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Chemical Rock The water in seas, lakes, swamps, and underground reservoirs often contains dissolved minerals. Chemical sediments form when these minerals precipitate, or fall out of solution. Precipitation can occur through evaporation or through chemical action—as, for example, when dissolved ions combine to form new minerals. Common chemical sedimentary rocks are rock salt, rock gypsum, and some limestones. Rock salt, or halite, occurs in thick layers in many parts of the world. Rock gypsum occurs in layers or as nearly pure veins of the mineral gypsum. Although they are uncommon, limestones of chemical origin may form when tiny grains of calcite are deposited on the bottoms of seas or lakes. These limestones are often gray or tan, compact and dense, and smooth to the touch. Image not available. Please refer to the image in the textbook or in the eEdition CD. ROCK SALT FLAT at Bonneville, Utah Image not available. Please refer to the image in the textbook or in the eEdition CD. ORGANIC LIMESTONE CLIFFS in Dover, England Organic Rock An organic sedimentary rock forms from sediments consisting of the remains of plants and animals. Common organic sedimentary rocks are limestone and coal. Although coal consists primarily of carbon, it is formed from the fossilized remains of plants. The formation of coal is discussed in more detail in Chapter 7. Organically formed limestones contain the mineral calcite. Limestone formation begins when water dissolves calcite out of rocks on land and carries it in the form of calcium ions to an ocean or lake. There, certain organisms use the ions to produce calcium carbonate shells or other support structures. Clams, corals, and some algae are just a few of these organisms, many of which live in shallow water near ocean shores. When the organisms die, their calcium-rich remains pile up on the ocean floor. The type of sedimentary rock formed from these sediments depends on the type of structures and on the environment in which it forms. For example, the rock coquina develops when masses of whole or mostly whole shells are cemented together by minerals in ocean water. More often, waves break the shells into fragments. In time these fragments may become cemented into limestones. Limestones that form near shore may contain large amounts of clay. Those that form farther from shore may be almost pure calcite. Chapter 6 Rocks 129