Tachylite ( ; also spelled tachylyte) is a form of basaltic volcanic glass. This glass is formed naturally by the rapid cooling of molten basalt. It is a type of mafic igneous rock that is decomposable by acids and readily fusible. The color is a black or dark-brown, and it has a greasy-looking, resinous luster. It is very brittle and occurs in dikes, veins, and intrusive masses. The word originates from the Ancient Greek , meaning "swift".
Tachylites have the appearance of pitch and are often more or less vesicular and sometimes spherulitic. They are very brittle and break down readily under a hammer. Small crystals of feldspar or olivine are sometimes visible in them with the unaided eye. All tachylites weather rather easily and by oxidation of their iron become dark brown or red. Three modes of occurrence characterize this rock. In all cases they are found under conditions which imply rapid cooling, but they are much less common than acid volcanic glasses (or obsidians), the reason being apparently that the basic rocks have a stronger tendency to crystallize, partly because they are more liquid and the molecules have more freedom to arrange themselves in crystalline order.
Tachylite can be distinguished from obsidian and pitchstone by determining its fusibility, as splinters of tachyykite will fuse together when heated.
Geologic occurrences
Scoria sources
The fine scoria ashes or "cinders" thrown out by basaltic volcanoes are often spongy masses of tachylite with only a few larger crystals or phenocrysts imbedded in black glass. Such tachylite volcanic bombs and scoria are frequent in Iceland, Auvergne, Stromboli and Etna, and are very common also in the ash beds or tuffs of older date, such as occur in Skye, Midlothian and Fife, Derbyshire, and elsewhere. Basic pumices of this kind are exceedingly widespread on the bottom of the sea, either dispersed in the pelagic red clay and other deposits or forming layers coated with oxides of manganese precipitated on them from the sea water. These tachylite fragments, which are usually much decomposed by the oxidation and hydration of their ferrous compounds, have taken on a dark red color. This altered basic glass is known as "palagonite"; concentric bands of it often surround kernels of unaltered tachylite, and are so soft that they are easily cut with a knife. In the palagonite the minerals are also decomposed and are represented only by pseudomorphs. The fresh tachylite glass, however, often contains lozenge-shaped crystals of plagioclase feldspar and small prisms of augite and olivine, but all these minerals very frequently occur mainly as microlites or as skeletal growths with sharply-pointed corners or ramifying processes. Palagonite tuffs are found also among the older volcanic rocks. In Iceland a broad stretch of these rocks, described as "the palagonite formation," is said to cross the island from south-west to north-east. Some of these tuffs are fossiliferous; others are intercalated with glacial deposits. The lavas with which they occur are mostly olivine-basalts. Palagonite tuffs are found in Sicily, the Eifel, Hungary, Canary Islands, and other places.
Lava flow sources
A second mode of occurrence of tachylite is in the form of lava flows. Basaltic rocks often contain a small amount of glassy ground-mass, and in the limburgites this becomes more important and conspicuous, but vitreous types are far less common in these than in the acid lavas.
Tachylite may form at the edge of sills or thin dikes of basalt or diabase that rapidly cooled. Such edges may be as little as a millimetre thick. It merges internally into crystalline basalt.