Talc, in its pure form, is a hydrated magnesium silicate and has the chemical formula 3MgO.4SiO2·3H2O. The exact composition of talc depends on the location of mining, since talc from different mining locations may contain significantly different proportions of MgO, SiO2, and H2O. Talc has a layered platey structure consisting of a magnesium hydroxide layer sandwiched between two silica layers. These three layers are held together by weak chemical bonds of Van der Waals type and, therefore, they can slide past each other relatively easily when subjected to shearing action.

Talc is the softest mineral filler used in the plastics industry. It has high resistivity, low gas permeability, low abrasiveness, and high lubricity. All of these qualities are attributed to talc׳s platey structure. The aspect ratio of talc fillers is as high as 20:1. Talc with a high aspect ratio has shown reinforcement ability in high-density polyethylene, polypropylene, and other thermoplastics, especially when they are treated with coupling agents to promote better bonding with these polymers.