Limestone is a common sedimentary rock composed primarily of the calcium carbonate mineral, calcite (CaCO3). Limestone constitutes approximately 10 percent of the sedimentary rocks exposed on the earth’s surface.
Limestone is formed either by direct crystallization from water (usually seawater), or by the accumulation of sea animal shells and shell fragments. In the direct crystallization case calcium ions in the seawater combine with atmospheric or dissolved carbon dioxide to form calcium carbonate, which being insoluble, precipitates out. Over time, layers of the calcium carbonate form, and with sufficient time and pressure from overlying materials, are transformed to solid rock. The chemical composition of seawater and its minerals contaminants will be recorded in the limestone impurities. Limestone frequently also contains the minerals dolomite or calcium magnesium carbonate (CaMg(CO3)2, and a second crystalline form of calcium carbonate, aragonite.
In the seas that covered much of the earth in earlier geological times lived many clams and other shelled forms of animal life. When the animals died, their shells, which are composed of calcium carbonate, fell to the bottom. Thick layers, which can be a mile or 2 kilometers deep, gradually were changed into rock. If the shells were only compressed, with little crystallization occurring, soft chalk results. The White Cliffs of Dover in England are made of this type of chalk. With more time and pressure, a coarsely crystallized rock can form, which is limestone. In limestone, most traces of the animal shells have disappeared.
Pure calcite, dolomite, and aragonite which are in the limestones, are clear or white. However, with impurities, they can take on a variety of colors, commonly white, tan or gray. Limestone can also have gone through metamorphosis, with high pressure, heat and time creating marble. This process tends to intensify the color of the stone as well as changing the nature of some of the impurities. Metamorphosis creates the beautiful marbles seen in architectural applications.
Uses For Limestone
Limestone and marble are important building materials in most parts of the world. They are used as cut dimension stone (blocks and slabs) for building. In the form of coarsely crushed stone or aggregate limestone is used for general building purposes, roadbeds and as a component in concrete. When heated, the calcium carbonate in limestone decomposes to lime, or calcium oxide, which also has many industrial applications, including the manufacture of precipitated calcium carbonate (PCC). Click on the links below to learn more about lime and precipitated calcium carbonate.
Either limestone or marble may be used as the basis for crushed or ground calcium carbonate. Ground calcium carbonate, commonly referred to as GCC, is very widely used as an industrial mineral. Three primary attributes: particle size, color and chemical purity define the quality of the GCC, and define the suitability of use for any given application.
Ground limestones are available in a broad range of sizes. When specifying the size of a ground calcium carbonate, either the top size (the size of the largest particles) or the median particle size (essentially the average) can be used. Commercially, top size is generally specified for coarser industrial minerals which may be as big as beach sand, which is 10 mesh or 2 millimeters or approximately 0.1 inch in size, or as fine as flour; which is 200 mesh, or 75 microns. Finely ground calcium carbonates can range down in particle size to just a few microns, 1 to 3, with the top size controlled for high performance applications to as fine as 5 to 10 microns.
Limestone color can range from brilliant white to light gray and chemical purity can range from only 80-90% calcium carbonate to well over 99.9% purity.
Calcium carbonate is not a particularly hard mineral, with pure calcite falling at about 3 on the Mohs scale of 1 (talc) to 10 (diamond). Silica impurities in the limestone can raise the hardness of some ground calcium carbonates to a Mohs value of 4. Calcium carbonate is subject to attack by acidic environments, which has led to the damage of limestone buildings and monuments by acid rain. For industrial uses, the effect of an acid can be somewhat reduced by treating the ground limestone powder with fatty acids to put a non-polar, organic material on the surface. Few industrial minerals can match GCC for its combination of properties and cost.