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Encyclopedia Britannica - Main :: NUM-ORC |
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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
drawn
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
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 i9 96 I of the glass and the rapidity with which it was cooled. Another type of incipient crystallization which is excessively common in obsidian is 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 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 eye-piece 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
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
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
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
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
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