The Description
Limestone often contains variable amounts of silica in the form of chert or siliceous skeletal fragment, as well as varying amounts of clay, silt and sand sized terrestrial detritus carried in by rivers. The primary source of the calcite in limestone is most commonly marine organisms. These organisms secrete shells made of aragonite or calcite and leave these shells behind after the organism dies. Some of these organisms can construct mounds of rock known as reefs, building upon past generations. Below about 3,000 meters, water pressure and temperature causes the dissolution of calcite to increase non-linearly so that limestone typically does not form in deeper waters. Secondary calcite may also be deposited by supersaturated meteoric waters. This produces speleothems such as stalagmites and stalactites. Another form taken by calcite is that of oolites (oolitic limestone) which can be recognized by its granular appearance.
Coastal limestones are often eroded by organisms which bore into the rock by various means. This process is known as bioerosion. It is most common in the tropics, and it is known throughout the fossil record.
Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestones 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 limestones more precisely.
During regional metamorphism that occurs during the mountain building process limestone recrystallizes into marble.
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