OBSIDIAN, a glassy volcanic rock of acid composition. Rhy olitic lavas frequently are vitreous, and when the glassy matter greatly predominates and the crystals are few and inconspicuous the rock becomes an obsidian; the chemical composition is essen tially 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 or a ferromagnesian mineral, and when these are numerous the rock is called a porphyritic obsidian. These crystals have, as a rule, 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 be cause 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. 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 dama scened steel. 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 imper fect crystallizations (microlites, etc.). 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 f orming mineral.
These microlites or 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 com mon 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 honey-comb. In polarized light they show a weak grey colour with a black cross, the arms of which are parallel to the cross-wires 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 phe nomena appear in consequence. Porphyritic crystals often con tract less than the surrounding glass, which accordingly becomes strained, and in polarized light may show a weak double refrac tion 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 glob ular bodies have occasionally a sub-pearly lustre, and glassy rocks which possess this structure have been called perlites (q.v.).