Tachylite

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.

[2] 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.

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.

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.

Great caves are found where the crust has solidified and the liquid interior has subsequently flowed away, and stalactites and stalagmites of black tachylite adorn the roofs and floors.

On section these growths show usually a central cavity enclosed by walls of dark brown glass in which skeletons and microliths of augite, olivine and feldspar lie imbedded.

[1][4] A small outcrop at Spring Hill in Victoria, Australia[5] has tachylite which has been exploited as a material for making Aboriginal flaked stone implements.

In these situations tachylite is rarely vesicular, but it often shows very pronounced fluxion banding accentuated by the presence of rows of spherulites that are visible as dark brown rounded spots.

The ultimate product of decomposition in this case also is a red palagonitic substance, but owing to the absence of steam cavities the tachylite selvages of dikes are more often found in a fresh state than the basic lapilli in ash-beds.