Silica

SILICA, in chemistry, the name ordinarily given to amor phous silicon dioxide, Si02. This chemical compound is widely and most abundantly distributed in nature, both in the free state and in combination with metallic oxides, constituting about 6o% of the solid crust of the earth. 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, cat's eye and jasper. Tridymite (q.v.) is a rarer form, crystallo graphically different from quartz, and cristobalite is still rarer. 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, evaporat ing 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 C, and in the electric furnace it may be distilled, the vapours condensing to a bluish-white powder. The melting point is not that of amorphous silica or ordinary quartz, for above C this is unstable and is converted to tridymite; this in turn becomes unstable above C, being then converted into cristobalite, and it is this form which is stable at the melting point. Under the very high pressures prevailing during the crystallization of igneous rocks, however, the limit of stability of quartz would be greatly raised, since its density (2.65) is greater than that of tridymite (2.3), and this probably accounts for the comparative rarity of the latter in spite of the rocks having crystallized at above I,000° C. On the crystal structure of these forms of silica see Sir W. H. Bragg and R. E. Gibbs, Proc. Roy. Soc., 1925-26; R. W. G.

Wyckoff, Amer. J. Sci., 1925.

Quartz which has been heated above etches differently from ordinary quartz and is therefore called ;3-quartz, the un heated variety being a. Since quartz crystals have no plane or cen tre of symmetry, they can exist in two forms which rotate the plane of polarization of light in opposite directions and are related to one another as an object and its image.

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 con centrated 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 of approximate composition, The colloidal 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 salts, especially those of multivalent metals. (See CoLLoms.) The natural silicates may be regarded as fall ing into five classes, viz., orthosilicates, derived from Si(OH)4; metasilicates, from SiO(OH)2, disilicates, from Si203(OH)2; trisilicates, from 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 syn thetical production of these compounds, see also PETROLOGY.

Certain naturally occurring silicates, e.g., kaolin, and specially prepared amorphous silica, have the power of absorbing colour ing matters from oils and thus improving their appearance.

Solutions of sodium silicate containing excess of silica. are used as adhesives and possess great advan tages over ordinary adhesives in certain cases, e.g., sticking paper to metals.