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).
All obsidians have a low specific gravity (about 2.4) both because they are acid rocks and because they are non-crystalline.
In banded obsidians these microlites may be numerous in some parts but few or absent in others.
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.
Yet among the older rocks there are many which, though finely crystalline, have the chemical composition of modern obsidians and possess structures, such as the perlitic and spherulitic, which are very characteristic of vitreous rocks.
By many lines of 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.
Obsidians from Lipari often xIX.
Obsidians occur in many parts of the world along with rhyolites and pumice.
Very fine obsidians are also obtained in Mexico, at the Yellowstone Park, in New Zealand, Ascension and in the Caucasus.
It is certain, however, that most obsidians are very acid or rhyolitic. The dark, semiopaque glassy forms of the basic igneous rocks are known as tachylytes.
The typical obsidians exhibit the chemical peculiarities of the acid igneous rocks (viz.
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, spear-heads, knives and razors.
By the ancient Greeks and Romans obsidian was worked as a gem-stone; and in consequence of its having been often imitated in glass there arose among collectors of gems in the 18th century the practice of calling all antique pastes "obsidians."
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