Marble Rock: Geology, Properties, Uses

How Marble Forms

Limestone, the source material for marble, forms when calcium carbonate precipitates out of water or when organic debris (shells, coral, skeletons) accumulates. Marble forms when limestone experiences metamorphism. Usually, this happens at a convergent tectonic plate boundary, but some marble forms when hot magma heats limestone or dolomite. The heat or pressure recrystallizes calcite in the rock, changing its texture. Over time, the crystals grow and interlock to give the rock a characteristic sugary, sparkling appearance.

Other minerals in marble also change during metamorphism. For example, clay recrystallizes to form mica and other silicates.

Marble is found all over the world, but four countries account for half of its production: Italy, China, Spain, and India. Probably the most famous white marble comes from Carrara in Italy. Carrara marble was used by Michelangelo, Donatello, and Canova for their masterpiece sculptures.

Properties

The visible crystals in marble give it a characteristic granular surface and appearance, but there are other properties used to identify the rock.

Marble is considered to be a strong, hard stone, even though its primary mineral, calcite, only has a Mohs hardness of 3. Marble can be scratched with a metal blade.

Marble tends to be light in color. The purest marble is white. Marble that contains a lot of bituminous material may be black. Most marble is pale gray, pink, brown, green, yellow, or blue.

Marble fizzes upon contact with dilute hydrochloric acid.

Uses

Because of the way marble forms, it occurs in large deposits worldwide. It's economical to mine this common, useful rock on a large scale.

Most marble is used in the construction industry. Crushed marble is used to build roads, foundations of buildings, and railroad beds. Dimension stone is made by cutting marble into blocks or sheets. Dimension stone is used to make buildings, sculptures, paving stones, and monuments. The statue of Lincoln in the Lincoln Memorial is made of white marble from Georgia, while the floor is pink Tennessee marble, and the exterior facade is marble from Colorado. Marble is susceptible to acid rain and weathering, so it wears down over time.

White marble is ground to make "whiting," a powder used as a brightener and pigment. Powdered marble, along with limestone, may be used as a calcium supplement for livestock. Crushed or powdered marble is used in the chemical industry to neutralize acid, as a pill filler, and to remediate acid damage in water and soil.

Marble may be heated to drive off carbon dioxide, leaving calcium oxide or lime. Lime is used in agriculture to reduce the acidity of soil.

The Other Definition of Marble

In the stone trade and common usage, any crystalline carbonate that takes a high polish might be called "marble." Sometimes limestone, travertine, serpentine (a silicate), and breccia are called marble. Geologists use the narrow definition of a metamorphic rock formed from limestone or dolomite.

Are Marbles Made of Marble?

The original toy called "marbles" bears the mark "Made in Germany." These playthings were made by rolling clay or another pottery material into balls, then glazing and firing it so that it resembled imitation agate. The marbles featured round "eyes" from the firing process, giving them a sort of marbled appearance.

Glass marbles entered mass production in 1846, with the German invention of marble scissors. Toys resembling marbles have been found in excavations of ancient Egyptian and Mesopotamian sites. Early marbles were rounded stones, nuts, or clay. While a few marbles are indeed made of marble, the stone is too soft to be an ideal material for the modern game. The name of the toy reflects the appearance of the balls, not their composition.

Key Points

• Marble is a metamorphic stone formed by subjecting limestone to heat or pressure.
• In pure form, marble consists of calcium carbonate (calcite) and is sparkling white. Impurities produce pale gray, brown, or variegated colored rock. Black marble also occurs.
• Marble takes a high polish. In common usage, any stone that takes a high polish may be called marble, but this is technically incorrect.
• Marbles aren't made of marble. The toy got its name from its appearance rather than its composition. Ancient toys resembling marbles were made of smooth stone, clay, or nuts.

Sources

• Acton, Johnny, et al. Origin of Everyday Things. Sterling Publishing Company, 2006.
• Baumann, Paul. Collecting Antique Marbles: Identification & Price Guide. Krause Publications, 1999.
• Kearey, Philip. Dictionary of Geology. Penguin Group, 2001.