LIMESTONE

Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3). Limestone 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. These organisms secrete shells that settle out of the water column and are deposited on ocean floors as pelagic ooze or alternatively is conglomerated in a coral reef. Secondary calcite may also be deposited by supersaturated meteoric waters (groundwater that precipitates the material in caves). 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. Limestone makes up about 10% of the total volume of all sedimentary rocks.

Limestone is especially popular in architecture, and many landmarks around the world, especially in North America and Europe, are made primarily of the material. So many buildings in Kingston, Ontario, Canada were constructed from it that it is nicknamed the 'Limestone City'. 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.

Limestone was most popular in the early 20th and late 19th centuries. Train stations, banks and other structures from that era are normally made of limestone. Limestone was also a very popular building block in the Middle Ages in the areas where it occurred since it is hard, durable, and commonly occurs in easily accessible surface exposures. Many medieval churches and castles in Europe are made of limestone.
Limestone and marble are very reactive to acid solutions, making acid rain a significant problem. Many limestone statues and building surfaces have suffered severe damage due to acid rain. Acid-based cleaning chemicals can also etch limestone, which should only be cleaned with a neutral or mild alkaline-based cleaner.