Photoglossary

Silicates   All rocks are composed of minerals, and minerals are composed of one or more chemical elements. The primary elements that make up rock-forming minerals are silicon (Si), oxygen (O), iron (Fe), magnesium (Mg), potassium (K), sodium (Na), and calcium (Ca). Various combinations of these elements form the most common mineral group, the silicates. The most important silicate minerals are quartz, feldspar, mica, amphibole, pyroxene, and olivine. All silicate minerals contain oxygen and silicon atoms, and these atoms organize into a structure called the silicon-oxygen tetrahedron. This unit is made up of one silicon atom surrounded by four oxygen atoms. The silicon-oxygen tetrahedron is the fundamental building-block of silicate minerals, and all silicates contain this structure. Ninety-two percent of the Earth's crust is composed of rocks that are made up of silicate minerals. Most of these rocks formed when molten material from the Earth's interior solidified. Rocks that form in this manner are catagorized as igneous rocks. There are many types of igneous rocks, although basalt and granite are the most familiar.     Molten rock, or magma, contains the chemical elements that form silicate minerals. Once these elements begin to form chemical bonds within the magma, crystals start to develop. Some minerals, such as pyroxene and plagioclase feldspar (which form basalt), crystallize at relatively high temperatures. Seventy percent of the Earth's surface is basalt, with most of it found on the ocean floors. Other silicate minerals, such as potassium feldspar and quartz, crystallize at lower temperatures. These minerals are the primary constituents of granitic rocks. Granite is the rock that makes up the majority of the continental crust. Granite is pictured (top) with its primary constituent silicate minerals--(left to right) plagioclase feldspar, mica, potassium feldspar, hornblend (amphibole), and quartz.   Photo Credit: Janet Wood   Illustration Credit: Lutgens, F./Tarbuck, E., FOUNDATIONS OF EARTH SCIENCE, 2/e, © 1999 Electronically reproduced by permission of Prentice Hall, Upper Saddle River, New Jersey.        Io Home Page Photoglossary Home Page