PERLITE, or PEARLSTONE, a glassy volcanic rock which, when struck with a hammer, breaks up into small rounded masses that often have a pearly lustre. The 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 groups, one within another. The circular cracks bound the little 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 light from enclosed films of 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 limonite, which make them more obvious, or by other secondary minerals. The glass itself often undergoes change along the cracks by becoming finely crystalline or devitrified, dull in appearance and slightly opaque in section. In polarized light the perlitic glass is usually quite isotropic, but sometimes the internal part of some of the spheres has a slight double refraction which is apparently due to strain. The glass found on the 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 opal which is called hyalite. This forms small transparent rounded masses like drops of gum, and in microscopic section exhibits concentric systems of cracks. Hyalite, like perlitic obsidian, is amorphous or non-crystalline. It is easy to imitate perlitic structure by taking a little Canada balsam and heating it on a slip of glass till most of the volatile matters are driven out; then drop it in a basin of cold water and typical perlitic structure will be produced. The reason is apparently the sudden contraction when the mass is chilled. In the glaze on tiles and china rounded or polygonal systems of cracks may often be seen which somewhat resemble perlitic structure but are less perfect and 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 process of time been changed to a finely crystalline state by devitrification. Occasionally in olivine and quartz rounded cracks not unlike perlitic structure may be observed.
Many perlitic rocks contain well-developed crystals of quartz, feldspar, augite or magnetite, etc., usually more or less corroded or rounded, and in the fine glassy base minute crystallites cfften abound. Some of the rocks have the resinous lustre and the high percentages of combined water which distinguish the pitchstones; others are bright and fresh obsidians, and nearly all the older examples are dull, cryptocrystalline felsites. According to their chemical compositions they range from very acid rhyolites to trachytes and andesites, and the dark basaltic glasses or tachylytes are sometimes highly perlitic. It is probable that most perlites are of intrusive origin, and the general absence of 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 kind are found in Meissen, Saxony, as dikes of greenish and brownish pitchstone. Other examples are furnished by the Tertiary igneous rocks of Hungary (Tokai, etc.), the Euganean Hills (Italy) and Ponza Island (in the Mediterranean).
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 Ockotsk, Siberia, in association with a large mass of perlitic obsidian. These globular bodies are, in fact, 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.
In Great Britain Tertiary vitreous rocks are not common, but the pitchstone which forms the Scuir of Eigg is a dark andesitic porphyry with perlitic structure in its glassy matrix. A better example, however, is provided by a perlitic dacitic pitchstone porphyry that occurs near the Tay Bridge in Fifeshire. The tachylytic basalt dikes of Mull are occasionally highly perlitic. At Sandy Braes in Antrim a perlitic obsidian has been found, and the Lea Rock, near Wellington in Shropshire, is a devitrified obsidian which shows perlitic cracks and the remains of spherulites. (J. S. F.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)