What is marble?
Marble is a rock resulting from metamorphism of sedimentary carbonate rocks, most commonly limestone or dolomite rock. Metamorphism causes variable recrystallization of the original carbonate mineral grains. 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 dolostoneis subjected to heat and pressure.
The resulting marble rock is typically composed of an interlocking mosaic of carbonate crystals. Primary sedimentary textures and structures of the original carbonate rock (protolithic) have typically been modified or destroyed.
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
Most marble is made into either crushed stone or dimension stone. Crushed stone is used as an aggregate in highways, railroad beds, building foundations and other types of construction. Dimension stone is produced by sawing marble into pieces of specific dimensions. These are used in monuments, buildings, sculptures, paving and other projects. We have an article about "the uses of marble" that includes photos and descriptions of marble in many types of use.
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.
Hardness: 3 to 4 on Moh's Scale
Density: 2.55 to 2.7 Kg/cm3
Compressive Strength: 70 to 140 N/mm2
Water Absorption: Less than 0.5% (except Rainforest Green/Brown with 2-3%)
Porosity: Quite low
Weather Impact: Resistant
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.
Textures of Marbles
Marbles show variety of textures on account of existing minerals & re-crystallization patterns. Texture depends upon form, size and uniformity of grain arrangements. Marbles can be classified on the basis of the following factors :-
Calcite Marble - Mostly CaCo3; MgCo3<0.50%
Dolomite Marble - Having > 40% MgCo3
Magnesium Marble - MgCo3 between 5 to 40%
Serpentine Marble – remobilized marble due to the effect of Thermodynamic metamorphic wherein serpentine is prominent
Onyx Marble - Lime carbonate deposition on account of cold water solution activity
Impurities in Marble
The following are the major mineral impurities in marble:
Quartz, Tremolite Actinolite, Chert, Garnet, Biotite, Muscovite, Microcline, Talc, Fosterite.
The following are the major chemical impurities in marble:
SiO2, Fe2O3, 2Fe2O3, 3H2O, Limonite, Manganese, Al2O3, FeS2 (pyrite)
On account of the mineral composition of marble the color variations