TALC

    Class : Silicates
    Subclass : Phyllosilicates
    Crystal system : Monoclinic
    Chemistry : Mg3Si4O10(OH)2
    Rarity : Common


Talc is a magnesium phyllosilicate that can appear in three ways, in different geological contexts. It can first form by hydrothermal alteration of magnesium silicates : usually olivine (forsterite) of peridotites or chrysotile and antigorite of serpentinites (themselves derived from peridotites). Talc also appears by siliceous metasomatosis of dolomites, this context giving the purest qualities, highly sought after by industrialists. And finally, talc can crystallize following the light metamorphism of magnesian rocks. It is sometimes abundant to the point of being the major constituent of certain rocks (talcschists). Its name comes from the Arabic talq (pure, white) because of the color of its powder. Talc occurs in automorphic crystals forming flexible pseudohexagonal lamellae, sometimes centimeter-sized, or in cleavable flaky masses, in rosettes or in tufts of transparent lamellae. Its color is white to greenish when mixed with chlorite, sometimes grayish in massive varieties. Its low hardness (1 on the Mohs scale, the softest of minerals) gives it a creamy feel, which is the origin of most of its uses. It is in fact used in powder form in the pharmaceutical and cosmetic industries (soaps, powders and body creams), the rubber, paint, ceramics and food industries (anti-sticking of cereal cakes intended for livestock and confectionery, insecticides). Talc is also a mineral filler widely used in papermaking, particularly for coating magazines, and plays a role in the manufacture of thermal and electrical insulators. The compact, fine-grained varieties (steatite) are a highly sought-after ornamental material for sculpting art objects, particularly in China.

Main photo : Talc from Trimouns, Luzenac, Ariège, France © Guy Bernadi

Talc from Talggenkopf, Tyrol, Austria © Gerard van der Veldt
Talc from Verde Antique International quarry, Vermont, USA © Peter Cristofono
Talc from Trimouns, Ariège, France © Jordy Fabre
Talc from Trimouns, Ariège, France © Michel Bretheau

Talc in the World

Good talc crystals are rare. Centimeter-sized aggregates of transparent lamellae have been discovered in Coopstown (Maryland) and Chester (Massachusetts) while in Brumado (Brazil), sheets of several square centimeters included in magnesite are known. Good specimens are also reported in serpentinites from the Swiss (Zermatt), Austrian (Zillertal), and Italian (Val Malenco) Alps.

Talc in France

In France, there are among the best known samples with transparent lamellar crystals of hexagonal habit of 1 cm grouped in fascicles, they come from the largest talc deposit in the world: Trimouns near Luzenac (Ariège). It is also reported in many other occurrences.

Twinning

No twinning known for this mineral species.

Fakes and treatments

No fakes listed for this mineral species.



Hardness : 1
Density : 2.58 to 2.83
Fracture : Micaceous
Streak : White


TP : Translucent to transparent
RI : 1.538 to 1.600
Birefringence : 0.051
Optical character : Biaxial -
Pleochroism : Low
Fluorescence : None


Solubility : Insoluble

Magnetism : NoneRadioactivity : None