Igneous Rock Schaller Pegmatite

Associated Smithsonian Expert: Michael A. Wise, Ph.D.

Michael Wise investigates granite and pegmatite dikes that intruded into metasedimentary rocks near Mount Antero, Colorado.

Courtesy of Jennifer C. Kelly

Dr. Michael Wise is a geologist and an expert on pegmatites, which are coarse-grained igneous rocks rich in the minerals quartz and feldspar. A native of Smithfield in southeastern Virginia, Dr. Wise first got interested in mineralogy as an undergraduate at the University of Virginia, when rock-hunting trips to central Virginia and North Carolina strengthened his interest in this type of rock. He obtained his doctorate from the University of Manitoba in Canada in 1987 and joined the Smithsonian National Museum of Natural History Department of Mineral Sciences one year later. He has traveled widely in the Appalachian mountains of the eastern United States to study pegmatites, which are good sources of not only semiprecious gemstones such as aquamarine, rose quartz, and topaz, but also rare elements, such as beryllium, lithium, and cesium, with important economic applications. His research has also taken him to Nevada, Colorado, California, and the Northwest Territories of Canada. In the laboratory, he uses a scanning electron microscope, cathodoluminescence microscope, X-ray diffractometer, and other tools to investigate the composition, crystal structure, and evolution of pegmatites.

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Lava fountains erupt from Krafla volcano in Iceland
Photographed by Michael Ryan, U.S. Geological Survey, Public Domain

How Igneous Rocks Are Formed

Earth's crust, or outermost rocky layer, sits on top of a deeper layer called the mantle, which stores heat from two sources: the formation of the Earth 4.65 billion years ago and the radioactive decay of uranium, thorium, and potassium. When cracks between huge crustal plates open up, the gap causes the underlying mantle to rise up. The upwelling partially melts that region of the mantle; scientists call that decompression melting. The molten rock, or magma, is less dense than solid rock, so it moves upward, the way a cork bobs to the surface of water. As the magma reaches the upper layers of the crust or even Earth's surface, it cools and hardens into a solid known as igneous rock. Scientists categorize igneous rocks according to their chemical composition, the method of their formation, and their degree of crystallization.

Cripple Creek granite at Florissant Fossil Beds National Monument in Colorado
Photographed by Joseph Hall, U.S. National Park Service, Public Domain

About Intrusive Igneous Rocks

Molten rock, or magma, does not always reach Earth's surface. It may flow upward through cracks that end below the surface, where it gets trapped and cools slowly. Some intrusions, called plutons, are several kilometers or miles wide. During the slow cooling process, the magma freezes into crystals. Magma containing higher iron, magnesium, and calcium levels is the first to turn solid and forms dark, coarse-grained rocks such as gabbro. The lighter-colored granite comes from magma with high levels of silica (silicon dioxide) and relatively little iron and magnesium. Humans cannot witness the formation of intrusive igneous rock in the same way we see volcano eruptions. However, over millions of years, the crustal rocks above some intrusions wear away, leaving the solidified magma exposed to the environment in places such as the Sierra Nevada Mountains and Yosemite National Park in California.

Magma rich in silica was injected into cracks in the older gneiss, creating this silicic intrusion, mainly consisting of pink feldspar and white quartz at its core. Location: Morrison, CO.
Photographed by Donald E. Hurlbert, Smithsonian Institution

Crystals and Pegmatites

Pegmatites are extremely coarse-grained intrusive igneous rocks, containing crystals that are both large (at least 5 cm or 2 inches across) and packed closely together. Pegmatites crystallize during the final stages of granite formation. The same silicate minerals that form granite - quartz, feldspar, and mica - generally make up the bulk of pegmatites, too, but the individual minerals in pegmatites can be many centimeters or even several meters in diameter. Some pegmatites also contain less common minerals, such as garnet, albite, lepidolite, beryl, and fluorite. Geologists and miners sometimes find beautiful, gemstone-quality crystals of topaz, beryl (aquamarine), rose quartz, smoky quartz, and other minerals within pegmatites.