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pumice(pŭm`ĭs), volcanic glass formed by the solidification of lava that is permeated with gas bubbles. Usually found at the surface of a lava flow, it is colorless or light gray and has the general appearance of a rock froth. The viscosity of the lava, the quantity of water vapor and gas, and the rate of cooling together determine the fineness of the vesicular substance. Large amounts of gas result in a finer-grained variety known as pumicite. The chemical composition is that of granite. Coarser-grained rock, with fewer and larger air spaces, is called scoria; it is usually associated with dark-colored igneous rocks of diorite or gabbro composition. Pumice is used chiefly as an abrasive and is included in many scouring preparations. Ground pumice is also used in finishing furniture. Deposits are found in volcanic areas throughout the world. Because of its air chambers, pumice has a very low density and has been observed blowing off volcanic islands in strong winds. It usually floats and can be carried great distances by ocean currents.
a porous, spongelike, vitreous volcanic rock. It is formed during volcanic eruptions as a result of the swelling and rapid cooling of acidic lavas (60–73 percent SiO2) heavily saturated with vapors and gases.
The porosity of pumice reaches 80 percent. The volumetric mass is 400–900 kg/m3. Pumice has a hardness of about 6 and is white, gray, or yellow. It melts at 1300°-1400°C. It is a poor conductor of heat.
Pumice is used as an abrasive. In the chemical industry it is used in filters, drying devices, and elsewhere. Crushed pumice is used as a lightweight aggregate in concrete, and pumice sand and ash are used as hydraulic admixtures in cement. Pumice deposits are found in many volcanic areas and frequently occur together with various volcanic ashes and tuffs. High-grade pumice deposits are found abroad on the Lipari Islands of Italy. The chief deposits in the USSR are in the Armenian SSR (the Ani group) and the Northern Caucasus, in the vicinity of Nal’chik.