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The scientific study of rocks is called petrology, which is an essential component of geology. Rocks are composed of grains of minerals, which are homogeneous solids formed from a chemical compound arranged in an orderly manner. The aggregate minerals forming the rock are held together by chemical bonds. The types and abundance of minerals in a rock are determined by the manner in which it was formed. Most rocks contain silicate minerals, compounds that include silicon oxide tetrahedra in their crystal lattice, and account for about one-third of all known mineral species and about 95% of the earth’s crust. The proportion of silica in rocks and minerals is a major factor in determining their names and properties.