Soapstone structure

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. 

A bar of talc feels like a bar of soap which is why it is often called soapstone. Its exceptional softness (it is the softest of the Mohs minerals) is a direct result of its unusual crystal structure. This consists of sheets of silicate tetrahedra without metal ions between the sheets. Thus the sheets are bonded only by London polarization forces. The latter are particularly weak because silicate tetrahedra have relatively small polarizabilities. 

The crystal structure of talc, illustrated in Figure 4, consists of repeating layers of a sandwich of bmcite [1317-43-7], Mg(OH)2, between sheets of silica [7631-86-9], SiOj. The layers of silica are not strongly bonded to each other (except for van der Waals forces) and thus it is easy to fracture talc along this surface, which corresponds to delamination. This surface is covalent and hydrophobic. If talc is fractured across the bmcite layer, the surfaces generated are ionic and hydrophilic in nature. Thus talc has a natural balance of hydrophilic and hydrophobic surfaces, giving it surfactant properties and consequendy the name soapstone which is used in many parts of the wodd. 

Talc is the magnesium silicate structural analog to pyrophyllite. Its properties are nearly identical to pyrophyllite, except that Al3+ cations have been replaced by Mg2+ cations [25], Talc occurs in secondary deposits and is formed by the weathering of magnesium silicate minerals such as olivine and pyroxene [2], In bulk form, talc is also called soapstone and steatite. A typical composition for talc is given in Table 7 [22], Historically, talc has been used extensively in electrical insulator applications, in paints, and as talcum powder [2],... 

A basic study of the chemical composition and structure of two soapstone samples to improve scientific knowledge of this marvelous mineral is reported here. An investigation about the effects of a sulfuric acid aqueous solution on the physical integrity of the stone is also performed to provide some clues as to the main causes of corrosion (acid rain), and so to the possible approach to avoiding it. Since the acid attack begins at the surface, it is the surface chemical composition and structure that controls the acid corrosion. 

Sheet silicates find the bulk of the tetrahedra sharing three oxygens to form a Si205 repeating unit shown in Figure I5.13g. Chrysotile is a sheet silicate that rolls up into small, curly, silky, tubular structures possessing a hollow core. Talc, or soapstone, is a sheet silicate, as is the mineral petalite from which Arfwedson first isolated lithium (see Chapter 12). 

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