Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of the mineral calcite. It most commonly forms in clear, warm, shallow marine waters. It is usually an organic sedimentary rock that forms from the accumulation of shell, coral, algal and fecal debris. It can also be a chemical sedimentary rock formed by the precipitation of calcium carbonate from lake or ocean water.
Limestone is by definition a rock that contains at least 50% calcium carbonate in the form of calcite by weight. All limestones contain at least a few percent other materials. These can be small particles of quartz, feldspar, clay minerals, pyrite, siderite and other minerals. It can also contain large nodules of chert, pyrite or siderite.
The calcium carbonate content of limestone gives it a property that is often used in rock identification – it effervesces in contact with a cold solution of 5% hydrochloric acid.
Varieties of Limestone
There are many different names used for limestone. These names are based upon how the rock formed, its appearance or its composition and other factors. Here are some of the more commonly used.
- Chalk: A soft limestone with a very fine texture that is usually white or light gray in color. It is formed mainly from the calcareous shell remains of microscopic marine organisms such as foraminifers or the calcareous remains from numerous types of marine algae.
- Coquina: A poorly-cemented limestone that is composed mainly of broken shell debris. It often forms on beaches where wave action segregates shell fragments of similar size.
- Fossiliferous Limestone: A limestone that contains obvious and abundant fossils. These are normally shell and skeletal fossils of the organisms that produced the limestone.
- Lithographic Limestone: A dense limestone with a very fine and very uniform grain size that occurs in thin beds that separate easily to form a very smooth surface. In the late 1700’s a printing process (lithography) was developed to reproduce images by drawing them on the stone with an oil-based ink and then using that stone to press multiple copies of the image.
- Oolitic Limestone: A limestone composed mainly of calcium carbonate “oolites”, small spheres formed by the concentric precipitation of calcium carbonate on a sand grain or shell fragment.
- Travertine: A limestone that forms by evaporative precipitation, often in a cave, to produce formations such as stalactites, stalagmites and flowstone.
- Tufa: A limestone produced by precipitation of calcium-laden waters at a hot spring, lake shore or other location.
Uses of Limestone
Limestone is a rock with an enormous diversity of uses. It could be the one rock that is used in more ways than any other. Most limestone is made into crushed stone and used as a construction material. It is used as a crushed stone for road base and railroad ballast. It is used as an aggregate in concrete. It is fired in a kiln with crushed shale to make cement.
Some varieties of limestone perform well in these uses because they are strong, dense rocks with few pore spaces. These properties enable them to stand up well to abrasion and freeze-thaw. Although limestone does not perform as well in these uses as some of the harder silicate rocks it is much easier to mine and does not exert the same level of wear on mining equipment, crushers, screens and the beds of the vehicles that transport it.
Some additional but also important uses of limestone include:
- Dimension Stone: Limestone is often cut into blocks and slabs of specific dimensions for use in construction and in architecture. It is used for facing stone, floor tiles, stair treads, window sills and many other purposes.
- Roofing Granules: Crushed to a fine particle size, crushed limestone is used as a weather and heat-resistant coating on asphalt impregnated shingles and roofing. It is also used as a top coat on built-up roofs.
- Flux Stone: Crushed limestone is used in smelting and other metal refining processes. In the heat of smelting, limestone combines with impurities and can be removed from the process as a slag.
- Portland Cement: Limestone is heated in a kiln with shale, sand and other materials and ground to a powder that will harden after being mixed with water.
- AgLime: Calcium carbonate is one of the most cost-effective acid neutralizing agents. When crushed to sand-size or smaller particles limestone becomes an effective material for treating acidic soils. It is widely used on farms throughout the world.
- Lime: If calcium carbonate (CaC03 is heated to high temperature in a kiln the products will be a release of carbon dioxide gas (CO2) and calcium oxide (CaO). The calcium oxide is a powerful acid neutralization agent. It is widely used as a soil treatment agent (faster acting than aglime) in agriculture and as an acid neutralization agent by the chemical industry.