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Sedimentary RocksSedimentary rocks are formed from pieces of other existing rock or organic material. There are three different types of sedimentary rocks: clastic, organic (biological), and chemical. Clastic sedimentary rocks, like sandstone, form from clasts, or pieces of other rock. Organic sedimentary rocks, like coal, form from hard, biological materials like plants, shells, and bones that are compressed into rock. The formation of clastic and organic rocks begins with the weathering or breaking down, of the exposed rock into small fragments. Through the process of erosion, these fragments are removed from their source and transported by wind, water, ice, or biological activity to a new location. Once the sediment settles somewhere, and enough of it collects, the lowest layers become compacted so tightly that they form solid rock. Chemical sedimentary rocks, like limestone, halite, and flint, form from chemical precipitation. A chemical precipitate is a chemical compound—for instance, calcium carbonate, salt, and silica—that forms when the solution it is dissolved in, usually water, evaporates and leaves the compound behind. This occurs as water travels through Earth’s crust, weathering the rock and dissolving some of its minerals, transporting it elsewhere. These dissolved minerals are precipitated when the water evaporates. Metamorphic RocksMetamorphic rocks are rocks that have been changed from their original form by immense heat or pressure. Metamorphic rocks have two classes: foliated and non-foliated. When a rock with flat or elongated minerals is put under immense pressure, the minerals line up in layers, creating foliation. Foliation is the aligning of elongated or platy minerals, like hornblende or mica, perpendicular to the direction of pressure that is applied. An example of this transformation can be seen with granite, an igneous rock. Granite contains long and platy minerals that are not initially aligned, but when enough pressure is added