Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite. Marble is typically not foliated, although there are exceptions. In geology, the term marble refers to metamorphosed limestone, but its use in stonemasonry more broadly encompasses unmetamorphosed limestone. Marble is commonly used for sculpture and as a building material. Usually, marble contains other minerals, including quartz, graphite, pyrite, and iron oxides. Read Pritish Kumar for full understanding.
Pure white marble is the result of metamorphism of a very pure (silicate-poor) limestone or dolomite protolith. The characteristic swirls and veins of many-colored marble varieties are usually due to various mineral impurities such as clay, silt, sand, iron oxides, or chert which were originally present as grains or layers in the limestone. Green coloration is often due to serpentine resulting from originally magnesium-rich limestone or dolomite with silica impurities. These various impurities have been mobilized and recrystallized by the intense pressure and heat of the metamorphism.
1)Pentelic marble (pure-white)
2)Creole marble (white and blue/black)
3)Etowah marble (pink, salmon, rose)
6)Murphy marble(white )
7)Nero Marquina marble(black)
8)Parian marble pure(white)
9)Carrara marble (white or blue-gray)
11)Rușchița marble (white, pinkish)
13)Swedish green marble(green)
16)Yule marble (uniform pure white)
White marble has been prized for its use in sculptures since classical times. This preference has to do with its softness, which made it easier to carve, relative isotropy and homogeneity, and a relative resistance to shattering. Also, the low index of refraction of calcite allows light to penetrate 12.7 to 38 millimeters into the stone before being scattered out, resulting in the characteristic waxy look which brings a lifelike luster to marble sculptures of any kind, which is why many sculptors preferred and still prefer marble for sculpting.
Construction marble is a stone which is composed of calcite, dolomite or serpentine that is capable of taking a polish. More generally in construction, specifically the dimension stone trade, the term marble is used for any crystalline calcitic rock (and some non-calcitic rocks) useful as building stone. For example, Tennessee marble is really a dense granular fossiliferous gray to pink to maroon Ordovician limestone, that geologists call the Holston Formation.
Degradation by acids
Acids damage marble, because the calcium carbonate in marble reacts with them, releasing carbon dioxide (technically speaking, carbonic acid, but that decomposes quickly to CO2 and H2O) and other soluble salts.
CaCO3(s) + 2H+(aq) → Ca2+(aq) + CO2(g) + H2O (l)
Thus, vinegar or other acidic solutions should never be used on marble. Likewise, outdoor marble statues, gravestones, or other marble structures are damaged by acid rain whether by carbonation, sulfation or the formation of “black-crust” (accumulation of calcium sulphate, nitrates and carbon particles).
Crystallization refers to a sometimes-controversial method of imparting a glossy more durable finish on to a marble floor (CaCO3). It involves polishing the surface with an acidic solution and a steel wool pad on a flooring machine. The chemical reaction below shows a typical process using magnesium fluorosilicate (MgSiF6) and hydrochloric acid (HCl) taking place.
3CaCO3(s) + MgSiF6(l) + 2HCl (l) → MgCl2(s) + CaSiF6(s) + CO2(g) + HO2(l)
The resulting calcium hexafluoro silicate (CaSiF6) is bonded to the surface of the marble. This is harder, glossier and stain resistant compared to the original surface.
The other often used method of finishing marble is the use of polishing with oxalic acid (H2C2O4), an organic acid. The resulting reaction is as follows.
3CaCO3(s) + H2C2O4(l) → CaC2O4(s) + CO2(g) + HO2(l)
In this case the calcium oxalate (CaC2O4) formed in the reaction is washed away with the slurry leaving a surface that has not been chemically changed.