Talc geology

Talc is one of the common minerals in metamorphic rock. Although talc deposits can be found throughout the world in various geological contexts, economically viable concentrations of talc are not that common.

How talc deposits are formed

Talc deposits result from the transformation of existing rocks under the effect of hydro-thermal fluids carrying one or several of the components needed to form the mineral (MgO, SiO₂, CO₂). Tectonics plays a major role in the genesis of a talc deposit. It enables hydro-thermal fluids to penetrate the rock, creating a micro-permeability that facilitates reactions in the mass. The size and shape of talc deposits depend upon the intensity of the hydrothermal activity which corresponds to the climate of a low temperature metamorphism. Pressure and deformations, both concurrent with and subsequent to this transformation, determine the crystallinity of the talc ore in the deposit.

Types of talc deposit

Talc deposits differ according to the parent rock from which they are derived. Four main categories exist.

Magnesium carbonate derivative orebodies

The talc results from the transformation of carbonates (dolomite and magnesite) in the presence of silica. The carbonates fix in-situ the magnesium needed to form the mineral whereas the silica is provided by hydro-thermal circulation. This reaction results in a talc which, depending on the composition of the parent rock, is either mineralogically pure or associated with minerals such as carbonates (both residual and reactional), quartz and chlorite.

Deposits of this kind represent some 60% of world production and provide some of the whitest and purest talc ores. The Yellowstone (Montana, USA) and Respina (North-West Spain) talc deposits are good examples.

Yellowstone mine in Montana - US, example of a magnesium carbonate derivative talc orebody

Serpentinite derivative orebodies

About 20% of present world production comes from the transformation of serpentinite into a mixture of talc and reactional magnesium carbonates. This ore, commonly called "soapstone", is always grey and never pure. To be used as an industrial mineral, it is often upgraded by flotation to increase the talc content and whiteness.

This type of deposit is relatively common and widely distributed along ultra-mafic rock belts. Amongst those currently being worked are deposits in Vermont USA, Quebec and Ontario in Canada, and Finland.

Argonaut mine in Vermont - US, example of a serpentine derivative talc orebody

Siliceous or silico-aluminous rock derivative orebodies

Here, talc results from the transformation of siliceous rocks such as quartzite, which provides the silica needed for the mineral's formation. Magnesium is brought by the migration of hydro-thermal fluids.

If the parent rock has a silico-aluminous composition, e.g. pelitic schist or gneiss, and under the same conditions of formation, chlorite can be formed in addition to the talc, the resulting ore being a mixture of both talc and chlorite.

This type of deposit can be found in association with the magnesium-carbonate derivative type, as is the case of Trimouns in the French Pyrenees. This kind of deposit represents about 10% of world production.

Trimouns mine, French Pyrenees, example of a silico-aluminous rock derivative talc orebody

Magnesium sedimentary deposit derivative orebodies are only mentioned here to provide a complete picture. Although they can correspond to very large concentrations, the talc ore is generally found in association with impurities such as quartz, mica, clay, organic materials and iron hydroxides. No deposits belonging to this category are currently being mined.

The four types of deposit outlined above yield a wide variety of ores that differ in:

• their mineralogical composition,
• their color (measured in terms of whiteness or brightness),
• their crystalline structure (compact or platy).

and these three parameters govern the specific nature of each commercial talc grade as well as its industrial application.