Talc is a natural mineral (Mg3Si4 O10(OH)2 – molecular weight 379.26). As shown in the opposite figure, its elementary sheet is composed of a layer of magnesium-oxygen/hydroxyl octahedra, sandwiched between two layers of tetrahedral silica. The main surfaces, known as basal surfaces, of the elementary sheet contain neither hydroxyl groups nor active ions, making talc both hydrophobic and inert.
The size of an individual talc platelet i.e. a few thousand elementary sheets, can vary from approximately 1 micron to over 100 microns depending on the conditions of formation of the deposit. It is this individual platelet size that determines a talc’s lamellarity. A highly lamellar talc will have large individual platelets whereas a microcrystalline talc will have small platelets.
The elementary sheets are stacked on top of one another, like flaky pastry, and because the binding forces (known as Van de Waal’s forces) linking one elementary sheet to its neighbours are very weak, the platelets slide apart at the slightest touch, giving talc its characteristic softness.
Talc is odourless. It is insoluble in water and in weak acids and alkalis. Although talc has a marked affinity for certain organic chemicals, it generally has very little chemical reactivity. It is neither explosive nor flammable. Above 900°C, talc progressively loses its hydroxyl groups and above 1050°C, it re-crystallises into different forms of enstatite (anhydrous magnesium silicate). Talc’s melting point is at 1500°C.
Talcs differ according to their mineralogical composition, i.e. the type and proportion of associated minerals present. The most common mineral found with talc is chlorite, which is structurally and chemically very similar. Dolomite and magnesite are also often present. As we have seen above, talcs also differ in their degree of lamellarity.
People always think of talc as white but it can also be grey, green, blue, pink and even black.
Basically, talc is many talcs.
Talc shows an affinity for certain organic chemicals. It is therefore organophilic. In the case of talc’s affinity for polypropylene, a likely explanation is that the position of the oxygen atoms on the surface of the talc platelet corresponds to the carbon bonds on the surface of the polypropylene crystal.
Talc’s organophilic properties are used to great benefit in a number of applications, for instance, it helps polypropylene to crystallise. In papermaking, the talc particles attract the undesirable resin droplets, i.e. organic chemicals, in the pulp onto their surface. This is known as pitch control. In body powder applications, perfume, also organic, is adsorbed onto the talc surface and retained.