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PERLITE, or PEARLSTONE
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See also:PERLITE, or PEARLSTONE , a glassy volcanic See also:rock which, when struck with a See also:hammer, breaks up into small rounded masses that often have a pearly lustre. The See also:reason for this peculiarity is obvious in microscopic sections of the rock, for many small cracks may be seen traversing the glassy substance. These mostly take a circular course, and often occur in See also:groups, one within another. The circular cracks See also:bound the little See also:spheres into which the rock falls when it is struck, and the concentric fissures. are the cause of the pearly lustre, by the reflection of See also:light from enclosed films of See also:air. Longer straight cracks run across the sections separating areas in which the circular fissures preponderate. By decomposition the fissures may be occupied by deposits of See also:limonite, which make them more obvious, or by other secondary minerals. The See also:glass itself often undergoes See also:change along the cracks by becoming finely crystalline or devitrified, dull in See also:appearance and slightly opaque in See also:section. In polarized light the perlitic glass is usually quite isotropic, but sometimes the See also:internal See also:part of some of the spheres has a slight See also:double See also:refraction which is apparently due to See also:strain. The glass found on the See also:waste-heaps of glass-furnaces is sometimes very coarsely perlitic. Perlitic structure is not confined to glass, but may be seen also in that variety of See also:opal which is called hyalite. This forms small transparent rounded masses like drops of See also:gum, and in microscopic section exhibits concentric systems of cracks. Hyalite, like perlitic See also:obsidian, is amorphous or non-crystalline. It is easy to imitate perlitic structure by taking a little See also:Canada See also:balsam and See also:heating it on a slip of glass till most of the volatile matters are driven out; then drop it in a See also:basin of See also:cold See also:water and typical perlitic structure will be produced. The reason is apparently the sudden contraction when the See also:mass is chilled. In the glaze on tiles and See also:china rounded or polygonal systems of cracks may often be seen which somewhat resemble perlitic structure but are less perfect and See also:regular. Many rocks which are cryptocrystalline or felsitic, and not glassy, have perfect perlitic structure, and it seems probable that these were originally vitreous obsidians or pitchstones and have in See also:process of See also:time been changed to a finely crystalline See also:state by devitrification. Occasionally in See also:olivine and See also:quartz rounded cracks not unlike perlitic structure may be observed.
Many perlitic rocks contain well-See also:developed crystals of quartz, feldspar, See also:augite or See also:magnetite, &c., usually more or less corroded or rounded, and in the See also:fine glassy See also:base See also:minute crystallites often abound. Some of the rocks have the resinous lustre and the high percentages of combined water which distinguish the pitchstones; others are See also:bright and fresh obsidians, and nearly all the older examples are dull, cryptocrystalline felsites. According to their chemical compositions they range from very See also:acid rhyolites to trachytes and andesites, and the dark basaltic glasses or See also:tachylytes are sometimes highly perlitic. It is probable that most perlites are of intrusive origin, and the See also:general See also:absence of See also:steam cavities in these rocks would support this conclusion, but some perlitic Hungarian rhyolites are believed to be lavas.
Very well known rocks of this See also:kind are found in See also:Meissen, See also:Saxony, as dikes of greenish and brownish See also:pitchstone. Other examples are furnished by the See also:Tertiary igneous rocks of See also:Hungary (Tokai, &c.), the Euganean Hills (See also:Italy) and See also:Ponza See also:Island (in the Mediterranean).
In mineralogical collections rounded nodules of See also: They have See also:long been known to geologists and are found at Ockotsk, See also:Siberia, in association with a large mass of perlitic obsidian. These globular bodies are, in fact, the more coherent portions of a perlite; the See also:rest of the rock falls down in a fine powier, setting See also: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 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 See also: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. In See also:Great See also:Britain Tertiary vitreous rocks are not See also:common, but the pitchstone which forms the Scuir of Eigg is a dark andesitic See also:porphyry with perlitic structure in its glassy See also:matrix. A better example, however, is provided by a perlitic dacitic pitchstone porphyry that occurs near the See also:Tay See also:Bridge in Fifeshire. The tachylytic See also:basalt dikes of See also:Mull are occasionally highly perlitic. At Sandy Braes in See also:Antrim a perlitic obsidian has been found, and the See also:Lea Rock, near See also:Wellington in See also:Shropshire, is a devitrified obsidian which shows perlitic cracks and the remains of See also:spherulites. (J. S. Additional information and CommentsThere are no comments yet for this article.
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