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an acid volcanic glass with fine, concentric cracks along which a specimen breaks into small pebbles that sometimes have a pearly luster; such cracks are characteristic of perlitic structure.
The composition of perlite resembles that of acid lavas, for example, liparite and dacite, which are chiefly composed of 65 to 75 percent SiO2 and 10 to 15 percent Al2O3 and which also include admixtures of Fe2O3, CaO, MgO, SO3, and R2O in proportions that range from fractions of a percent to single percentage units. Perlite can also contain as much as 3 to 6 percent constitutional water, that is, water which is bound into hydrated crystals. When crushed perlite is rapidly heated, the enclosed water evaporates, thus swelling the softened rock and increasing the specimen’s volume ten to 20 times. The swelling temperature of perlite, which depends on the water content and chemical composition, ranges from 850°C to 1000°C and sometimes reaches 1200°C. Expanded perlite grains have a low density that ranges from 70 to 600 kg/m3; they are thus suitable for use as sand or rubble in aggregates for lightweight concrete as well as in heat-insulating products, for example, those made from perlite and bitumens, perlite and silicates, and perlite and ceramics. Expanded perlite is also used in the chemical, petroleum-refining, food, and pharmaceutical industries and in glassmaking and agriculture.
Perlite is widely used in many countries throughout the world. The USSR has aggregate geological reserves of perlite that are estimated at approximately 500 million m3. More than 600,000 m3 of perlite was extracted in 1974, including 427,000 m3 from the Aragats deposit in the Armenian SSR, 110,000 m3 from the Beregovo deposit in the Ukrainian SSR, and 66,000 m3 from the Mukhor-Tala deposit in the Buriat ASSR.
V. M. BORZUNOV