Previously, we talked about Pietra Grey Marble and its’ wide vast range of grey marble applications in various projects. In this paper, we will get deeper into marble features and characteristics.
THESAURUS dictionary defines marble as metamorphosed limestone, consisting chiefly of recrystallized calcite or dolomite. This occurs in a wide range of colors and variegations and used in sculpture and architecture. Marble is capable of taking a high polish.
Britannica, dictionary defines marble as granular limestone or dolomite that is rock composed of calcium-magnesium carbonate. It has been recrystallized under the influence of heat, pressure, and aqueous solutions. Commercially, it includes all decorative calcium-rich rocks that can be polished, as well as certain serpentines and antiques.
Therefore, as many dictionaries and sources defined it, marble is a metamorphic rock. It is composed of recrystallized carbonate minerals. Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means change in form.
The original rock called protolith is subjected to heat. By heat we mean temperatures greater than 150 to 200 °C and pressure 100 megapascals (1,000 bar) or more, causing profound physical or chemical change.
The protolith may be a sedimentary, igneous, or existing metamorphic rock. Metamorphic rocks make up a large part of the Earth’s crust and form 12% of the Earth’s land surface. They are classified by texture and by chemical and mineral assemblage or metamorphic faces.
Most of the white and gray marbles of Alabama, Georgia, and Western New England, and that from Yule, Colorado, are recrystallized rocks. Furthermore, a number of Greek and Italian statuary marbles famous from antiquity are recrystallized rocks, which are still being quarried.
These include the Parian marble, the Pentelic marble of Attica in which Phidias, Praxiteles, and other Greek sculptors executed their principal works, and the snow-white Carrara marble used by Michelangelo and Antonio Canova and favored by modern sculptors.
The exterior of the National Gallery of Art in Washington, D.C., is of Tennessee marble, and the Lincoln Memorial contains marbles from Yule, Colorado, Alabama (roof transparencies), and Georgia (Lincoln statue).
Marble may be formed simply by being deep beneath the Earth’s surface. Marble is subjected to high temperatures and the great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion. They are also formed when rock is heated by the intrusion of hot molten rock called magma from the Earth’s interior.
The study of metamorphic rocks provides information about the temperatures and pressures that occur at great depths within the Earth’s crust. Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.
Metamorphic minerals are those that form only at the high temperatures and pressures associated with the process of metamorphism. These minerals known as index minerals, include sillimanite, kyanite, staurolite and andalusite.
Other minerals, such as olivines, pyroxenes, amphiboles, micas, feldspars, and quartz, may be found in metamorphic rocks. But these are not necessarily the result of the process of metamorphism. These minerals formed during the crystallization of igneous rocks.
They are stable at high temperatures and pressures. Furthermore, they may remain chemically unchanged during the metamorphic process. However, all minerals are stable only within certain limits. The presence of some minerals in metamorphic rocks indicates the approximate temperatures and pressures at which they formed.
The change in the particle size of the rock during the process of metamorphism is called recrystallization. For instance, the small calcite crystals in the sedimentary rock limestone and chalk change into larger crystals in the metamorphic rock marble; in metamorphosed sandstone, recrystallization of the original quartz sand grains results in very compact quartzite, also known as metaquartzite, in which the often larger quartz crystals are interlocked.
Both high temperatures and pressures contribute to recrystallization. High temperatures allow the atoms and ions in solid crystals to migrate, thus reorganizing the crystals, while high pressures cause solution of the crystals within the rock at their point of contact.
Even the purest of the metamorphic marbles contain some accessory minerals, which, in many cases, form a considerable proportion of the mass. The commonest are quartz in small rounded grains, scales of colorless or pale-yellow mica, and dark shining flakes of graphite, iron oxides, and small crystals of pyrite.
Many marbles contain other minerals that are usually silicates of lime or magnesia. Diopside is very frequent and may be white or pale green; white bladed tremolite and pale-green actinolite also occur; the feldspar encountered may be a potassium variety but is more commonly a plagioclase (sodium-rich to calcium-rich) such as albite, labradorite, or anorthite.
Scapolite, various kinds of garnet, vesuvianite, spinel, forsterite, periclase, brucite, talc, zoisite, wollastonite, chlorite, tourmaline, epidote, chondrodite, biotite, titanite, and apatite are all possible accessory minerals. Pyrrhotite, sphalerite, and chalcopyrite also may be present in small amounts. Stay with us on Niayesh Stone blog. Many other aspects of natural stone, and especially Pietra Grey Marble are to be discussed.
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