Talc structural formulas

In a T-O-T layer (also termed a 2 1 layer), tetrahedral sheets attach to both sides of an octahedral sheet, thereby giving a structural formula of M2T40io(°h)4 to M3T4O10 (OH) 4. The T-O-T layer forms the basis of the sheet silicates pyrophyllite (M Al) and talc (M - Mg). 

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. 

Fig. 2.13 The 1 1 and 2 1 layer arrangements in the sheet structure minerals and the (010) view of the structures of the serpentine, clay, talc, pyrophyllite, mica, and chlorite minerals. X = layer charge per formula unit. [From Bailey (1980), Fig 1.1, p. 3 Fig 1.2, p. 6.1...

Although there are many chain silicates, we will consider only the pyroxene and amphibole families, and the sheet silicates related to talc. The pyroxenes contain single chains of Si04 tetrahedra linked by octahedra to give a formula /tSiOj, or (/l,.6)Si20e, where A and B are usually small octahedrally co-ordinated cations. A simplified diagram of the structure is shown in Figure 9 (a). Typical examples of... 

Many of the layer silicate clays common in soils are based on the mica structure (shown in Figure 2.9b) in which two tetrahedral sheets sandwich a single sheet of octahedrally coordinated cations. Consequently, they are termed 2 1 layer sihcates. Conceptually, it is useful to start with the neutral framework of the talc and pyro-phyllite structures, representing the trioctahedral (Mg in the octahedral sheet) and dioctahedral (AF in the octahedral sheet) members of the 2 1 group. These have the ideal formulae given below ... 

Smectites, which are based on either the trioctahedral 2 1 (talc) or dioctahedral 2 1 (pyrophyllite) structure, differ from these neutral structures by the presence of isomorphous substitution in the octahedral or tetrahedral sheet. For example, the dioctahedral smectite, montmorillonite, has the general formula... 

Natural talc is not pure, but contains Al, Ca and Fe. As already described (see section on montmorillonites) talc is a hydrated aluminium silicate having the formula Mg3Si40io(OH)2. Structurally it is composed of a layer of brucite, Mg(OH)2, sandwiched between two silica-type layers (see section on montmorillonites). 

It is necessary to identify some plate-shaped crystals that have appeared in a glass melt. X-ray diffraction shows that they are single phase and of a structure related to muscovite mica, talc or pyrophyUite. the chemical analysis gives an empirical formula of KF AlF BaO MgO AlaOs 5MgSi03. Suggest substitutions that could have been made in the muscovite mica, talc and pyrophylhte stmctures to produce the crystals. [Note answer is not provided at the end of this book.]... 

In Figure 22.34(c) each silicate tetrahedron is linked to three others, forming an infinite sheet structure. The simplest formula of this sheet is Si20s. The mineral talc, also known as talcum powder, has the formula Mg3(Si20s)2(0H)2 and is based on this sheet structure. The Mg and OH ions lie between the silicate sheets. The slippery feel of talcum powder is due to the silicate sheets sliding relative to one another. 

The structure of the clay talc, whose molecular formula is 1 935140,o(OH)2, showing the sheets of Si tetrahedrons (blue) separated from each other by sheets of Mg octahedrons (green). 

Talc is in the same acceptance class as saccharin. It is another case of familiarity breeding acceptance. Talcum has never caused me a problem, why should it cause me a problem now It has been claimed for many years that some talcum powders contain several percent amphibole asbestos. This is strange because, as noted above, the chemical formulas of talc and chrysotile are very similar. Neither talc nor chrysotile are amphiboles. It would be expected that if talc were to contain a fibrous component it would be one of similar composition, but this is not the case. The structure of chrysotile is almost unbelievable and perhaps talc does not provide a proper environment for this crystal growth. 

FIGURE 2 [010] view of structures of major clay mineral groups. 1 1 and 2 1 T 0 layers are represented by the primary units in serpentine-kaolin and talc-pyrophyllite, respectively. The c-sin (5 distance is the basal (001) c-axis d-spacing. Layer eharge per formula unit = X. (Reproduced with the permission of the Mineralogical Society of Great Britain and Ireland from Ref. 13.)... 

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