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talc is a mineral resulting either from the metamorphism (contact or regional) of
dolomitic sediments or from hydrothermal alteration of magnesium-bearing basic to
ultrabasic igneous rocks. It usually occurs in metamorphic rocks as a secondary mineral.
Formation and Deposits of talc
talc is formed by different processes on earth. Several types of talc deposits may be
distinguished according to the parent rock from which they are derived. There are four
types of talc deposits of which the two main ones contributing to the world's talc production
are presented below more detailed.
dolomite-hosted talc and talc-chlorite rocks
The talc is formed by the alteration of sedimentary magnesium carbonate rocks
(dolomite and magnesite) at high temperature and pressure under the earth's crust.
While the magnesium is fixed in-situ the silica is brought by heated waters to react
with the carbonate rocks. The altered rocks are then dolomitic marbles which contain the talc deposits.
These deposits deliver usually the purest talc. The mineral composition of
these rocks generally corresponds to 30 - 100 % talc, 0 - 70 % chlorite/carbonates and 0.1 - 0.5 % quartz.
Ultramafic-hosted talc magnesite rocks
The talc deposits result from hydrothermal alteration of magnesium-rich,
basic (mafic) to ultrabasic (ultramafic) parent rocks. Fluids react with mafic (magnesium + ferrous and/or ferric)
minerals such as olivine, pyroxene or amphibole in the host rocks transforming them to serpentine
which is then replaced by talc.
These rocks consist of talc, magnesite, chlorite and others with
no or very low quartz content. Since these talc deposits are less pure than in the previous
case, the crude ore needs to be upgraded by flotation to increase the talc content and
whiteness before its use as an industrial mineral. Such talc deposits occur in Finland, Norway,
Sweden, Canada, Russia and Italy.
talc may also be derived from alumino-silicate rocks. In this case talc is stable under high pressure conditions up to a temperature of about 700° C in
association with phengit, Mg-garnet, Mg-chloritoid and kyanite in meta pelites (Al-rich metamorphic rocks). The deposits are often found in combination with magnesium carbonate deposits.
Finally talc may be formed by direct transformation of magnesium clays. Due to the impurities associated with the talc ore no such deposits are currently being mined.
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