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. You can see the platy nature of talc from this Scanning Electron Micrograph (SEM) taken at a 5,000 times magnification. 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.
How is talc formed?
Talc is an alteration mineral. It is formed by geological modification of some host rock. Most talc is formed from the alteration of dolomite (CaMg(CO3)2 or 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.
Minerals commonly associated with talc are, tremolite [(CaMg3(SiO3)4)], serpentine [(3MgO·2SiO2·2H2O)], anthophyllite [Mg7·(OH)2·(Si4O11)2)], magnesite, mica and chlorite. The latter is a talc-like mineral in which varying proportions of aluminum are substituted for magnesium in the brucite layer. Other impurities such as dolomite, the calcite form of calcium carbonate (CaCO3), iron oxide, carbon, quartz, and manganese oxide may also be present.
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.
Uses for talc
Talc has many uses. The most commonly known application would be in cosmetics and dusting, body and baby powders, where the oleophilic (oil-loving, hydrophobic) nature of the mineral helps to absorb natural oils, and the slippery nature presents a pleasant feel. While this usage is well known it is a minor segment of the talc industry as a whole.
The paper industry is the largest consumer of talc, where it is used as a filler to enhance opacity and improve printing properties. It is also used as a pitch control agent in the production of wood pulp to remove resinous, sticky remains of wood, which otherwise would create dark or thin spots in the paper. In the ceramic industry, which is also a large consumer, talc prevents delayed glaze crazing, lowers firing temperatures, and reduces fired shrinkage. Ceramic wall tile contains up to 65 percent talc in the formulation. Another important application is in paint and coatings. Talc improves exterior durability, controls viscosity, brushing, and gloss properties. It also reduces paint formulation costs, by extending or replacing more expensive resins and solvents.
The use of talc for plastic filling and reinforcement has grown rapidly in the past decade. When used in plastics, the plates of talc make the resin more rigid and stronger. Polypropylene parts reinforced with as much as 40 percent of talc have replaced metal in many automotive applications, such as bumpers, interior plastic ductwork and fascias, as manufacturers have aimed to reduce weight, improve gasoline mileage, and reduce the number of separate parts that have to be made. Another usage in plastics is as an antiblock in blown polyethylene films. Without the use of an antiblock, it is difficult to pull the two faces of a plastic bag apart.
Other uses of talc are as chewing gum dusting, insecticide carriers, rubber-dusting and textile-filling materials, and as additive in asphalt roofing compounds.
Specialty Minerals Talc Products
Specialty Minerals Inc. (SMI) produces ground talc products in Barretts, Montana, using ore from its two nearby mines.
For plastic reinforcement, SMI products are offered as small as 0.8 micron for superior reinforcement. MicroTuff® AG and MicroTalc® products are widely used in polyolefins for automotive and appliance parts. For blown and cast films, ABT®, Polybloc® and the high clarity Optibloc® are talc-based antiblocks.
The Talcron® and Sericron® talc series are a range of products for paints and coatings. Cercron®talcs are used in specialty ceramic applications. Select MicroTalc® and UltraTalc® talc products meet high purity standards required for use in foods, pharmaceuticals and body powders.