Soapstone

Soapstone is a type of talc-schist metamorphic rock. Also other naming’s are steatite or soaprock. The composed primarily of talc, with varying amount of micas, chlorite, amphiboles, carbonates and other minerals. It is produced by dynamothermal metamorphism and metasomatism. It occurs in the regions where tectonic plates are present, replacing the rocks with heat and pressure, with the flow of liquids, but without melting. It has been an environment for carving for thousands of years. It is composed primarily of talc so it usually very soft. It is typically gray, bluish, green, or brown in color, often variegated. Its name is derived from its “soapy” feel and softness.

Texture: Non-foliated to weakly-foliated; Fine-grained

Composition: Talc

Index Minerals: Talc

Color: White, green or gray

Miscellaneous: Softer than fingernail; may be schistose in texture

Metamorphic Type: Hydrothermal

Metamorphic Grade: Low to Medium Grade

Parent Rock: Peridotite

Metamorphic Environment: Hydrothermal solutions concentrated during final stages of magma crystallization in batholiths or hot seawater solutions drawn down into subduction zones

Hardness: Very soft because of primary mineral is talc

Minerals: Micas, chlorite, amphiboles, carbonates, magnesite

Dominant Minerals: Talc

Soapstone Composition

Soapstone composed is predominantly talc and various amounts of chlorite and amphiboles (typically tremolite, anthophyllite and cummingtonite) and traces of small iron-chromium oxide. It may be schistose or massive. Soapstone is formed by metamorphism of ultramafic protoliths (eg dunite or serpentinite) and metasomatism of siliceous dolomites.

By mass, “pure” steatite is roughly 63.37% silica, 31.88% magnesia, and 4.74% water. It commonly contains minor quantities of other oxides such as CaO or Al2O3.

Soapstone Formation

Soapstone usually formed at convergent plate boundaries. These areas of the Earth’s crust are subject to direct pressure and heat, thus metamorphosing periodites, serpentinites and dunites into soapstones. It can also be formed from the altering of dolostones by hot and chemically active fluids through metasomatism.

Where is it found?

Soapstone can be found all over the world. At these days, much of soapstone comes from Brazil, Chine or India. Important deposits also exist in Australia and Canada, as well as in England, Austria, France, Italy, Switzerland, Germany and the United States. Stones from different countries have different properties, but all are geologically stable, solid and not affected by humidity, therefore objects carved from soapstone last a very long time.

Soapstone Characteristics and Properties

Soapstone is composed primarily of talc and shares many physical properties with that mineral. These physical properties make soapstone valuable for many different uses. These useful physical properties include:

• soft and very easy to carve
• nonporous
• nonabsorbent
• low electrical conductivity
• heat resistant
• high specific heat capacity
• resistant to acids and alkalis

Its composition depends upon the parent rock material and the temperature/pressure conditions of its metamorphic environment. As a result, the physical properties of the soapstone can vary from quarry to quarry and even within a single rock unit.

Soapstone Uses

Native Americans would mold cooking bowls out of soapstone. These bowls could be placed into a fire to cook meat and stews. The heat resistant nature of soapstone ensured that their cooking bowls could withstand the direct heat of the fire while ensuring their food was tenderly cooked.

Another handy cooking tool the Native Americans used was that of “boiling stones.” These stones were made from soapstone and lined with thick animal skin.

Since soapstone is easy to carve and drill, Native Americans would often make their smoking pipes and pipe bowls out of soapstone, which allowed for the heat burning their tobacco to be greater within and lower externally.

Other historical uses for soapstone, during the Revolutionary War, It was used to create bullet molds since it was easy to carve, heat resistant, and durable.

It has also been used as control panels for high voltage equipment. It is also used for carvings and sculptures.

It has also been known to be cut into small cubes and refrigerated as whiskey stones. These stones were used instead of ice as a means of chilling the whiskey drink for over thirty minutes while keeping it undiluted.

Modern soapstone uses, Soapstone continues to be used in a variety of means. As a countertop, soapstone remains unaffected by heat and stains. Although susceptible to scratches, a gentle sanding and mineral oil treatment help restore a soapstone countertop to better condition.

Conclusion

• It is primarily made of talc. It shares many physical properties with this mineral and makes it valuable for many different uses. It is non-porous, heat resistant, non-absorbent, soft and easy to process, high specific heat capacity, resistant to acids and bases.
• The mineral composition of this rock may vary. It depends on the main rock material and the pressure / temperature conditions of the metamorphic environment.
• Grain size is determined by the level of metamorphism. There are more durable hard varieties used in machine construction, and some have the desired fine particle size for high carvings.
• 8,000 years ago, Native Americans used the rock to make carved sculptures and cooking pots. In the Late Archaic Period, Native Americans from North America made bowls, smoking pipes, cooking plates and ornaments.
• During the Stone Age, the people of Scandinavia used molded patterns of soapstone to pour metal objects such as knife blades and spearheads. They discovered that they were able to heat the soapstone and then spread it slowly. This causes them to make cooking utensils, bowls, cooker liners and cooking plates from soapstone.
• The famous sculpture of the city of Rio de Janeiro, which overlooks the “Savior Christ”, is made of concrete and is faced with soapstone. It weighs 635 mt and stands at a height of 120 feet. The sculpture was founded between 1922 and 1931. It has become a cultural symbol.
• During the Revolutionary War, the army removed the molds from the soap molds because they were easily carved and heat resistant.
• Its electrical properties are used as an insulator for the body and electrical components due to its durability and can be pressed in complex shapes before firing.
• As an alternative natural stone kitchen counter instead of marble or granite, soapstone is often used because it is not stained with tomatoes, grape juice or wine. It is used in laboratories because it is not affected by acids and bases. Soapstone is not affected by heat, so a casserole can be placed on it without fear of burning or damaging the surface.
• Soapstone is applied to almost every object, mainly made of talc, leaving a white trace. Tailors, carpenters and other artisans use soap stones because they have been using soapstone as a marking tool for years. It is also used as a marker by welders during welding, because the powder is heat resistant and does not burn.
• Small chilled soapstone can be used instead of ice in a glass of whiskey. It is ideal because it does not dilute the alcohol and a few stones can keep a drink cold for more than 30 minutes because the temperature of the rock changes very slowly. Also because the rock is soft, it does not scratch the glass.

References

Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
Clark, Sarah. (2017, April 24). The Characteristics of Slate. Sciencing. Retrieved from https://sciencing.com/characteristics-slate-8199338.html
http://www.softschools.com/facts/rocks/soapstone_facts/3028/