Virginia State Stone
Unakite, also called epidotized granite, is a distinctive mottled pink and olive-green variety of granite that forms when epidote replaces the granite’s silica to at least some extent. Granite is generically a hard, crystalline, plutonic rock whose main components are silica and feldspar, in varying proportion, often with darker flecks of biotite, hornblende, muscovite, pyroxene, or other minerals. The most common colors of granite are white, gray, pink, and red. In unakite, the epidote accounts for the green to yellow-green hues. Unakite’s hardness is 6 to 7 on the Mohs scale, while granite generally ranges from 5 to 7.
A STATE SYMBOL
Unakite is widely considered Virginia’s "state rock," although no such designation has been made by the legislature. It occurs in Virginia principally in the counties of Augusta and Roanoke; the material from Roanoke County is of the highest grade, suitable for manufacture of gemstones. Hikers in Shenandoah National Park and in Virginia’s extensive and scenic state parks in the Blue Ridge area may find specimens of the rock ranging in size from pebbles to small boulders.
Because unakite forms by alteration of granite, it occurs in masses of granite, particularly in dikes, narrow intrusions where granite has pushed upwards through fissures or through colder, less malleable rock. Granite is plutonic, meaning that it forms underground. Dikes are one mechanism known to bring granite into the upper portion of the earth’s crust. It is likely that the pressure forcing the dike upward, combined with the presence of epidote in the surrounding rock, leads to the low-grade metamorphism of the granite and the incorporation of epidote into its mineral structure.
NAME AND GEOGRAPHIC DISTRIBUTION
Unakite is named for the place where it was first identified, the Unaka range in the Blue Ridge Mountains. In the United States, most unakite is associated with the Blue Ridge Mountains, mostly at higher elevations but also and in places where it has been tumbled downstream. Unakite is popular with rock collectors in Virginia, North Carolina, and Tennessee. Outside the U.S., notable sources of unakite are in South Africa, Brazil, and China.
Ordinary granite is one of the best stones for building construction because of its load-bearing capacity and resistance to weathering. If unakite were more common, it could be used as a building stone, too. Instead, its most massive application is generally tile for walls or paving. Most commonly, unakite is fashioned into small decorative objects such as jewelry or animal figurines.
The Smithsonian Institution’s Museum of Natural History in Washington, DC, uses unakite tiles for the floor of its main terrace and for trim on the stairs leading to the building from the Capitol Mall. Unakite’s generic counterpart, granite, is of course much more commonly used in architecture throughout the world. Aberdeen, Scotland, and Hyderabad, India, are two cities that have used great quantities of locally quarried granite in their buildings; Aberdeen is nicknamed "the Granite City."
Because granite endures weathering better than most rocks, it forms some of the nation’s most impressive stone outcrops. The famous mountainous formations of Yosemite National Park are made of granite.
Chemical Formula: SiO2
Color: White, gray, pink or red
|Author: World Trade Press|