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Marble


A non-foliated metamorphic rock that forms when limestone is subjected to heat and pressure.



What is Marble?



Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. It is composed primarily of the mineral calcite (CaCO3) and usually contains other minerals such as: clay minerals, micas, quartz, pyrite, iron oxides and graphite. Under the conditions of metamorphism the calcite in the limestone recrystallizes to form a rock that is a mass of interlocking calcite crystals. A related rock, dolomitic marble, is produced when dolostone is subjected to heat and pressure.

Photo Gallery:   The Many Uses of Marble


How Does Marble Form?



Most marble forms at convergent plate boundaries where large areas of Earth's crust are exposed to regional metamorphism. Some marble also forms by contact metamorphism when a hot magma body heats adjacent limestone or dolostone.

Before metamorphism, the calcite in the limestone is often in the form of lithified fossil material and biological debris. During metamorphism, this calcite recrystallizes and the texture of the rock changes. In the early stages of the limestone-to-marble transformation the calcite crystals in the rock are very small. In a freshly-broken hand specimen they might only be recognized as a sugary sparkle of light reflecting from their tiny cleavage faces when the rock is played in the light.

As metamorphism progresses the crystals grow larger and become easily recognizable as interlocking crystals of calcite. Recrystallization obscures the original fossils and sedimentary structures of the limestone. It also occurs without forming foliation which normally is found in rocks that are altered by the directed pressure of a convergent plate boundary.

Recrystallization is what marks the separation between limestone and marble. Marble that has been exposed to low levels of metamorphism will have very small calcite crystals. The crystals become larger as the level of metamorphism progresses. Clay minerals within the marble will alter to micas and more complex silicate structures as the level of metamorphism increases.


Physical Properties and Uses of Marble



Marble occurs in large deposits that can be hundreds of feet thick and geographically extensive. This allows it to be economically mined on a large scale with some mines and quarries producing millions of tons per year.

Marble is crushed and used as an aggregate in highways, railroad beds, building foundations and other types of construction. It is also cut into pieces of specific size to produce dimension stone. These are used in monuments, buildings, sculptures, paving and other projects.

Color:   Marble is usually a light-colored rock. When it is formed from a limestone with very few impurities it will be white in color. Marble that contains impurities such as clay minerals, iron oxides or bituminous material can be bluish, gray, pink, yellow or black in color.

Marble of extremely high purity with a bright white color is very useful. It is often mined, crushed to a powder and then processed to remove as many impurities as possible. The resulting product is called "whiting". This powder is used as a coloring agent and filler in paint, whitewash, putty, plastic, grout, cosmetics, paper and other manufactured products.

Acid Reaction:   Being composed of calcium carbonate, marble will react in contact with many acids, neutralizing the acid. It is one of the most effective acid neutralization materials. Marble is often crushed and used for acid neutralization in streams, lakes and soils.

It is used for acid neutralization in the chemical industry. A pharmaceutical product known as "Tums" is a small calcium carbonate pill, sometimes made from powdered marble, that is used by people who suffer from acid reflux or acid indigestion. Powdered marble is used as an inert filler in other pills.

Hardness:   Being composed of calcite, marble has a hardness of three on the Mohs hardness scale. As a result, marble is easy to carve and that makes it useful for producing sculptures and ornamental objects. The translucence of marble makes it especially attractive for many types of sculptures.

The low hardness and solubility of marble allows it to be used as a calcium additive in animal feeds. Calcium additives are especially important for dairy cows and egg-producing chickens. It is also used as a low hardness abrasive for scrubbing bathroom and kitchen fixtures.

Ability to Accept a Polish:   After being sanded with progressively finer abrasives, marble can be polished to a high luster. This allows attractive pieces of marble to be cut, polished and used as floor tiles, architectural panels, facing stone, window sills, stair treads, columns and many other pieces of decorative stone.


Another Definition of Marble



The name "marble" is used in a different way in the dimension stone trade. Any crystalline carbonate rock that has an ability to accept a polish is called "marble". The name is sometimes used for other soft rocks such as travertine, verd antique, serpentine and some limestones.

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More Metamorphic Rocks
  Schist
  Slate
  Soapstone
  Quartzite
  Novaculite
  Gneiss
  Marble
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