Talc - the Soapstone

Published: 08.11.2018 10:46
Last updated: 13.02.2019 05:32

Although most mineral collectors are not attracted by talc, it is still an interesting mineral. It has set of quite unique physical properties, which create endless options for various industrial applications.

Structure of Talc

The composition of talc is Mg3(Si4O10)(OH)2. The only common substitution is small amount of Fe and Al replacing Mg. The replacement of Si by Al is very limited. Talc has a triclinic crystal system but some monoclinic polytypes also exist. It belongs to phyllosilicates and the talc-pyrophyllite group. The internal microstructure is layered and the layers have zero valence - that means that the layers are kept together only by very poor van der Waals bonds. This is the primary reason for the softness and perfect cleavage of talc.

Rare talc crystals from Trimouns Talc Mine, Luzenac, Ariège, Midi-Pyrénées, France
Rare crystals of talc from Trimouns Talc Mine, Luzenac, Ariège, Midi-Pyrénées, France. Size 14 x 6 cm. Photo: Didier Descouens.

Most of the time, talc occurs in a foliated form, which means that it is actually hundreds of very small, leaf-shaped crystals that combine into larger sheets or plates. It may also occur in masses that are micaceous in nature; this means that they are compact and flat, and have layers that may be easily peeled apart. Other talc masses may be botryoidal, radiating, or fibrous. Very rarely, talc may form tiny tabular crystals.

Massive pale talc from Lassing, Austria
Typical example of pale colored talc aggregate from Lassing, Austria. Size 7 x 7.5 cm. Photo: Zbyněk Buřival.

Physical Properties of Talc

Talc is usually white, light gray or very pale green. But it may occur in a variety of pale colors, including gray, yellow, beige, brown, purple, pink, green, and blue. It may present a very slight fluorescence, in some instances, and it shows a white streak.

Talc aggregates from Mutnik, Slovakia
Talc aggregates from Mutnik, Slovakia. FOV 30 cm. Photo: Pelex.

When broken, talc shows an uneven fracture. Talc is very soft and registers at only 1 on the Mohs scale of hardness. Even a fingernail can easily scratch the surface of this mineral. Because of its extreme softness, it often has a greasy, almost oily surface. Talc has a pearl or greasy luster and is very resistant to chemicals, including strong mineral acids.

The low hardness level of talc also allows it to be crushed with ease. Talc is commonly used in talcum powder, which serves as an inert filling in paper, plastic or even medicine pills. It is also present in electronics and paint, because it is resistant to both electricity and high levels of heat. It is used in heat resistant devices in metallurgy, glass production and labs.

Talc with pyrite from Habachtal, Austria
Pyrite crystal in talc from Sedl, Habachtal, Austria. Size 5 x 5 mm. Photo: Vítězslav Snášel.

Massive talc may be used in the carving of figures and statues, because it is easy to sculpt. Talc was very popular in the ancient times for carving statues, vases, urns, pots and many other things. The massive (often pale green) form with chlorite and pyroxene inclusions is known as soapstone. However, sopastone may be also composed of pyrophyllite or its mixture with talc.

Historic pot made of talc - soapstone
Reconstruction of historic pot made of soapstone. Photo: Lysippos.

Occurrence and Origin

Talc originates by alteration of ultrabasic rocks with olivine and pyroxenes, especially serpentinites. It can also form by the metamorphosis of ultrabasic rocks or dolomites. Hydrothermal talc is much less common.

Talc and actinolite from Habachtal, Austria
Talc with green actinolite crystals from Sedl, Habachtal, Austria. Size 6 x 6 cm. Photo: Zbyněk Buřival.

Although talc can be found the world over, microscopic talc crystals may be found in Brosso, Italy, and Luzenac, France. Talc with nice pyrite and actinolite crystals occurs in Habachtal, Austria. Pseudomorphs of talc from quartz can be found in Bavaria, Germany. Brumado, Brazil is known for its quality talc, and green talc crystals can be found at the Vermont Verde Antique Quarry in Vermont, in the United States. Chester and Ludlow, two other mining locations in Vermont, are home to talc deposits as well, and the Serpentine Belt between Maryland and Pennsylvania boasts excellent talc specimens. White talc with black manganese inclusions grows in Madison County, Montana. Talc with pyrite, actinolite and sometimes emerald crystals occurs in Habachtal, Austria.

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