The Softest Mineral in the World: Mineralogy and Geology
Talc is a platy mineral belonging to the group of silicates. It is formed either through regional or contact metamorphosis of carbonate sediments or through hydrothermal alteration of magnesium-rich magmatic rocks. Basically, all rocks containing talc are metamorphic rocks and talc is thus present as a secondary mineral.
Talc is also called steatite – or, in chemical terms, hydrated magnesium silicate. It is the main component of soapstone. Its crystals usually develop massive, leafy aggregates with laminar particles. Ground talc is called talcum.
Talc is the softest mineral, occupying place 1 in the Mohs` hardness scale. Its silicate layers lie on top of one another without having a chemical bond but are bound to each other by weak van der Waals forces. This structure gives talc the platy appearance and its characteristic greasy or soapy feeling – hence the name "soapstone”. In its pure form, talc is colourless or appears white, and often it has a mother-of-pearl sheen. When it contains other substances, it can also appear light grey, green, yellowish or pink.
Accompanying minerals include magnesite, dolomite, chlorite, serpentine, calcite, chlorite, and in some cases sulfides, quartz, tremolite or vermiculite.
In naturally occurring crystals, the magnesium in talc can be replaced by iron or nickel and the silicium by aluminium or chromium.
Talc has a high capacity for absorbing organic substances. The opposite applies to water: talc is hydrophobic and insoluble. It is also acid-resistant, chemically inert and non-toxic. Talc has neither aroma nor taste.
Formation and deposits of talc
Talc can be formed in different geological environments or processes. Several types of talc deposits may be distinguished according to the present composition and parent rock from which they are derived. There are four types of talc deposits; the two main ones contributing to the world's talc production are described below.
Dolomite-hosted talc and talc-chlorite rocks
The talc is formed by the alteration of sedimentary magnesium carbonate rocks (dolomite and magnesite) at elevated temperature and pressure below the earth's surface. While the magnesium is fixed in situ, the silica is transported by silica-containing hot fluids to react with the Magnesium- bearing carbonates to form talc. These altered rocks are then talc-rich dolomites or magnesites. This type of deposits usually deliver the massive talcs accompanied by carbonates, chlorite and some quartz. The mineral composition is generally 30 - 100% talc, 0 - 70% chlorite/carbonates and 0.1 - 0.5% quartz.
Ultramafic-hosted talc magnesite rocks
These talc deposits result from hydrothermal alteration of magnesium-rich magmatic parent rocks. They are called ultramafic rocks while they consist mostly of mafic (dark Magnesium-rich) minerals. Alteration process is two-fold: first hydration of these mafic minerals such as olivine or pyroxene by H2O influx into serpentine which is a hydrated Magnesium-silicate. Second step is alteration of serpentine into talc and magnesite by CO2- addition.
These rocks consist of talc, magnesite, chlorite, sulfides and other minerals with no or very low quartz content.
Since talc in these deposits is not massive but occurs as talc-magnesite rock, the crude ore must be crushed and grinded prior to refining by flotation to increase the talc content and whiteness before this talc can be used as an industrial mineral.
This kind of talc deposits occur in Finland, Norway, Sweden, Canada and Russia.
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 phengite, Mg-garnet, Mg-chloritoid and kyanite in meta pelites (Al-rich metamorphic rocks). The deposits are often found in combination with magnesium carbonate deposits. As fourth type, 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.