SILICA, in chemistry, the name ordinarily given to amorphous silicon dioxide, SiO2. This chemical compound is widely and most abundantly distributed in nature, both in the free state and in combination with metallic oxides. Free silica constitutes the greater part of sand and sandy rocks; when fairly pure it occurs in the large crystals which we know as quartz (q.v.), and which, when coloured, form the gem-stones amethyst, cairngorm, cats'-eye and jasper. Tridymite (q.v.) is a rarer form, crystallographically different from quartz. Amorphous forms also occur: chalcedony (q.v.), and its coloured modifications agate, carnelian, onyx and sard, together with opal (qq.v.) are examples. Amorphous silica can be obtained from a silicate (a compound of silica and a metallic oxide) by fusing the finely powdered mineral with sodium carbonate, decomposing the sodium silicate thus formed with hydrochloric acid, evaporating to dryness to convert the colloidal silicic acid into insoluble silica, and removing the soluble chlorides by washing with hot water. On drying, the silica is obtained as a soft white amorphous powder, insoluble in water and in all acids except hydrofluoric; it dissolves in hot solutions of the caustic alkalis and to a less extent in alkali carbonates. It melts at a high temperature, and in the electric furnace it may be distilled, the vapours condensing to a bluish-white powder. By heating a solution of sodium silicate in a glass vessel the glass is attacked (an acid silicate being formed) and silica separates at ordinary temperatures in a hydrated amorphous form, at higher temperatures but below 180° as tridymite, and above 180° 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 great importance since they constitute the commonest rock-forming and many other minerals, and occur in every petrographical species. The parent acid, silicic acid, was obtained by T. Graham by dialysing a solution of hydrochloric acid to which sodium silicate had been added; a colloidal silicic acid being retained in the dialyser. This solution may be concentrated until it contains about 14% of silica by open boiling, and this solution on evaporation in a vacuum gives a transparent mass of metasilicic acid, H2SiO3. 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⋅H2O, 3SiO2⋅H2O, 4SiO2⋅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 Si3O6(OH)2; trisilicates, from Si2O3(OH)2; and basic silicates. These acids may be regarded as derived by the partial dehydration of the ortho-acid. Another classification is given in Metallurgy; a list of mineral silicates is given in Mineralogy, and for the synthetical production of these compounds see also Petrology.