Glass, devitrification of volcanic

Composition and Structure of Glassy Rocks

Volcanic glasses have been classified descriptively as follows: obsidians, those glasses with a vitreous luster; pitchstones, those with a resinous luster; and perlites, those that contain a multitude of irregular tension fractures. Chemically, all three types may range in composition from basalt through andesite, to rhyolite or trachyte. Compositions that do not fall on the main lines of liquid development during fractional crystallization of basalt are not represented. The chemical distribution as a function of silica content shows a dominant peak at about 73% of silica, corresponding to rhyolite glass. In addition, a distinct maximum at about 52% corresponds to basaltic glasses. No volcanic glasses are known with less than 35% or more than 80% of silica. Rhyolite glass seems about 3 times more abundant than basalt or andesite glass, despite the general 2:1 preponderance in nature of crystalline basalt over rhyolite. The greater stability of...