Limestone is a sedimentary rock with several variations in formation.
Although rocks are nearly everywhere and therefore seem commonplace and even dull, they are the textbooks of the planet. Rocks hold the key to the formation of landforms, give us clues to our past and help to date the major environmental events of Earth. They are broken into three classifying groups based on their formation, and each of these groups is further broken down. For instance, fossiliferous limestone is one variety of limestone, which is one variety of carbonate, which is one variety of chemical sedimentary rock, which is one variety of sedimentary rock, which is one of the three main classification groups. A closer look will help explain what exactly fossiliferous limestone is and how it fits into the rock classification system. Does this Spark an idea?
With a few exceptions, rocks are naturally occurring solids that are made up of minerals or other mineral-like matter. Exceptions to this include substances such as mercury, coquina and coal, which are classified as rocks but don’t meet all the criteria. Depending on how they are formed, rocks are classified as igneous, metamorphic or sedimentary, and then further categorized according to chemical or mineral content, texture and grain size. Igneous rock is formed when magma cools and crystallizes. Metamorphic rocks are those that are changed by heat and pressure. Marble, for instance, is limestone that has been changed by heat.
Sedimentary rocks are formed by three different processes, and are classified accordingly as clastic, chemical and organic. Clastic sedimentary rocks are formed when sediments from other rocks weather and are deposited other places by wind, gravity or water. The sediments settle according to grain size (larger grains settle first; smallest grains settle last) and are turned into new rocks through one of two methods of lithification. Weight either compacts the sediment particles, or dissolved minerals cement particles together. Chemical sedimentary rocks are formed from chemicals dissolved in water. They are classified as evaporites, carbonates or siliceous rocks. Evaporites, such as rock salt or gypsum, are formed when water evaporates and becomes saturated with chemicals. When the water can no longer hold the elements in solution, they crystallize and form rocks. Carbonates, such as limestone and dolostone, are formed by chemical and biochemical processes, and siliceous rocks are silica-based rocks formed by silica-secreting organisms. Organic sedimentary rocks, such as coal, are made up of organic plant fragments. Coal is categorized as either lignite or bituminous.
Chemical sedimentary rocks occur when dissolved minerals are separated (precipitated) from water, such as when water evaporates. White deposits left on kitchen or bathroom faucets after water drains are a visible example of the process. Some examples of chemical sedimentary rocks are crystalline or fossiliferous limestone, chalk, chert, gypsum, rock salt and bituminous coal.
Limestone is a chemical sedimentary rock that is at least 50 percent calcium carbonate in the form of the mineral calcite. Limestone may also contain small particles of other materials, such as quartz, feldspar, clay minerals, pyrite and siderite, and it may contain larger nodules of chert, pyrite or siderite. Different types of limestone are classified based on the texture, mineral content, origin and geological age, but all limestone is part or wholly organic and may contain fossilized shells and plants. If it does, it is called fossiliferous limestone.
Where Limestone Forms
Limestone usually forms in areas where organisms with calcium carbonate shells and skeletons live, which is generally shallow, calm, warm marine waters such as the Bahamas Platform, the Caribbean Sean, the Indian Ocean, Persian Gulf, Gulf of Mexico, around the Pacific Ocean islands and within the Indonesian archipelago. When the animals die, their shells and bones layer on the ocean ground and are lithified into rock. Limestone can also form by precipitation, as it does in caves to form stalactites, stalagmites or flowstone. This type of limestone is called travertine.
Limestone forms in many varieties which differ in hardness and composition, and so have different uses. Most limestone is crushed and used as construction material, either for road base or railroad ballast. It is also used in concrete, or combined with shale to make cement. Limestone can also be cut into slabs or blocks and be used in architecture as facing stone, floor tiles, window sills and other places. When crushed fine, limestone is a heat- and weather-resistant coating that is very good on shingles and roofing, and because limestone is primarily calcium carbonate, it also has uses based on those attributes. For instance, the use of calcium carbonate as a low-cost agent that can neutralize acids is seen on farms around the world and in the chemical industry. Limestone powder is also used as animal filler to help milking cows replenish their calcium levels or chickens to lay stronger eggs. The powder is also used as a filler in paper, paint, rubber and plastics and as a pollutant absorber at coal-burning facilities.