Marble is a non-foliated metamorphic rock that forms when limestone is subjected to heat and pressure of metamorphism. Under the conditions of metamorphism, the calcite (CaCO3) in the limestone recrystallizes to form a rock that is a mass of interlocking calcite crystals.
Marble is primarily composed of the mineral calcite (CaCO3) and 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. Marble is often the host rock for corundum, spinel, and other gem minerals. This is why marble is often used as decorative stone in designing plush and rich interiors.
Marble ores are formed from limestone with heat and pressure over years in the earth’s crust. These pressure or forces cause the limestone to change in texture and make up. The process is called recrystallization. Fossilized materials in the limestone, along with its original carbonate minerals, recrystallize and form large, coarse grains of calcite.
Geologically, location of marble in Earth’s crust occurs usually at convergent plate boundaries where large areas of Earth’s crust are exposed to regional metamorphism.
Impurities present in the limestone during the recrystallization period affect the mineral and colour composition of the marble which is formed. At relatively low temperatures, silica impurities in the carbonate minerals form masses of chert or crystals of quartz. At higher temperatures, the silica reacts with the carbonates to produce diopside and forsterite. At a very high temperatures, rarer calcium minerals, such as larnite, monticellite, and rankinite, forms in the marble. If water is present, serpentine, talc, and certain other hydrous minerals may be produced. The presence of iron, alumina, and silica may result in the formation of hematite and magnetite.
The minerals that result from impurities give marble a wide variety of colours. The purest calcite marble is white in colour. Marble containing hematite are reddish in color. Marble that has limonite is yellow, and marble with serpentine is green in colour.
Marble does not split easily into sheets of equal size and must be mined with care. Blocks of marble are mined with channelling machines, which cut grooves and holes in the rock. Miners outline a block of marble with rows of grooves and holes. They then drive wedges into the openings and separate the block from the surrounding rock. The blocks are cut with saws to the desired shape and size.
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