Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration of obsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an industrial mineral and a commercial product useful for its light weight after processing. When it reaches temperatures of 850-900 C, perlite softens (because it is a glass). Water trapped in the structure of the material vapourises and escapes, and this causes the expansion of the material to 7-16 times its original volume. The expanded material is a brilliant white, due to the reflectivity of the trapped bubbles. Unexpanded ('raw') perlite has a bulk density around 1100 kg/m (1.1 g/cm), while typical expanded perlite has a bulk density of about...'
Contributions by FocalPoint, Vsmith, and Mikenorton.