The term talc covers a wide range of natural rocks and minerals, most of which are magnesium silicates. The pure talc mineral is a hydrous magnesium silicate, Mg3Si3O10(OH)2, which theoretically is 31.7 percent MgO, 63.5 percent SiO2, and 4.8 percent H2O. The crystal structure of pure talc is a brucite sheet (Mg12O12H4) sandwiched between two silica (SiO2) sheets, to form talc layers that are superimposed indefinitely. Each layer is electrically neutral. Adjacent layers are held together only by weak van der Waals forces. This crystal structure has two consequences. First, talcs tend to form in plates. This platy structure gives talc many of its reinforcing performance properties in plastics. Second, the weak van der Waals forces between the layers in the crystal can be easily overcome by rubbing. When you rub talc, those brucite and silica layers slide over each other, and the talc feels slippery. This is one of the reasons it is used in body and baby powders.
Talc is an alteration mineral. It is formed by geological modification of some host rock. Most talc is formed from the alteration of magnesite (MgO) in the presence of excess dissolved silica (SiO2). Altering serpentine or quartzite can also form talc. The different alteration routes form talcs that have significant differences in chemistry, color, morphology and impurities. Because of the alteration method of formation and the multiple routes of talc formation into talc deposits - even if they are close in distance - can be very different. Thus, in any general discussion of talc care must be taken in applying all attributes to all deposits. Most commercial talc properties can be readily identified by their chemistry and mineralogy. Not all deposits are suited for all applications. Talc is characterized by softness, hydrophobic surface properties, chemical inertness and a slippery feeling. Some commercial talc may be harder because of the presence of impurities and associated minerals such as dolomite, calcite, tremolite and quartz. Talc is inert in most chemical reagents.
Talc – the “secret ingredient”: colourless, odourless, imperceptible – insoluble and inert. Yet highly effective for so many applications. Most people use products made from talc every day, however, they don't realize that talc is in the product or the special role that it plays.
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