What is talc? Talc is a pulverized, natural, foliated, hydrated magnesium silicate that has the approximate chemical formula of Mg3(Si2O5)2(OH)2.12 There are many talc deposits scattered throughout the world, and every talc deposit is unique with regard to both chemistry and morphology. In general, the companies that produce the talc that is used for pleurodesis do not know the origin of their talc. There are variable amounts of calcium, aluminum, and iron present in different talc preparations. Non-talc minerals associated with commercial talc vary from deposit to deposit and may include calcite, magnesite, dolomite, chlorite, serpentine, quartz, and others. Talc is processed by being passed through mesh to eliminate the larger talc particles. Accordingly, the mesh size sets the upper limit for the size of the talc particles in the final preparation.