Soapstone Rock Type, Composition, Formation, Occurrence & Uses

Soapstone is a non-foliated metamorphic rock. The name “soapstone” is derived from soapy feel and texture. It is also known as steatite or soaprock and one of the most commonly used rocks throughout history and in the modern world.

An interesting fact about soapstone is that many names around the world know it. Soapstone is known as Combarbalite stone in Chile. It is mined in Combarbalá, and is known for its many colors. These colors are not visible during mining but appear after the stone is refined and treated. It is called Palewa and Gorara in India. Soapstone is often named after regional terms for marketing purposes.

Table of Contents

Rock Composition

Soapstone is mainly made up of talc with varying amounts of amphiboles and chlorite. The amphiboles are typically anthophyllite, tremolite, and cummingtonite. Trace amounts of minor iron-chromium oxides are also present in soapstone.

A pure soapstone sample contains 64% silica, 31% magnesia, and 35% water. The quantities may vary from one mine to another depending upon the rock’s impurities.

You may see pyrophyllite (mineral), similar to talc, being called soapstone. This confusion exists because pyrophyllite has similar physical properties and uses compared to soapstone. On the other side, pyrophyllite doesn’t have a soapy texture like soapstone.

Soapstone Physical Properties

Soapstone is a softer stone compared to other metamorphic rocks. The softness is due to high talc content. You won’t find a fixed hardness for soapstone because the talc content varies from one specimen to another. Soapstone with 30% talc content is used for architectural purposes and 80% for carving. Some specimens are so soft that they feel like bathing soap when you touch them.

Soapstone is usually gray, bluish, brown, or green. The rock often displays different colors with irregular patches or streaks.

Its physical properties can be summarized in the following points.

Soft & easy to carve
Nonporous and nonabsorbent
Electrical insulator and low heat conductivity
Resistant to acids and alkalis (placed above marble in the quality)
High specific heat capacity

The outer layers of the Christ the Redeemer sculpture in Rio de Janeiro are made of soapstone.

Artistic Soapstone vs. Architectural Soapstone

Soapstone is divided into two categories based on talc content.

Artistic Soapstone:

Artistic soapstone is made up of 80% talc. It is so soft that it can be scratched with a fingernail. It usually comes in brown, yellow, green, blue, and white colors. Artistic soapstone is usually used to make sculptures and handicrafts.

Architectural Soapstone:

Architectural soapstone is used in the building and construction industry. It has 15 to 50 percent talc content, but some specimens with up to 60% talc content are also used for countertops.

Soapstone Formation

Soapstone is produced by regional metamorphism and metasomatism at convergent plate boundaries. The parent rocks or protoliths are usually dunites, peridotites, and serpentines.

Soapstone can also form from siliceous dolostones through metasomatism. Hot, chemically active fluids alter the parent rock in this process.

The level of metamorphism or metasomatism the rock receives determines its grain size. Soapstone exposed to high levels of metamorphism has a fine grain size and is used for highly detailed carvings. The presence of minerals in the rock can influence the rock’s hardness.

Where is Soapstone Found?

Soapstone is a common rock and is found worldwide. Its biggest reserves are located in Finland and Brazil. US reserves are located in the Appalachian range from Maine to Georgia. Significant deposits are also found in Vermont and Virginia.

Significant reserves are also located in India and Pakistan. China and Australia also have huge soapstone reserves.

Historical Uses

Africa:

Ancient Egyptians made scarab signets and amulets from glazed steatite (soapstone). Ancient Nigerian people also used it to make statues.

Americas:

Native Americans used soapstone to make bowls, cooking slabs, and smoking pipes. Smoking pipes are still made from soapstone in some parts of the world. It was also used to make cemetery markers in the 19th century.

Asia:

The ancient city of TepeYahya in Iran was a center for the production and sale of soapstone. It made statues, sculptures, and ceremonial knives in India and China.

Europe:

Vikings used soapstone to make cooking pots.

Nidaros Cathedral in Trondheim, Norway, constructed mainly of soapstone

Present Day Uses

Its present day uses has made this rock valuable for mankind. It is even rated better than marble in some applications. The common uses are given below.