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OBSIDIAN
Online EncyclopediaOriginally appearing in Volume
V19,
Page 962
of the 1911 Encyclopedia Britannica.
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OBSIDIAN , a glassy volcanic See also:
As the rock was highly viscous and the surface over which it moved was often irregular the See also: motion was disturbed and fluctuating; hence the sinuous and contorted appearance frequently assumed by the banding. When crystals are See also: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 See also: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 See also:light, have angular outlines like those of crystals, and may even show twinning and definite See also:optical properties by which they can be identified as belonging to felspar, See also:augite or some other rock-forming See also:mineral. The variety of their shapes is endless. Some are black, very thin and curved like threads or hairs (trichites); often a See also:group of these is seated on a small crystal of augite or See also:magnetite and spreads outwards on all sides. Others have hollow or See also:funnel-shaped ends and are constricted at the See also:middle like a See also:dice See also:cup. In some rocks small rod-like microlites are grouped together in a See also:regular way to form growths which resemble See also:fir branches, See also:fern leaves, brushes or networks, in the same manner as minute needles of See also:ice produce See also:star-like See also:snow crystals or the See also:frost growths on a window See also:pane. These crystallites (q.v.) show that the glassy rock has a tendency to crystallize which is inhibited only by the very viscous See also:state i9 96 I of the glass and the rapidity with which it was cooled.Another type of incipient See also: crystallization which is excessively See also:common in obsidian is See also:spherulites (q.v.), 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 See also:Teneriffe and Lipari the whole rock consists of them, so closely packed together that they assume Polygonal shapes like the cells of a See also:honeycomb. In polarized light they show a weak grey See also:colour with a black See also:cross, the arms of which are parallel to the cobwebs in the See also:eye-piece of the See also:microscope and remain stationary when the See also: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 See also:place before the rock ceased to flow. As cooling progresses the glassy rock contracts and See also:strain phenomena appear in consequence. Porphyritic crystals often See also:contract less than the surrounding glass, which accordingly becomes strained, and in polarized light may show a weak See also:double See also:refraction in a limited See also: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 See also:system of rounded fissures arises. These surround little spherules of glass which are detached when the rock is struck with a See also:hammer. There may be concentric See also: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 See also:Canada See also:balsam and See also:heat it strongly for a little See also:time most of the volatile See also:oils are driven out of it.When it cools it becomes hard, but if before it is quite See also: cold we plunge it into cold See also:water a very perfect perlitic structure will arise in it. Occasionally the rounded cracks extend from the See also: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 See also:size of a See also:pea to that of an See also:orange, may often be seen labelled marekanite. They have long been known to geologists and are found at See also:Okhotsk, See also:Siberia, in association with a large See also:mass of perlitic obsidian. These globular bodies are, in fact, merely the more coherent portions of a See also:perlite; the See also:rest of the rock falls down in a See also:fine See also:powder setting See also:free the glassy See also:spheres. They are subject to considerable See also:internal strain, as is shown by the fact that when struck with a hammer or sliced with a See also: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 See also:Prince See also: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 See also:property and no longer exhibit any tendency to sudden disintegration. Although rocks wholly or in large See also:part vitreous are known from very See also:ancient See also:geological systems, such as the Devonian, they are certainly most frequent in See also:recent volcanic countries. Yet among the older rocks there are many which, though finely crystalline, have the chemical composition of See also:modern obsidians and possess structures, such as the perlitic and spherulitic, which are very characteristic of vitreous rocks.By many lines of See also: 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 See also:analysis leaves little doubt as to the real nature of 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 See also:change such as percolating water have had easiest See also:access (as along See also:joints, perlitic cracks and the margins of dikes and sills). Obsidians from Lipari often have felsitic bands alternating with others which are purely glassy. In See also:Arran there are See also: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 See also:Roman glass which have been dug out of the ruins of See also:Pompeii. Obsidians occur in many parts of the See also:world along with rhyolites and See also:pumice. In See also:Europe the best-known localities for them are the Lipari Islands, Pantellaria, See also:Iceland and See also:Hungary. Very fine obsidians are also obtained in See also:Mexico, at the Yellowstone See also:Park, in New See also:Zealand, See also:Ascension and in the See also:Caucasus.Included in this group are some rocks which are more properly to be regarded as vitreous forms of See also: 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, semi-opaque glassy forms of the basic igneous rocks are known as tachyl tes. The typical obsidians exhibit the chemical peculiarities of the acid igneous rocks (viz. high percentage of See also:silica, low iron, See also:lime and See also: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, See also: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 See also:las Navajas, or " See also:
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