Obsidian
From Wikipedia, the free encyclopedia
For other uses, see Obsidian (disambiguation).
| Obsidian | |
|---|---|
 |
|
| General | |
| Category | Volcanic glass |
| Formula (repeating unit) |
70–75% SiO2, plus MgO, Fe3O4 |
| Identification | |
| Color | Deep black |
| Fracture | Conchoidal |
| Mohs scale hardness | 5–6[1] |
| Luster | Vitreous |
| Specific gravity | ~2.4[2] |
| Optical properties | Translucent |
| References | [3] |
It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerization degree of the lava. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Obsidian is hard and brittle; it therefore fractures with very sharp edges, which had been used in the past in cutting and piercing tools, and has been used experimentally as surgical scalpel blades.[4]
Contents
Origin and properties
... among the various forms of glass we may reckon Obsian glass, a substance very similar to the stone found by Obsius in Ethiopia.[5]The translation into English of Natural History written by the elder Pliny of Rome shows a few sentences on the subject of a volcanic glass called Obsian, so named from its resemblance to a stone found in Ethiopia by Obsius (obsiānus lapis).[6][7][8]
Obsidian is the rock formed as a result of cooled lava, which is the parent material.[9][10][11] Having a low water content when newly formed typically less than 1% water by weight,[12] becomes progressively hydrated when exposed to groundwater, forming perlite. Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis.
Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too complex to comprise a single mineral. It is sometimes classified as a mineraloid. Though obsidian is usually dark in color similar to mafic rocks such as basalt, obsidian's composition is extremely felsic. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite. Because obsidian is metastable at the Earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age. This breakdown of obsidian is accelerated by the presence of water.
Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark brown to black color. Very few samples are nearly colorless. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). It may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden sheen (sheen obsidian). An iridescent, rainbow-like sheen (rainbow obsidian) is caused by inclusions of magnetite nanoparticles (Nadin, 2007).[13]
Occurrence
There are only four major deposit areas in the central Mediterranean: Lipari, Pantelleria, Palmarola and Monte Arci.[16]
Ancient sources in the Aegean were Melos and Giali.[17]
Acigöl town and the Göllü Dağ volcano were the most important sources in central Anatolia, one of the more important source areas in prehistoric Near East.[18][19][20]
Historical use
Middle East
In Ubaid in the 5th millennium BC, blades were manufactured from obsidian mined in what is now Turkey.[28] Ancient Egyptians used obsidian imported from the eastern Mediterranean and southern Red Sea regions. Obsidian was also used in ritual circumcisions because of its deftness and sharpness.[29] In the east of the Mediterranean the material was used to make tools, mirrors and decorative objects.[30]Americas
See also: Obsidian use in Mesoamerica
Pre-Columbian Mesoamericans' use of obsidian was extensive and sophisticated; including carved and worked obsidian for tools and decorative objects. 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.
In Chile obsidian tools from Chaitén Volcano have been found as far away as in Chan-Chan 400 km north of the volcano and also in sites 400 km south of it.[33][34]
Easter Island
Obsidian was also used on Rapa Nui (Easter Island) for edged tools such as Mataia and the pupils of the eyes of their Moai (statues).Current use
Though not approved by the US Food and Drug Administration (FDA) for use on humans, obsidian is used by some surgeons for scalpel blades, as well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels, the cutting edge of the blade being only about 3 nanometers thick.[35] Even the sharpest metal knife has a jagged, irregular blade when viewed under a strong enough microscope; when examined even under an electron microscope an obsidian blade is still smooth and even.[36] One study found that obsidian incisions produced fewer inflammatory cells and less granulation tissue at 7 days, in a group of rats.[37] Don Crabtree produced obsidian blades for surgery and other purposes,[35] and has written articles on the subject. Obsidian scalpels may currently be purchased for surgical use on research animals.[38]
Plinths for audio turntables have been made of obsidian since the 1970s; e.g. the grayish-black SH-10B3 plinth by Technics.
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