In this passage we will discuss physical deformation and chemical decomposition of metamorphic marbles. Do you know the expression metamorphic marble? If you don’t, stay with us.
The term marble originally comes from the Greek word marmaros. Greek word marmaros means shinning stone. There are hundreds of different types of marble, each with different cutting and shaping properties. This stone is very popular and is commonly used in architecture and for decorative carvings and statuary.
Not all marble is pure white or pure black. Many forms have swirls or veins of color. Pietra Grey is one the most appealing famous marble types with striking white veins. To get more information on this stone, go to the previous articles on Niayesh Stone blog or contact us.
Marble’s reputation as an architectural material has been known since Augustus, the first Emperor of Rome, covered the public buildings there in it. However, marble doesn’t handle Midwestern winters well, so they soon had to change it to white granite.
As said in previous article, marble, granular limestone or dolomite is rock composed of calcium-magnesium carbonate that 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.
Petrographically, marbles are massive rather than thin-layered and consist of a mosaic of calcite grains that rarely show any traces of crystalline form under the microscope. They are traversed by minute cracks that accord with the rhombohedral cleavage (planes of fracture that intersect to yield rhombic forms) of calcite. In the more severely deformed rocks, the grains show stripes and may be elongated in a particular direction or even crushed.
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, talc, vesuvianite, periclase, spinel, forsterite, brucite, wollastonite, zoisite, chlorite, tourmaline, chondrodite, epidote, biotite, titanite, and apatite are all possible accessory minerals. Pyrrhotite, sphalerite, and chalcopyrite also may be present in small amounts.
These minerals represent impurities in the original limestone, which reacted during metamorphism to form new compounds. Alumina represents an admixture of clay. The silicates derive their silica from quartz and from clay. Iron came from limonite, hematite, or pyrite in the original sedimentary rock. In some cases the original bedding of the calcareous sediments can be detected by mineral banding in the marble.
The silicate minerals may color the marble. Bands of calc-silicate rock may alternate with bands of marble or form nodules and patches, sometimes producing interesting decorative effects, but these rocks are particularly difficult to finish because of the great difference in hardness between the silicates and carbonate minerals.
Later physical deformation and chemical decomposition of the metamorphic marbles often produces attractive colored and variegated varieties. Decomposition yields hematite, brown limonite, pale-green talc, and, in particular, the green or yellow serpentine derived from forsterite and diopside, which is characteristic of the ophicalcites or verd antiques.
Movements of earth may shatter the rocks, producing fissures that are afterward filled with veins of calcite; in this way the beautiful brecciated, or veined, marbles are produced. Sometimes the broken fragments are rolled and rounded by the flow of marble under pressure.
The so-called onyx marbles consist of concentric zones of calcite or aragonite. This is deposited from cold-water solutions in caves and crevices and around the exits of springs. They are, in the strict sense, neither marble nor onyx, for true onyx is a banded chalcedony composed largely of silicon dioxide.
Onyx marble was the alabaster of the ancients, but alabaster is now defined as gypsum, a calcium sulfate rock. These marbles are usually brown or yellow because of the presence of iron oxide.
Well-known examples include the giallo antico (antique yellow marble) of the Italian antiquaries, the reddish-mottled Siena marble from Tuscany, the large Mexican deposits at Tecali near Mexico City and at El Marmol, California, and the Algerian onyx marble used in the buildings of Carthage and Rome and rediscovered near Oued-Abdallah in 1849.
Unmetamorphosed limestones showing interesting color contrasts or fossil remains are used extensively for architectural purposes. The Paleozoic rocks from 251 million to 542 million years in age of Great Britain, for example, include madrepore marbles rich in fossil corals and encrinital marble containing.
The shelly limestones of the Purbeck Beds, England, and the Sussex marble, both of Mesozoic Era, consist of masses of shells of freshwater snails embedded in blue, gray, or greenish limestone.
They were a favorite material of medieval architects and may be seen in Westminster Abbey and a number of English cathedrals. Black limestones are containing bituminous matter. They commonly emit a fetid odor when struck, are widely used; the well-known petit granit of Belgium is a black marble containing crinoid stem plates.
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