Silicate << SIHL uh kiht or SIHL uh kayt >> is any of a group of minerals that contain silicon, oxygen, and one or more metallic elements. Silicates make up about 95 percent of Earth’s crust. Soil consists chiefly of silicates, as do most rocks.
All silicates have a crystal structure composed of units called silicon-oxygen tetrahedrons. Each of these units consists of one silicon ion surrounded by four oxygen ions, which form a pyramidlike figure with four triangular faces—that is, a tetrahedron. Such units may exist independently. But they also may be linked with others, forming more complex structures. These structures, in turn, may be loosely held together by cations (positively charged atoms) of aluminum, iron, or other metals present in a given silicate mineral.
Silicates are classified according to the way their silicon-oxygen units are arranged. An independent tetrahedral silicate consists of isolated groups of silicon-oxygen tetrahedrons held together by metal cations. Olivine is a mineral of this type. A double tetrahedral silicate is composed of two tetrahedral groups. The mineral epidote contains such double tetrahedrons. A ring silicate consists primarily of rings of three or six tetrahedrons. The mineral beryl is an example of a six-membered ring silicate. A chain silicate has numerous tetrahedrons linked together in either single or double chains. Pyroxenes have single chains. Amphiboles have double chains. A sheet silicate, such as mica, consists of sheets of tetrahedrons with metal cations sandwiched between them. A framework silicate is comprised of tetrahedrons linked in three-dimensional networks that extend in all directions. Feldspar and quartz, the most abundant minerals in Earth’s crust, belong to the framework silicate group.
