaminar water layers. Because these cations are not structural components, they can be exchanged with other charge-balancing cations under the proper conditions. 

Smectite clay - Like mica, smectite cl (commonly called bentonite) has either a pyrophyllite or talc structure. Montmorillonite, a common high-aliunimun smectite, can be characterized by die pyrophyllite crystal structure with a small amount of octahedral Al replaced by Mg. The resulting charge imbalance is compensated by exchangeable cations, usually Na or Ca, between the laminae. In addition to diese counterions, oriented water, similar to that in vermiculite, occupies the interlaminar space. When Ca is the exchangeable cation, there are two water l ers, as in vermiculite when Na is the counterion, there is usually just one water layer. Figure 18 shows the montmorillonite structure. 

Saponite, a high magnesium smectite, is similar in strueture to talc but with limited substitution of tetrahedral Sf by while heetorite has the talc structure but with limited substitution of Li for octahedral Mg and F for OH". As with montmorillonite, the resulting charge imbalanee is eompensated by Na or Ca residing with oriented water in the interlaminar spaces. Saponite and heetorite have swelling, ion exchange, and absorbent properties similar to those of montmorillonite. 

At the other end of the smectite series are the high-magnesium members, hectorite and saponite. These clays possess a talc structure, with a trioctahedral magnesia l er sandwiched between the silica layers. Swellability results from minor substitution of aluminum for silicon in saponite or Hthimn for magnesium in hectorite. As with montmorillonite, the type of exchangeable cation determines the degree of swelling. Hectorite also has partial substitution of lattice hydroxyls by fluorine. 

The formation of chrysotile was never observed. However, evidence of Mg or Co talc could be detected with the characteristic line at 0.95 nm. in the absence of magnesium, a badly crystallized Co talc was obtained together with CuO. in the presence of Mg, the Mg talc structure seemed to be favoured but the degree of crystallization remained low. 

A series of magnesium silicates has been prepared according to the general recipe of hydrothermal synthesis, with 3Si/Mg ratios variing from 0 (brucite) to 4 (talc) with intermediate ratios corresponding to mixtures of brucite, chrysotile and talc structure as reported in table 3. The various Mg silicates... 

This group comprises the minerals pyrophyllite and talc—the aluminian and magnesian end members—along with minnesotaite which has Fe -for-Mg substitution in the talc structure. No thermal data for the last-named can be traced, so only pyrophyllite and talc will be considered here. 

A number of substances such as graphite, talc, and molybdenum disulfide have sheetlike crystal structures, and it might be supposed that the shear strength along such layers would be small and hence the coefficient of friction. It is true... 

The layers in the plate-like structure of talc are Joined by very weak van der Waals forces, and therefore delamination at low shear stress is produced. The plate-like structure provides high resistivity, and low gas permeability to talc-filled polymers. Furthermore, talc has several other structure-related unique properties low abrasiveness, lubricating effect, and hydrophobic character. Hydrophobicity can be increased by surface coating with zinc stearate. 

Various additives and fillers may be employed. Calcium carbonate, talc, carbon black, titanium dioxide, and wollastonite are commonly used as fillers. Plasticizers are often utilized also. Plasticizers may reduce viscosity and may help adhesion to certain substrates. Thixotropes such as fumed silica, structured clays, precipitated silica, PVC powder, etc. can be added. Adhesion promoters, such as silane coupling agents, may also be used in the formulation [69]. 

Figure 9.11 Alternative representations of the layer structures of (a) kaolinite, (b) pyrophyllite, and (c) talc. (After H. J. Emeleus and J. S. Anderson, 1960 and B. Mason and L. G. Berry, 1968.)...

Talc, unlike Che micas, consists of electrically neutral layers without the interleaving cations. It is valued for its softness, smoothrtess and dry lubricating properties, and for its whitertess, chermcal inertness and foliated structure. Its most important abdications are in ceramics, insecticides, paints and paper manufacture. The more familiar use in cosmetics and toilet preparations accounts for only 3% of world production which is about 5 Mt per annum. Half of this comes from Japan and the USA. and other major producers are Korea, the former Soviet Union, France and China. Talc and its more massive mineral form soapstone or steatite arc widely distributed throughout the world and many countries produce it for domestic consumption either by open-cast or underground mining. 

Where, /(k) is the sum over N back-scattering atoms i, where fi is the scattering amplitude term characteristic of the atom, cT is the Debye-Waller factor associated with the vibration of the atoms, r is the distance from the absorbing atom, X is the mean free path of the photoelectron, and is the phase shift of the spherical wave as it scatters from the back-scattering atoms. By talcing the Fourier transform of the amplitude of the fine structure (that is, X( )> real-space radial distribution function of the back-scattering atoms around the absorbing atom is produced. 

More complex (and more common) structures result when some of the sili-con(IV) in silicates is replaced by aluminum(III) to form the aluminosilicates. The missing positive charge is made up by extra cations. These cations account for the difference in properties between the silicate talc and the aluminosilicate mica. One form of mica is KMg (Si1AlO10)(OH)2. In this mineral, the sheets of tetrahedra are held together by extra K+ ions. Although it cleaves neatly into transparent layers when the sheets are torn apart, mica is not slippery like talc (Fig. 14.40). Sheets of mica are used for windows in furnaces. 

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