Obsidian
Obsidian is an igneous rock that is produced during the rapid cooling of volcanic lava and takes a glassy form. It is sometimes classified as a mineraloid, implying that it is mineral-like but not a true mineral because it is not crystalline.
Because of the lack of crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as arrowheads and its modern use as surgical scalpel blades.
Formation and occurrence
Obsidian is produced from volcanoes when felsic[1] lava cools rapidly and freezes without sufficient time for crystal growth. It is commonly found in areas where the cooling of lava was rapid—such as at the margins of the lava flows and in places where the lava entered a lake or sea. This rock can be classified as an extrusive igneous rock—that is, igneous rock formed by the cooling of lava on the Earth's surface.
This glassy rock is metastable—over time, it becomes fine-grained, mineral crystals. The breakdown of obsidian is accelerated by the presence of water. Consequently, no obsidian has been found that is older than the Cretaceous age. Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, but few scientists currently endorse this hypothesis.
Among other places, Yellowstone National Park has a mountainside containing large quantities of obsidian, located between Mammoth Hot Springs and the Norris Geyser Basin. It can also be found in Scotland.
Characteristics
Obsidian consists mainly of silicon dioxide (SiO2), usually 70% or more. Its composition is very similar to that of granite and rhyolite. It is relatively soft, with a typical hardness of 5 to 5.5. Its relative density is approximately 2.6.
Pure obsidian is usually dark in appearance, but the color varies depending on the presence of impurities. Iron and magnesium typically give the material a dark green to brown to black color. Some rare samples are nearly clear. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produces a blotchy or snowflake pattern known as "snowflake obsidian." It may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock flowed before solidifying. These bubbles can produce interesting effects such as a golden ("sheen obsidian") or rainbow sheen ("rainbow obsidian").
Historical uses
Obsidian was highly valued in certain Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads. Like all glass and some other types of naturally occurring rocks, it breaks with a characteristic conchoidal fracture. It was also polished to create early mirrors.
Pre-Columbian Mesoamericans used obsidian extensively. For instance, they carved and worked obsidian for tools and decorative objects. The ancient Mesoamericans also made a type of sword with obsidian blades mounted in a wooden body. Called a macuahuitl, the weapon was capable of inflicting terrible injuries, combining the sharp cutting edge of an obsidian blade with the ragged cut of a serrated weapon that brought fear.
Native Americans traded obsidian throughout North America. Each volcano, and in some cases each volcanic eruption, produces a distinguishable type of obsidian, making it possible for archeologists to trace the origins of a particular artifact.
Obsidian was also used in construction of the Moai—the large stone statues on Easter Island.
Current usage
Obsidian is used in cardiac surgery, as well-crafted obsidian blades have a cutting edge up to five times sharper than high-quality steel surgical scalpels, with the edge of the blade reaching veritable molecular thinness. It produces a cleaner cut and less tissue trauma, which translates to faster healing and less scar tissue.[2]
Obsidian is also used for ornamental purposes and as a gemstone, for it possesses the peculiar property of presenting a different appearance according to the manner in which it is cut. When cut in one direction it is a beautiful jet black; when cut across another direction it is glistening gray. "Apache tears" are small rounded obsidian nuggets embedded within a grayish-white perlite matrix.
See also
- Crystal
- Glass
- Rock (geology)
- Tektite
- Volcano
Notes
- ↑ The term "felsic" combines the words "feldspar" and "silica." It refers to silicate minerals, magmas, and rocks that are enriched in lighter elements such as oxygen, aluminum, sodium, and potassium.
- ↑ J.J. Disa, J. Vossoughi, and N.H. Goldberg, "A comparison of obsidian and surgical steel scalpel wound healing in rats," Plast. Reconstr. Surg. 92 (5) (1993): 884-87. Retrieved April 11, 2007.
ReferencesISBN links support NWE through referral fees
- Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.
- Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 978-0471721574.
- Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.
- Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.
- Mineral Gallery. 2006. The Mineraloid Obsidian. Amethyst Galleries. Retrieved April 11, 2007.
External links
- Photo Glossary of Volcano Terms: Obsidian. U.S. Geological Survey. Retrieved April 11, 2007.
- Obsidian is Hot Stuff. Volcano World. Retrieved April 11, 2007.
- Obsidian. Mindat.org. Retrieved April 11, 2007.
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