Limestone

Limestone, a sedimentary rock consisting largely of calcium carbonate (CaCO₃), usually in the form of the mineral calcite. It may be produced biologically, chemically, or physically. Most of the world’s ocean floors contain limestone that formed from the shells of dead marine organisms (such as foraminifera) that drifted downwards through the water and settled on the sea floor. A particular form of biological limestone, generated mainly in shallow waters, is chalk, but not all limestone is chalk. Coquina and oolite are also organic forms of limestone. However, limestone may also be produced chemically, being forced to precipitate out from saturated seawater that can dissolve no more carbonate. In rarer instances it may also be produced physically, by the deposition of pre-existing limestone particles that have been washed down by rivers, although rivers would probably dissolve much of the limestone that entered them.

Limestone may contain a small percentage of the calcium-magnesium carbonate mineral dolomite, CaMg(CO₃)₂, and still be called a limestone, or sometimes dolomitic limestone. Moreover, unlike chalk, a particularly pure form of limestone, “limestone” may contain significant amounts of non-carbonate material such as silica, feldspar, clay, or pyrite.

When heated to a high temperature in a furnace, limestone is converted to lime (calcium oxide, CaO), one of the chief uses of which is as a fertilizer. However, limestone is also useful in its own right, for example, as Portland stone, in building. Metamorphosed limestone is known as marble, and is used for building and as an ornamental stone. However, not all the so-called marble used by builders is of true limestone origin. See also Limestone Features.