Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3Limestone often contains variable amounts of silica in the form of chert or flint, as well as varying amounts of clay, silt and sand as disseminations, nodules, or layers within the rock. The primary source of the calcite in limestone is most commonly marine organisms. Pure limestone is almost white. Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestone exhibit different colors, especially on weathered surfaces. Limestone may be crystalline, clastic, granular, or massive, depending on the method of formation. Crystals of calcite, quartz, dolomite or barite may line small cavities in the rock. Folk and Dunham classifications are used to describe limestone more precisely.
Limestone contains a number of distinguishable natural characteristics, including calcite streaks or spots, fossils or shell formations, pit holes, reedy formations, open texture streaks, honeycomb formations, iron spots , travertine-like formations and grain formation changes. One or a combination of these characteristics will affect the texture.
Classification
LOW DENSITY - Limestone having a density ranging from 110 through 135/lb/ft3 (1760 through 2160 kg/m3)..
MEDIUM DENSITY - Limestone having a density greater that 135 and not greater than 160 lb/ft3 (2160 through 2560 kg/m3).
HIGH DENSITY - Limestone having a density greater than 160 lb/ft3 (2560 kg/m3).
Uses of limestone
Limestone is especially popular in architecture, and many landmarks around the world, especially in North America, Europe, and the pyramids in Egypt are made of limestone. Limestone is readily available and relatively easy to cut into blocks or more elaborate carving. It is also long-lasting and stands up well to exposure. However, it is a very heavy material, making it impractical for tall buildings, and relatively expensive as a building material.
Limestone is used as a facade on some skyscrapers, but only in thin plates for covering rather than solid blocks. In the United States, Indiana, most notably the Bloomington area, has long been a source of high quality quarried limestone, called Indiana limestone. Many famous buildings in London are built from Portland limestone.
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