Sedimentary rocks are first classified not by their elemental composition but rather how they are formed. The first type is clastic sedimentary rock, which is any rock formed by particles being cemented together. The most common example of this is sandstone, which is made from tiny particles of sand. Larger particles like gravel can come together to form breccia or conglomerate. Smaller particles like clay can come together to form shale. Mineral cements act as the glue in these rocks when they form, or lithify.
Sometimes particles of other rock will dissolve completely in water and be carried away in solution. When these chemicals settle and the water evaporates, or the chemicals precipitate from solution and lithify underground, they can create chemical sedimentary rocks. One common example of chemical sedimentary rock is halite, also known as rock salt, formed from dissolved salt. Another common example is limestone, which forms when calcium carbonate precipitates out of solution. The main difference between chemical sedimentary rocks and clastic sedimentary rocks is that chemical rocks form from dissolved substances while clastic rocks form from particles that are cemented together.
Sometimes sedimentary rocks form from substances that are not even rocks. Organic sedimentary rocks, which are also called biochemical sedimentary rocks, form from the accumulation of plant and animal debris. Over time, the dead animals and plants form rock, often concentrating their organic elements. The most common example of this is coal, which is formed from plants that died millions of years ago. Because of the high organic components to these rocks, they are sought for fuel and farming purposes.
Even though sedimentary rocks can be formed from a variety of sediments, most of them are made from only a few minerals. Most sandstone and other clastic rocks are made from common igneous rocks and minerals like quartz, feldspar, mica, hornblende, actinolite, tremolite, olivine, calcite, dolomite and chlorite. Chemical sedimentary rocks include calcium carbonate for limestone and various dissolved salts. Organic sedimentary rocks do not contain much pre-existing rocks but rather are formed from hydrocarbon chains from living plants and animals. Some organic sedimentary rocks on the ocean floor and stream beds contain high levels of calcium from the calcium carbonate shells of dead sea creatures.