silica

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A white or colorless crystalline compound, SiO₂, occurring abundantly as quartz, sand, flint, agate, and many other minerals and used to manufacture a wide variety of materials, especially glass and concrete.

Century Dictionary and Cyclopedia

1. n. Silicon dioxid (SiO₂), or silicic anhydrid, a white or colorless substance, nearly insoluble in water and in all acids except hydrofluoric acid. Silica is extremely hard, and fuses with difficulty in the oxyhydrogen flame to a colorless amorphous glass. In nature, as quartz, it is universally distributed, and is the commonest of minerals; here belong the varieties rock-crystal, amethyst, chalcedony, agate, carnelian, onyx, jasper. flint, hornstone, etc., which differ in degree of crystallization and in purity, and hence in color. Silica in the form of quartz makes the sand of the seashore, and rock-masses as quartzite and sandstone. It also occurs us the rare mineral tridymite, known only in volcanic rocks anil in a few meteorites, and as the amorphous opal, which is softer and more soluble than quartz and contains more or less water. (See quartz, tridymite, opal, also asmanite, cristobalite, melanophlogite.) Silica also forms the material of the spicules of many sponges and of the frustules of diatoms; deposits of the latter are not uncommon under peat-swamps, and in some regions vast beds have been accumulated. (See infusorial earth, under infusorial.) Silica combines with bases to form compounds called silicates, which constitute the rocky crust of the globe. It occurs in solution in the waters of many mineral springs, and sometimes is deposited in enormous quantities about geyser-basins. From the silicates taken up by plants silica is often deposited on the surface or in the interior of their stems. The value of the equisetum, or scouring-rush, is due to the silica contained in it, which sometimes amounts to 18 per cent. of the fresh plant. Sand is extensively used for the manufacture of glass and mortar. The prominent silicates recognized among minerals are the metasilicates, salts of meta-silicic acid (H₂SiO₃), and orthosilicates, salts of orthosilicic acid (H₄SiO₄). Examples are rhodonite, or manganese metasilicate (MnSi0₃), anil willemite, or zinc orthosilicate (Zn₂SiO₄). There are also disilicates, polysilicates, etc., but they are rarer, and their nature is less clearly understood. See glass, mortar, and s and. Also called silex.
2. n. Silica fused by the oxyhydrogen blowpipe may be worked in the plastic state, very much as glass is, and retains its amorphous character on cooling. Extremely delicate threads of this material may be drawn, and are used to suspend minute, readily mobile mirrors or other such parts of physical apparatus; they are also valuable as electrical insulators, being less affected by atmospheric moisture than threads of glass. Tubes, flasks, and beakers of moderate size have also been made from fused silica, and are useful on account of the hardness of the material, the high temperature it will bear without melting, the rapid changes of temperature it will sustain without cracking, and its superior resistance to most chemical reagents. Its behavior as respects dilatation by heat is also important. Berthelot has, however, recently shown that it is permeable by gases, particularly at high temperatures.
3. n. Same as siliqua, 6.

Wiktionary

1. n. Silicon dioxide.
2. n. Any of the silica group of the silicate minerals.

GNU Webster's 1913

1. n. (Chem.) Silicon dioxide, SiO�. It constitutes ordinary quartz (also opal and tridymite), and is artifically prepared as a very fine, white, tasteless, inodorous powder.

WordNet 3.0

1. n. a white or colorless vitreous insoluble solid (SiO2); various forms occur widely in the earth's crust as quartz or cristobalite or tridymite or lechatelierite