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SILICA

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Originally appearing in Volume V25, Page 93 of the 1911 Encyclopedia Britannica.
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SILICA , in See also:

chemistry, the name ordinarily given to amorphous See also:silicon dioxide, SiO2. This chemical See also:compound is widely and most abundantly distributed in nature, both in the See also:free See also:state and in See also:combination with metallic oxides. Free silica constitutes the greater See also:part of See also:sand and sandy rocks; when fairly pure it occurs in the large crystals which we know as See also:quartz (q.v.), and which, when coloured, See also:form the See also:gem-stones See also:amethyst, See also:cairngorm, See also:cats'-See also:eye and See also:jasper. See also:Tridymite (q.v.) is a rarer form, crystallographically different from quartz. Amorphous forms also occur: See also:chalcedony (q.v.), and its coloured modifications See also:agate, See also:carnelian, See also:onyx and See also:sard, together with See also:opal (qq.v.) are examples. Amorphous silica can be obtained from a silicate (a compound of silica and a metallic See also:oxide) by fusing the finely powdered See also:mineral with See also:sodium carbonate, decomposing the sodium silicate thus formed with hydrochloric See also:acid, evaporating to dryness to convert the colloidal silicic acid into insoluble silica, and removing the soluble chlorides by washing with hot See also:water. On drying, the .silica is obtained as a soft See also:white amorphous See also:powder, insoluble in water and in all acids except hydrofluoric; it dissolves in hot solutions of the See also:caustic alkalis and to a less extent in See also:alkali See also:carbonates. It melts at a high temperature, and in the electric See also:furnace it may be distilled, the vapours condensing to a bluish-white powder. By See also:heating a See also:solution of sodium silicate in a See also:glass See also:vessel the glass is attacked (an acid silicate being formed) and silica separates at See also:ordinary temperatures in a hydrated amorphous form, at higher temperatures but below 18o° as tridymite, and above 18o° as quartz. Silicates.—These compounds are to be regarded as salts of silicic acid, or combinations of silicon dioxide and metallic basic oxides; they are of See also:great importance since they constitute the commonest See also:rock-forming and many other minerals, and occur in every petrographical See also:species. The See also:parent acid, silicic acid, was obtained by T. See also:Graham by dialysing a solution of hydrochloric acid to which sodium silicate had been added; a colloidal silicic acid being resained in the dialyser.

This solution may be concentrated until it contains about 14°f° of silica by open boiling, and this solution on evaporation in a vacuum gives a transparent See also:

mass of metasilicic acid, H2SiOa. The solution is a tasteless liquid having a slight acid reaction; it gradually changes to a clear transparent jelly, which afterwards shrinks on drying. This coagulation is brought about very quickly by sodium carbonate, and may be retarded by hydrochloric acid or by a solution of a caustic alkali. Several hydrated forms have been obtained, e.g. 2SiO2•See also:H2O, 3SiO2•H2O, 4Si02•H2O, 8SiO2•H2O; these are very unstable, the first two losing water on exposure whilst the others absorb water. The natural silicates may be regarded as falling into 5 classes, viz. orthosilicates, derived from Si(OH)4; metasilicates, from SiO(OH)2; disilicates, from Si2Oa(OH)2; trisilicates, from Sia06(OH)2; and basic silicates. These acids may be regarded as derived by the partial dehydration of the ortho-acid. Another See also:classification is given in METALLURGY; a See also:list of mineral silicates is given in See also:MINERALOGY, and for the synthetical See also:production of these compounds see also See also:PETROLOGY.

End of Article: SILICA

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SILHOUETTE, ETIENNE DE (1709-1767) 		[next]
SILICON [symbol Si, atomic weight 28.3 (0= 16)]

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