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Definition of PERLITE
: volcanic glass that has a concentric structure, appears as if composed of concretions, is usually grayish and sometimes spherulitic, and when heated expands to form a lightweight aggregate used especially in concrete and plaster and as a medium for potting plants
Natural glass with concentric cracks such that the rock breaks into small, pearl-like bodies. It is formed by the rapid cooling of viscous lava or magma. Perlite is porous and has a waxy to pearly lustre and is commonly gray or greenish but may be brown, blue, or red. Since c. 1950, large deposits have been worked in New Mexico, Nevada, California, and other western states. Heat-treated perlite is a substitute for sand in lightweight wall plaster and concrete aggregate. Perlite is used for heat and sound insulation, lightweight ceramic products, and filters.