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Obsidian

Obsidian from Lake County, Oregon
General
Category Volcanic glass
Chemical formula 70–75% SiO2,
plus MgO, Fe3O4
Identification
Color Black, gray, dark green, red, yellow, pink
Fracture Conchoidal
Mohs scale hardness ~ 5 to 5.5
Luster Vitreous
Specific gravity ~ 2.6
Optical properties Translucent

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools without 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. Because of this lack of crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as projectile points, and its modern use as surgical scalpel blades.[1][2]

Contents

Origin and properties

Pliny's Natural History features volcanic glass called "Obsidianus", so named from its resemblance to a stone found in Ethiopia by one Obsius.[3]

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 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. Obsidian has low water content when fresh, typically less than 1% water by weight,[4] but 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.

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 green to brown to black color. A 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) or a rainbow sheen (rainbow obsidian).

Occurrence

Obsidian can be found in locations which have experienced rhyolitic eruptions. It can be found in Armenia, Canada, Chile, Greece, Iceland, Italy, Kenya, Mexico, New Zealand, Peru, Scotland, Argentina and United States. Obsidian flows which may be hiked on are found within the calderas of Newberry Volcano and Medicine Lake Volcano in the Cascade Range of western North America, and at Inyo Craters east of the Sierra Nevada in California. Yellowstone National Park has a mountainside containing obsidian located between Mammoth Hot Springs and the Norris Geyser Basin, and deposits can be found in many other western U.S. states including Arizona, Colorado, New Mexico, Texas, Utah, Washington[5], Oregon[6] and Idaho. Obsidian can also be found in the eastern U.S. state of Virginia.

Historical use

Obsidian talus at Obsidian Dome, California.
Obsidian arrowhead.

Obsidian was valued in 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, obsidian breaks with a characteristic conchoidal fracture. It was also polished to create early mirrors.

Modern archaeologists have developed a relative dating system, Obsidian hydration dating, to calculate the age of Obsidian artifacts.

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Americas

Lithic analysis can be instrumental in understanding prehispanic groups in Mesoamerica. A careful analysis of obsidian in a culture or place can be of considerable use to reconstruct commerce, production, distribution and thereby understand economic, social and political aspects of a civilization. This is the case in Yaxchilán, a Maya city where even warfare implications have been studied linked with obsidian use and its debris.[7] Another example is the archeological recovery at coastal Chumash sites in California indicating considerable trade with the distant site of Casa Diablo in the Sierra Mountains.[8]

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.

Native American people traded obsidian throughout the Americas. Each volcano and in some cases each volcanic eruption produces a distinguishable type of obsidian, making it possible for archaeologists to trace the origins of a particular artifact. Similar tracing techniques have allowed obsidian to be identified in Greece also as coming from Melos, Nisyros or Yiali, islands in the Aegean Sea. Obsidian cores and blades were traded great distances inland from the coast.[citation needed]

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.[9] [10]

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

Pig carved in snowflake obsidian, 10 centimeters (4 in) long. The markings are spherulites.

Obsidian has been used for blades in surgery, 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.[11] 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. One study found that obsidian produced narrower scars, fewer inflammatory cells, and less granulation tissue in a group of rats.[12]

Obsidian is also used for ornamental purposes and as a gemstone. It possesses the property of presenting a different appearance according to the manner in which it is cut: when cut in one direction it is jet black; in another it is glistening gray. "Apache tears" are small rounded obsidian nuggets embedded within a grayish-white perlite matrix.

Plinths for audio turntables have been made of obsidian since the 1970s; e.g. the greyish-black SH-10B3 plinth by Technics.

See also

References

  1. ^ Primitive Technology: A Book of Earth Skills David Wescott
  2. ^ Supplier of modern obsidian surgical scalpels with information on use
  3. ^ Encyclopedia.com
  4. ^ "Perlite - Mineral Deposit Profiles, B.C. Geological Survey". http://www.em.gov.bc.ca/mining/GeolSurv/MetallicMinerals/MineralDepositProfiles/profiles/r12.htm. Retrieved 2007-11-20. 
  5. ^ Washington Obsidian Source Map
  6. ^ Oregon Obsidian Sources
  7. ^ Brokmann, Carlos, Tipología y análisis de la obsidiana de Yaxchilán, Chiapas, Colcción Científica, no.422, INAH, 2000, 284p.
  8. ^ C.Michael Hogan (2008) Morro Creek, ed. by A. Burnham
  9. ^ (Spanish) Pino, Mario and Navarro, Rayen. Geoarqueología del sitio arcaico Chan-Chan 18. Revista Geológica de Chile, 2005.
  10. ^ Naranjo, José A; Stern, Charles R (December 2004). Holocene tephrochronology of the southernmost part (42°30'-45°S) of the Andean Southern Volcanic Zone. 31. Revista geológica de Chile. pp. 225-240. ISSN 0716-0208. OCLC 61022562. http://www.scielo.cl/scielo.php?pid=S0716-02082004000200003&script=sci_arttext. Retrieved 2008-05-02. 
  11. ^ Buck, BA (March 1982). "Ancient technology in contemporary surgery". The Western journal of medicine 136 (3): 265–269. ISSN 0093-0415. OCLC 115633208. PMID 7046256. 
  12. ^ Disa, JJ; Vossoughi, J; Goldberg, NH (October 1993). "A comparison of obsidian and surgical steel scalpel wound healing in rats". Plastic and reconstructive surgery 92 (5): 884–887. doi:10.1097/00006534-199392050-00015. ISSN 0032-1052. OCLC 121212765. PMID 8415970. http://www.ncbi.nlm.nih.gov/pubmed/8415970?dopt=AbstractPlus. Retrieved 2007-11-20. 

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

OBSIDIAN, a glassy volcanic rock of acid composition. A similar rock was named obsianus by medieval writers, from its resemblance to a rock discovered in Ethiopia by one Obsius. The early printed editions of Pliny erroneously named the discoverer Obsidius, and the rock obsidianus. Rhyolitic lavas frequently are more or less vitreous, and when the glassy matter greatly predominates and the; crystals are few and inconspicuous the rock becomes an obsidian; the chemical composition is essentially the same as that of granite; the difference in the physical condition of the two rocks is due to the fact that one consolidated at the surface, rapidly and under low pressures, while the other cooled slowly at great depths and under such pressures that the escape of the steam and other gases it contained was greatly impeded. Few obsidians are entirely vitreous; usually they have small crystals of felspar, quartz, biotite or iron oxides, and when these are numerous the rock is called a porphyritic obsidian (or hyalo-liparite). These crystals have, as a rule, very good crystalline form, but the quartz and felspar are often filled with enclosures of glass.

All obsidians have a low specific gravity (about 2.4) both because they are acid rocks and because they are non-crystalline. Their lustre is vitreous except when they contain many minute crystals; they are then velvety or even resinous in appearance. Thin splinters and the sharp edges of fragments are transparent. Black, grey, yellow and brown are the prevalent colours of these rocks. In hand specimens they:often show a well-marked banding which is sometimes flat and parallel, but may be sinuous and occasionally is very irregular, resembling the pattern of damascened steel. In such cases the molten rock cannot have been homogeneous, and as it flowed along the ground the different portions of it were drawn out into long parallel streaks. As the rock was highly viscous and the surface over which it moved was often irregular the motion was disturbed and fluctuating; hence the sinuous and contorted appearance frequently assumed by the banding. When crystals are present they generally have their long axes parallel to the fluxion.

Even when conspicuous and well formed crystals are not visible in the rock there is nearly always an abundance of minute imperfect crystallizations (microlites, &c.). They are often so small that high magnifications may be necessary to ascertain their presence. Some are globular and others are rod-shaped; they may be grouped in clusters, stars, rosettes, rows, chains or swarms of indefinite shape. In banded obsidians these microlites may be numerous in some parts but few or absent in others. The larger ones polarize light, have angular outlines like those of crystals, and may even show twinning and definite optical properties by which they can be identified as belonging to felspar, augite or some other rock-forming mineral. The variety of their shapes is endless. Some are black, very thin and curved like threads or hairs (trichites); often a group of these is seated on a small crystal of augite or magnetite and spreads outwards on all sides. Others have hollow or funnel-shaped ends and are constricted at the middle like a dice cup. In some rocks small rod-like microlites are grouped together in a regular way to form growths which resemble fir branches, fern leaves, brushes or networks, in the same manner as minute needles of ice produce star-like snow crystals or the frost growths on a window pane.

These crystallites (q.v.) show that the glassy rock has a tendency to crystallize which is inhibited only by the very viscous state of the glass and the rapidity with which it was cooled. Another type of incipient crystallization which is excessively common in obsidian is spherulites, or small rounded bodies which have a radiating fibrous structure. They are of globular shape, less frequently irregular or branching, and may be elongated and cylindrical (axiolites). In some obsidians from Teneriffe and Lipari the whole rock consists of them, so closely packed together that they assume polygonal shapes like the cells of a honeycomb. In polarized light they show a weak grey colour with a black cross, the arms of which are parallel to the cobwebs in the eyepiece of the microscope and remain stationary when the section is rotated. Often bands of spherulites alternate with bands of pure glass, a fact which seems to indicate that the growth of these bodies took place before the rock ceased to flow.

As cooling progresses the glassy rock contracts and strain phenomena appear in consequence. Porphyritic crystals often contract less than the surrounding glass, which accordingly becomes strained, and in polarized light may show a weak double refraction in a limited area surrounding the crystal. Minute cracks are sometimes produced by the contraction; they are often more or less straight, but in other cases a very perfect system of rounded fissures arises. These surround little spherules of glass which are detached when the rock is struck with a hammer. There may be concentric series of cracks one within another. The minute globular bodies have occasionally a sub-pearly lustre, and glassy rocks which possess this structure have been called perlites (q.v.). If we take a thin layer of natural Canada balsam and heat it strongly for a little time most of the volatile oils are driven out of it. When it cools it becomes hard, but if before it is quite cold we plunge it into cold water a very perfect perlitic structure will arise in it. Occasionally the rounded cracks extend from the matrix into some of the crystals especially those of quartz which have naturally a conchoidal fracture. If the matrix, however, is originally crystalline it does not seem probable that perlitic structure can develop in it. Hence it may be regarded as diagnostic of rocks which were vitreous when they consolidated.

In mineralogical collections rounded nodules of brown glass, varying from the size of a pea to that of an orange, may often be seen labelled marekanite. They have long been known to geologists and are found at Okhotsk, Siberia, in association with a large mass of perlitic obsidian. These globular bodies are, in fact, merely the more coherent portions of a perlite; the rest of the rock falls down in a fine powder setting free the glassy spheres. They are subject to considerable internal strain, as is shown by the fact that when struck with a hammer or sliced with a lapidary's saw they often burst into fragments. Their behaviour in this respect closely resembles the balls of rapidly cooled, unannealed glass which are called Prince Rupert's drops. In their natural condition the marekanite spheres are doubly refracting, but when they have been heated and very slowly cooled they lose this property and no longer exhibit any tendency to sudden disintegration.

Although rocks wholly or in large part vitreous are known from very ancient geological systems, such as the Devonian, they are certainly most frequent in recent volcanic countries. Yet among the older rocks there are many which, though finely crystalline, have the chemical composition of modern obsidians and possess structures, such as the perlitic and spherulitic, which are very characteristic of vitreous rocks. By many lines of evidence we are led to believe that obsidians in course of time suffer devitrification, in other words they pass from the vitreous into a crystalline state, but as the changes take place in a solid mass they require a very long time for their achievement, and the crystals produced are only of extremely small size. A dull stony-looking rock results, the vitreous lustre having entirely disappeared, and in microscopic section this exhibits a cryptocrystalline structure, being made up of exceedingly minute grains principally of quartz and felspar. Often this felsitic devitrified glass is so fine-grained that its constituents cannot be directly determined even with the aid of the microscope, but chemical analysis leaves little doubt as to the real nature cf the minerals which have been formed. Many vitreous rocks show alteration of this type in certain parts where either the glass has been of unstable nature or where agencies of change such as percolating water have had easiest access (as along joints, perlitic cracks and the margins of dikes and sills). Obsidians from Lipari often xIX. 31 have felsitic bands alternating with others which are purely glassy. In Arran there are pitchstone dikes, some of which are very completely vitreous, while others are changed to spherulitic felsites more or less silicified. The pitchstone of the Scuir of Eigg is at its margins characterized by a dull semi-opaque matrix which seems to be the result of secondary devitrification. In the same way artificial glass can be devitrified if it be kept at a temperature slightly below the fusing point for some days. Window glass exposed to alkaline vapours often shows a thin iridescent surface film which is supposed to be due to crystallization; the same change is found in pieces of Roman glass which have been dug out of the ruins of Pompeii.

Obsidians occur in many parts of the world along with rhyolites and pumice. In Europe the best-known localities for them are the Lipari Islands, Pantellaria, Iceland and Hungary. Very fine obsidians are also obtained in Mexico, at the Yellowstone Park, in New Zealand, Ascension and in the Caucasus. Included in this group are some rocks which are more properly to be regarded as vitreous forms of trachyte than as glassy rhyolites (Iceland), but except by chemical analyses they cannot be separated. It is certain, however, that most obsidians are very acid or rhyolitic. The dark, semiopaque glassy forms of the basic igneous rocks are known as tachylytes. The typical obsidians exhibit the chemical peculiarities of the acid igneous rocks (viz. high percentage of silica, low iron, lime and magnesia, and a considerable amount of potash and soda).

The chemical composition of typical obsidians is shown by the following analyses Obsidian, when broken, shows a conchoidal fracture, like that of glass, and yields sharp-edged fragments, which have been used in many localities as arrow-points, spear-heads, knives and razors. For such purposes, as also for use as mirrors, masks and labrets, it was extensively employed, under the name ,of itztli, by the ancient Mexicans, who quarried it at the Cerro de las Navajas, or "Hill of Knives," near Timapan. The natives of the Admiralty Islands have used it for the heads of spears. By the ancient Greeks and Romans obsidian was worked as a gem-stone; and in consequence of its having been often imitated in glass there arose among collectors of gems in the 18th century the practice of calling all antique pastes "obsidians." At the present time obsidian is sometimes cut and polished as an ornamental stone, but its softness (H = 5 to 5.5) detracts from its value. Certain varieties, notably some from Russia, possess a beautiful metallic sheen, referable to the presence of either microscopic fissures or enclosures. The substance known as moldavite, often regarded as an obsidian, and the so-called obsidian bombs, or obsidianites, are described under MOLDAVITE. U. S. F.)


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Simple English

[[File:|thumb|right|180px|Obsidian from Oregon]]

File:Rainbow
A piece of rainbow obsidian.
File:Glass
Glass Mountain, a large obsidian flow at Medicine Lake Volcano.

[[File:|thumb|right|200px|Counterclockwise from top: obsidian, pumice and rhyolite (light color)]]

Obsidian is a natural volcanic glass found as a kind of igneous rock. It is produced when lava high in silica (SiO2) cools rapidly, and solidifies without time for crystal growth.[1] Because there is no 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.[2]

Contents

Origin and properties

Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline. Obsidian is dark in color similar to rocks such as basalt. It consists mainly of SiO2 (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite.

Obsidian is metastable at the earth's surface: over time the glass becomes fine-grained crystals. So, no obsidian is older than Cretaceous age. This breakdown of obsidian is accelerated by the presence of water. Obsidian has a low water content when fresh, typically less than 1% water by weight [3], but takes it in when exposed to groundwater.

Pure obsidian is usually dark in appearance, though the color varies depending on the presence of different materials. Iron and magnesium typically give the obsidian a dark green to brown to black color. A few samples are nearly colorless. In some stones, inclusions of small crystals produce a snowflake pattern (snowflake obsidian). It may contain patterns of gas which produce effects such as a golden sheen (sheen obsidian) or a rainbow sheen (rainbow obsidian).

Historical use

[[File:|thumb|right|180px|Obsidian arrowhead.]] Obsidian was valued in Stone Age cultures because, like flint, it could be fractured to produce hand axes, sharp blades or arrowheads. Like all glass and flint, obsidian breaks with a characteristic shell-shaped fracture.

Pre-Columbian Mesoamericans' used obsidian a lot. It was carved and worked for tools and decorative objects. It was also polished to create early mirrors. Mesoamericans also made a type of sword with obsidian blades mounted in a wooden body. Called a macuahuitl, the weapon could cause terrible injuries, because it combined the sharp cutting edge of an obsidian blade with the ragged cut of a serrated edge.

Native American people traded obsidian throughout North America. Each volcano and in some cases each volcanic eruption produces a distinguishable type of obsidian. So archaeologists can trace the origins of a particular artifact. Similar tracing techniques have allowed obsidian to be identified in Greece also as coming from different islands in the Aegean Sea. Obsidian cores (unworked lumps) and blades were traded great distances inland from the coast.[4]

Modern archaeologists have developed a dating system to calculate the age of obsidian artifacts depending from the content of water in the object.

Occurrence

Obsidian can be found in locations which have experienced rhyolitic (high silicate) eruptions. Obsidan flows which you can hike on are found within the calderas of Newberry Volcano and Medicine Lake Volcano in the Cascade Range of western North America, and at Inyo Craters east of the Sierra Nevada in California. Yellowstone National Park has a mountainside containing obsidian between Mammoth Hot Springs and the Norris Geyser Basin, and deposits can be found in many other western US states including Arizona, Colorado, Texas, Utah, Oregon and Idaho. Obsidian can also be found in Armenia, Turkey, Italy, Mexico, Iceland, Greece and Scotland.

Current use

File:Pig.
Pig carved in snowflake obsidian, 10 centimeters (4 in) long. The markings are spherulites.

Obsidian is used in heart surgery, as well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels, with the edge of the blade being only about 3 nm wide.[5] Even the sharpest metal knife has a jagged, irregular blade when viewed under a strong enough microscope. When examined under an electron microscope an obsidian blade is still smooth and even. One study found that obsidian produced narrower scars, fewer inflammatory cells, and less granulation tissue in a group of rats.[6]

Obsidian is also used for ornamental purposes and as a gemstone. It possesses the 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.

References

  1. Obsidian is found in rhyolitic lava flows known as obsidian flows, where cooling of the lava is rapid.
  2. Primitive technology: a book of Earth skills David Wescott
  3. "Perlite - Mineral Deposit Profiles, B.C. Geological Survey". http://www.em.gov.bc.ca/mining/GeolSurv/MetallicMinerals/MineralDepositProfiles/profiles/r12.htm. Retrieved 2007-11-20. 
  4. 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).
  5. Buck B.A. 1982. Ancient technology in contemporary surgery. The Western Journal of Medicine, 136, 265-269
  6. "A comparison of obsidian and surgical steel scalpe...[Plast Reconstr Surg. 1993 - PubMed Result"]. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8415970&dopt=Abstract. Retrieved 2007-11-20. 

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