Marble is a highly valued rock known for its strength, aesthetics, ability to be polished and resistance to most weathering. It has a variety of uses and has been used by current or ancient civilizations for building applications and statues. This material comes in a variety of textures depending on the composition and how it is formed. However, what exactly is marble formed and what is its composition?
Marble is type of metamorphic rock that forms from limestone, dolomite, or older marble under certain conditions. These conditions are heat and pressure over a period of time inside the earth’s crust. The application of heat and pressure force the limestone to change. In a process called recrystallization, the limestone is altered to form coarse grained calcite. The composition of the resulting marble will be affected by the different impurities that may be present in the limestone before recrystallization takes place.
Depending on the temperature and silica impurities, quartz, diopside or fosterite may be formed in the calcite. If water is present during the formation then talc, serpentine and a variety of other minerals may form. Should iron, alumina, and silica be present during the recrystallization, then this could result in the formation of magnetite and hematite.
Marble is composed primarily of calcite, dolotimite, or perhaps serpentine and other similar minerals. The exact chemical composition of marble will greatly vary dependong on the location and the minerals or impurities present in the limestone during recrystallization. Typically, marble is composed of the following major constituents: 38-42% Lime (CaO), 20-25% Silica (SiO2), 2-4% Alumina (Al2O3), 1.5-2.5% various oxides (NaO and MgO), and 30-32% various carbonates (MgCO3 and others).
In addition to the major constituents, marble can have many different mineral impurities of various percentages. These include: Chert, Garnet, Hematite, Mircroline, Talc, Fosterite, Muscovite, Biotite, Termolite Actinolite, and Quartz. The presence of some of these impurities are responsible for giving marble its color. Very pure calcite marble will always be white in color so if few impurities are present in the marble then it will typically be white. Impurities of hematite can give marble a reddish color. Limonite will give marble a yellow color and serpentine will give marble a green color.
While marble is a strong material, there are some conditions that it cannot stand. Unlike granite, marble can’t take chemical weathering such as acid rain or any type of acid. The marble will begin breaking down as a result of the acid. It is also not as easily mined as granite as it is difficult to cut into large sheets.
A type of synthetic marble can also be used for many different applications and is cheaper than the original marble. This kind of material is typically called ‘cultured’ marble and is created by taking crushed marble or marble dust and adding it to a fiberglass resin. The resulting material can then be molded into just about any shape and will resemble marble as well as take a polish.