|
The term "Talc" has four complementary meanings:
Talc is a mineral: it is a hydrated magnesium silicate.
Talc is a rock, it is known as steatite or soapstone. It is composed of varying proportions of the mineral talc often combined with other minerals (e.g. chlorite and carbonate).
Talc is an industrial raw material in powder form, which is used as a commodity or
specialty in a wide range of applications.
Talc is a cosmetic powder. This is its most widely known application, even though it is one of the smallest in terms of consumption.
Geology & Occurrence
Talc deposits always result from the transformation of existing rocks under hydrothermal activity. Through this process, the components (MgO,
SiO2, H2O) required for forming the parent rock into talc are brought by the hydrothermal water. The size and the geometry of the final deposit depend upon the size and nature of the parent rock, and the intensity and scale of the phenomenon. The geological context required for such a transformation to occur is known as a low to medium temperature and pressure metamorphism. Tectonic movements always play a major role: earth movements allow the hydrothermal fluid to penetrate into the mother-rock, generating permeability that makes reactions inside the rock mass possible. The surrounding pressure, either at the time of the transformation or later, determines the mineral's degree of lamellarity (low pressure/low lamellarity, high pressure/high lamellarity).
The nature of the mineralisation depends essentially on the nature of the parent rock. Several types of rocks can undergo transformation of this type, and so be the source of talc. Talc deposits are classified according to the parent rock from which they derive. There are four types of talc deposits:
Deriving from magnesium carbonates. This kind of deposit provides >50% of world production. It is found in ancient metamorphosed carbonate sequences. This talc is generally pure and white.
Deriving from serpentines. This type of deposit provides about 40% of talc supplies. The crude ore is always grey and, to be commercially viable, can be up-graded to improve mineralogy and whiteness (generally by flotation).
Deriving from alumino-silicate rocks. About 10% of world production is mined from these deposits. They are sometimes found in combination with magnesium carbonate deposits. The crude ore is generally grey due to the presence of chlorite (another phyllosilicate), but no up-grading is generally necessary as chlorite performs adequately in the applications of interest.
Deriving from magnesium sedimentary deposits. Talc is formed by direct transformation of magnesium clays. No such deposit is currently mined.
This wide diversity of origins and types of deposits naturally gives rise to a wide variety of ores and product grades, which differ according to their mineralogical composition, colour and crystalline structure (micro-crystalline or lamellar).
Physico-chemical properties
Talc is a natural mineral (chemical formula Mg3Si4O10(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 (= 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 according to their degree of lamellarity.
People always think of talc as white but it can also be grey, green, blue, pink and even black.
Organophylic properties
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 is adsorbed onto the talc surface and retained.
Functions & Applications
The five main characteristics of talc that make it a unique material for industrial and domestic applications are:
* Lamellarity (made of platelets that easily slide on each other)
* Softness (unctuous and not abrasive)
* Chemical inertness
* Affinity for organic chemicals
* Whiteness
The main technical functions talc is used for are anti-sticking, anti-caking,
lubricant, carrier, thickener, strengthening
filler, smooth filler, and absorbent.
|
