Silica, (SiO₂)_n

With the exception of the little-known monoxide, the dioxide is the only oxide of silicon. It occurs naturally both in the free state and combined with metallic oxides, forming silicates, and is the most abundant substance in the crust of the earth.

Silica occurs not only in the mineral kingdom, but in the living tissues and structures of plants and animals. Thus silica is found in the straw of cereals such as wheat and oats, these plants having absorbed it in a combined state from the soil. It was at one time thought that the silica served to stiffen the straw, but it has been found possible to remove all of this substance from it without diminishing its rigidity. Silica is also found in palifi-leaves, and " Tabasheer," which occurs at the nodes of bamboos and consists largely of hydrated silica. Moreover, the skeletons of the microscopic plants known as diatoms constitute the siliceous earth Kieselguhr, which is used as a basis for dynamite and for packing purposes.

In the animal kingdom silica is found in living tissues, and is contained to the extent of 40 per cent, in the feathers of birds. Sponges are the chief marine organisms in which silica occurs. In these it is found in the shape of minute and beautiful spicules, which have been formed by the organisms during cell-production, and may be regarded as the result of definite growths of protoplasm. The beautiful Venus's Flower Basket is a delicate siliceous skeleton, which formed the greater part of the sponge.

It is suggested by Emerson Reynolds that these occurrences of silica show that silicon is able to play the part of an "organic element"; that whilst carbon is the element of the world of life, and silicon the chief element of the inorganic crust of the earth, these elements overlap in their functions, so that silicon can replace carbon to a limited extent in organisms; and that it is possible that at a much higher temperature a silicon protoplasm analogous to carbon protoplasm might form the basis of living organisms.

Silica occurs in the crust of the earth in two distinct crystalline forms, known respectively as quartz and tridymite; a third crystalline form is also recognised, and known as cristobalite.

Further, these three crystalline forms exist in a and (3 varieties; and the transition temperatures between the six forms have been determined by Fenner from the heating curves to be as follow:

α-quartz (tetartohedral hexagonal) ⇔ β-quartz (hemihedral hexagonal), 575° C.

β-quartz ⇔ β-tridymite (holohedral hexagonal), 870° + 10° C.

β-tridymite ⇔ β-cristobalite (cubic), 1470° ± 10° C.

On cooling, β-tridymite and β-cristobalite quickly pass at the following temperatures into metastable forms possessing lower optical symmetry:

β-tridymite ⇔ α-tridymite (biaxial, perhaps orthorhombic), 115°-120° C.

β-cristobalite ⇔ α-cristobalite (biaxial), 180°-270° C.

According to Endell and Rieke1 quartz and amorphous silica are converted into cristobalite by heating them above 800° C.; and Smits and Endell find that β-cristobalite, which is stable between 1685° C. and 800° C., is converted by slow cooling in presence of mineralisers into β-quartz, which in turn changes into a-quartz at 575° C. Under ordinary conditions of cooling, however, β-cristobalite persists below 800° C. and passes at 230° C. into α-cristobalite. The whole subject of the relations between the different crystalline forms of silica is still, however, in an uncertain state.

Quartz is by far the most plentiful form of natural silica; indeed, next to the felspars, it is the most abundant mineral in the earth's crust, for it has been estimated that about 12 per cent. of the entire lithosphere consists of quartz.

The purest form of quartz is known as rock-crystal, a name which originally implied its supposed formation from water by intense cold amidst the Alps. Rock-crystal is colourless and transparent, but other forms of quartz are coloured with traces of metallic oxides or organic matter. Thus amethyst is quartz coloured violet with oxide of manganese; jasper is an opaque, red variety of quartz containing ferric oxide; smoky-quartz contains organic matter; cairngorm is a yellow variety of quartz; besides which there are rose-quartz, coloured possibly with titanium oxide, and milky-quartz, which is opalescent.

Chalcedony is a variety of quartz, which may be called crypto-crystalline; it is colourless or white, being transparent or translucent. Varieties of chalcedony are: carnelian, which is red; sard, brownish-red; chrysoprase, apple-green; agate and onyx, banded and variegated; sardonyx, a kind of onyx containing carnelian or sard.

Flint and chert are mixtures of quartz and amorphous silica of aqueous and organic origin.

Quartz occurs as an essential part of granite and other primary rocks, of which it is the acidic constituent, and where it has solidified in the free state after the bases have combined with their full complement of silica. It is also formed as a secondary deposition after being removed in solution from the primary rocks, and thus it occurs in veins which may be of aqueous or igneous origin and auriferous, as well as in crystals attached at one end to other rocks or lying free in clay or gypsum. Such crystals may vary greatly in size; they may be almost microscopic, or they may be even a yard in length. Frequently, also, quartz crystals enclose other minerals such as rutile or haematite, or have within them cavities containing water, saline solutions, or petroleum. Immense masses of crystalline silica occur also as quartzite, which is often snowy-white, and has been formed from loose fragments of quartz cemented together and rendered homogeneous by percolating water. Sand in its purest form, i.e. "silver sand," consists of fragments of quartz remaining over from granite after its disintegration; and sandstone is composed chiefly of quartz grains cemented together by some binding material, but nevertheless porous.

Physical Properties of Quartz

Enantiomorphous quartz crystals.

Quartz crystallises in the hexagonal system. Superficial examination of a well-formed quartz crystal shows it to possess prismatic and pyramidal faces; sometimes, though rarely, the prismatic faces are entirely absent; generally the pyramidal faces are unequally developed, three alternate faces being large, and often almost crowding out the other three. The three alternate pyramidal faces are the faces of a rhombohedron, which is a hemihedral form derived from the hexagonal pyramid; and a quartz crystal really consists of two such hemihedral forms combined. It is therefore said to belong to the rhombohedral division of the hexagonal system. Another feature of quartz crystals is the horizontal striae on their prism faces, and a further one the frequent presence of other small faces which lie between the pyramid and prism faces on alternate corners of the crystal. These small faces cause the crystals to appear asymmetric, i.e. to be without a plane of symmetry; and there are two kinds of.such asymmetric crystals, which are related to one another as object and image, or as right and left hand. The crystals are therefore said to be enantio- morphous, and are conveniently called right- and left-handed crystals (see Fig.)

Now enantiomorphous crystals are known amongst organic compounds, and are there associated with optical activity of the compounds, as, for instance, in the case of the tartaric acids. Quartz, too, is optically active, i.e. it rotates the plane of polarisation of abeam of polarised light that passes through it in the direction of the principal axis; and the opposite crystals are opposite in their optical activity. In the case of organic compounds optical activity is a molecular property which persists when the compound is in the state of fusion or solution; it arises from the presence within each molecule of one or more " asymmetric " carbon atoms. In the case of quartz, however, optical activity is a property of the crystal rather than of the molecule of silica; it depends upon the way in which these molecules are built up within the crystal.

A section of quartz, 1 mm. thick, cut perpendicular to the principal axis of the crystal, rotates the plane of yellow (D) light through 22° and of blue (G) light through 43°.

Such a section shows interference colours in the polariscope.

The refractive index of quartz for sodium (D) light μ₀ = 1.544147; its density at 0° C. is 2.6507, and falls to 2.6409 at 100° C.; its hardness on Moh's scale is 7; its specific heat from 0° to 400° C. is given by the formula:

γ_t = 0.1737 + 0.000394t – 0.0₇27t³;

between 400° C. and 1200° C. it becomes constant = 0.3.

Quartz melts in the oxyhydrogen flame (1600°-1750° C.), producing a glass whose density at atmospheric temperature is 2.20; it begins to vaporise between 1700° C. and 1750° C. Before reaching its melting- point, however, it undergoes modification. Above 575° C. ordinary quartz, which may be called α-quartz, undergoes a reversible change into β-quartz, which differs from the former in optical properties and in the kind of etching produced upon it by hydrofluoric acid; whilst above 800° C. quartz is converted into tridymite. These facts throw some light on the temperature conditions of formation of quartz and tridymite.

Artificial Production of Quartz

Many experiments have been made upon the production of quartz in the presence of water, and in the dry way. Chrustschoff, basing his experiments on previous work of Schafhautl and Senarmont, obtained quartz by heating an aqueous solution of colloidal silica to 250° C. for some months. Also he added hydrofluoboric acid to the silics solution and obtained crystals of cristobalite at 180° C. to 228° C., quartz at 240°-300° C., and tridymite at 310°-360° C. Quartz has also been obtained by heating gelatinous silica with caustic potash and alumina, and with ammonium fluoride. Allen produced fine crystals of quartz by heating a mixture of magnesium ammonium chloride, sodium metasilicate, and water at 400°-450° C. for three days in a steel bomb.

Methods for the production of quartz in the dry way include the heating of amorphous silica with sodium or lithium tungstate to 750°, and of quartz glass to 700°-750° C. in the vapours of alkali chlorides; but in most of the attempts to obtain quartz in the dry way tridymite was formed because the temperature attained was above 800° C., the transition temperature of quartz into tridymite.

Uses of Quartz

Quartz has been valued from the earliest times on account of its transparency and hardness. It is employed in the form of gems and ornaments, for making spectacle-glasses (from "pebbles"), lenses and prisms, and unalterable chemical weights; whilst the commoner kinds are used for cutting and polishing purposes and in the manufacture of porcelain and glass. Agate is employed for making mortars and pestles for grinding hard solids, and for the bearings of chemical balances.

A recently developed and important use of quartz is for the manufacture of quartz glass. Transparent quartz of good quality is fused in an electric furnace, and the liquid worked like glass into various forms of chemical apparatus or drawn out into threads of extraordinary delicacy, which on account of their strength are employed for suspensions in galvanometers and other instruments (Boys). Quartz glass has several advantages over glass containing basic oxides. Its coefficient of linear expansion is only 0.000000449; or, according to Harlow, the cubical coefficient of expansion of fused silica is 99.8×10^-8 for the temperature interval 0°-100° C.,and 144.7×10^-8 for the interval 0°-184° C. In consequence of this very small coefficient of expansion, quartz glass can be heated to a red heat and plunged into water without fracture; it may therefore be employed in the distillation of valuable liquids and of sulphuric acid, as well as for thermometry; whilst the fact that it contains no alkali to be extracted by water makes it of service in chemical analysis. The density of quartz glass is 2.21, its hardness 5; its refractive index for the D line, μ_D = 1.45848.

A non-transparent kind of silica glass, known as "vitreosil," is prepared by passing a powerful electric current through a carbon rod or plate embedded in sand. The sand fuses round the carbon, and when the latter is removed an object of translucent silica remains, which may be glazed externally by subsequent fusion or worked on a lathe. Vitreosil is employed for making crucibles, dishes, combustion tubes, plates, and other articles.

Tridymite (τριδυμος, threefold) is a second crystalline form of silica, discovered by G. von Rath, which occurs in trachyte and other volcanic rocks. It forms hexagonal plates which are combinations of prisms and pyramids and probably orthorhombic; frequently it is found in trillings or triplets, whence its name is derived. That tridymite has been formed at high temperature is shown by its artificial production under these conditions. Indeed G. Rose observed the conversion of quartz into tridymite by simple ignition, and Brun found that quartz glass is converted into tridymite when heated between 800° C. and 1000° C. in the vapours of alkali chlorides. Tridymite can be conveniently prepared by fusing sodium silicate with three times its weight of sodium phosphate for six hours at 1000° C., extracting with water and washing. The residue consists of microscopic hexagonal tablets of density 2.310. As stated under quartz, 800° C. is the transition temperature, below which quartz is stable, above which tridymite. Quartz and tridymite are both polymeric forms of SiO₂, but tridymite has probably the simpler molecule of the two, and so is more stable at high temperatures.

The density of tridymite is 2.30, its hardness 7, its refractive indexμ_D = 1.476, its melting-point about 1625° C.

Cristobalite, another crystalline modification of silica described as pseudo-cubic, may be obtained by heating powdered transparent silica glass in a porcelain furnace. It has a density of 2.319, and is distinguished from tridvmite by becoming isotropic when heated for a short time at 250° C.

Amorphous silica occurs naturally as opal and hyalite. Opal is colourless or variously coloured, has a vitreous fracture, a density of 2.2, and contains a proportion of water which ranges between 2 and 13 per cent. There is no doubt that opal is of a colloidal nature, and has been formed by the drying up of gelatinous silica; for an amorphous mass, practically identical with opal, can be produced from the colloidal silicic acid separated from an alkali-silica solution by strong acid. Moreover, the siliceous sinter deposited in the neighbourhood of hot siliceous springs is a kind of opal; and at Plombieres, in France, opal has been formed by the action of water on Roman cement. Precious, iridescent opal has been formed in the cavities of igneous rocks by the action of hot water.

Artificial amorphous silica is prepared by decomposing silicon tetrachloride with water or an alkali silicate with acid. When, for example, concentrated hydrochloric acid is added to a solution of soluble glass, which is sodium metasilicate, Na₂SiO₃, "gelatinous silica" or metasilicic acid separates. The mass is carefully evaporated and ignited, and the silica is thereby rendered insoluble by a process of polymerisation which accompanies dehydration. The residue is then digested with dilute hydrochloric acid to dissolve oxide of iron or other impurity, as well as sodium chloride, and finally washed with water, dried, and ignited.

Thus obtained, amorphous silica is a white, mobile powder of density 2.22 and melting-point 1750°-1780° C. When it is strongly heated for some time the density gradually rises owing to the formation of tridymite.

The different forms of silica differ in chemical reactivity. This is shown by heating each variety finely powdered, with 1 per cent, hydrofluoric acid for an hour. Under these conditions 5.2 per cent, of the quartz dissolves, 20.3 per cent, of the tridymite, 25.8 per cent, of the cristobalite, and 52.9 per cent, of the amorphous silica. All kinds, however, are insoluble in water and all acids except hydrofluoric acid, which forms silicon tetrafluoride according to the reaction:

SiO₂ + 4HF = SiF₄ + 2H₂O.

If pure silica is mixed with excess of aqueous hydrofluoric acid in a platinum dish and the liquid is evaporated, the tetrafluoride passes away with the steam. Since, however, water hydrolyses silicon tetrafluoride, it is necessary to add a little concentrated sulphuric acid to prevent this action, and then the whole may be vaporised without residue. Quartz crystals are slowly etched by hydrofluoric acid, their different faces showing different characteristic markings. Dried gelatinous silica reacts with molten metaphosphoric acid to form a silicophosphoric oxide, but silica does not displace phosphoric oxide from a metaphosphate; hence particles of this substance float in a bead of fused microcosmic salt. Amorphous silica dissolves in solutions of caustic alkalis, which, however, scarcely act on quartz. Silica displaces carbon dioxide from sodium carbonate when it is fused with this salt, thereby forming a soluble glass. A convenient test for silica consists in introducing it into a sodium carbonate bead on a loop platinum wire. When sodium carbonate is fused alone it crystallises, and becomes opaque on cooling; when silica is fused with it the bead on cooling appears as a transparent glass. Hydrated amorphous silica dissolves in alkali carbonate solutions; this fact accounts for the appearance of this substance in spring waters, and furnishes the means by which silica circulates in nature.

Silica is reduced when heated with potassium, calcium, or magnesium, forming silicon and a metallic silicide and sometimes silicate; it appears, moreover, that silicon monoxide is produced when silica is heated electrically in an inert atmosphere with sufficient carbon or carborundum to remove half its oxygen. When silica is mixed with carbon and heated in hydrogen sulphide, silicon disulphide (q.v.) is formed.

The reduction of silica by hydrogen at 1350°-1400° C. has been studied by von Wartenberg.

It has already been seen that hydrated silica exists naturally as opal, and is formed artificially by decomposing silicon tetrachloride with water, or soluble glass with hydrochloric acid.

Theoretically two varieties of monosilicic acid are derivable from SiO₂: orthosilicic acid, Si(OH)₄, and metasilicic acid, SiO(OH)₂. That these simple formulae represent actual chemical compounds, however, cannot be affirmed, though products closely corresponding to them in composition may be obtained by suitable means.

Thus, if the gelatinous product of the action of water on silicon tetrachloride is washed with benzene and dry ether and then quickly dried at atmospheric temperature, the amorphous white powder obtained corresponds closely to the composition of orthosilicic acid, H₄SiO₄, but it rapidly loses water when left in the air.

A product corresponding approximately to the formula H₂SiO₃, and which is therefore called metasilicic acid, is obtained in the following way.

A dilute solution of soluble glass or sodium metasilicate, Na₂SiO₃, is acidified with dilute hydrochloric acid. No precipitate is formed, but a form of silicic acid is present in the clear liquid. This liquid is then submitted to dialysis. It is placed in a dialyser - that is, a flat drum with a bottom consisting of parchment paper - which is floated on the surface of running water in a large dish. Parchment is a "semipermeable membrane" through which the small molecules or ions of "crystalloids," such as hydrochloric acid and sodium chloride, can pass, but which the molecules of "colloids," of which silicic acid is an example, are too large to penetrate. Thus the sodium chloride and hydrochloric acid are washed away, and pure, dialysed silicic acid remains. Pure aqueous silicic acid may also be conveniently obtained by bringing silicon tetrachloride vapour into water through mercury and dialysing the solution. Such an aqueous colloid is called a "hydrosol" or "sol"; it is not a true solution, but a suspension of ultra-microscopic particles. It, however, possesses a faintly acid reaction, and therefore contains some silicic acid in true solution. According to Ebler the specific conductivity of a solution of silicic acid obtained from 40 grams of silicon tetrachloride and 2000 c.c. of water is K_18° = 1.7×10^-5 after dialysing for twenty-two days. Aqueous silicic acid is colourless and tasteless, and may be concentrated by boiling until it reaches a strength of 14 per cent. It is inclined, however, to gelatinise during evaporation, and when the concentrated liquid stands for a few days the whole is converted into a transparent jelly, the "hydrogel" or "gel." Thus the hydrosol and the hydrogel are two forms of the same product, the former being converted into the latter by catalytic agents. The structure of silicic acid "gel" has been elucidated by Zsigmondy. It serves as a medium in which certain inorganic salts may be crystallised.

If the sol of silicic acid evaporates over sulphuric acid at atmospheric temperature in a vacuum, a transparent glass remains which corresponds very nearly to the formula H₂SiO₃; and a product of similar composition is obtained by washing precipitated "gelatinous silica" with 90 per cent, alcohol.

Nevertheless it cannot be said that definite compounds corresponding to the formulae H₄SiO₄ and H₂SiO₃ exist. Since silicic acid is colloidal its molecules must be large, and the formulae mSiO₂.2nH₂O and mSiO₂.nH₂O, where m and n are approximately equal, large whole numbers more nearly represent the facts. Tschermak, who has studied the velocities of dehydration of silicic acids derived from various minerals, has shown that the curves indicating these velocities change abruptly in direction at certain compositions; this, it is claimed, indicates the existence of definite hydrates of silica. Recent research, however, has thrown doubt on the existence of any definite hydrate of silica. Hydrated silica performs the functions of a very weak acid. Its only soluble salts are those of the alkali metals, and these are hydrolysed in aqueous solution into alkali hydroxides and colloidal silicic acid; this is proved by determination of the freezing-points and electric conductivities of the solutions. Hydrolysis of the silicates M₂SiO₃ and MHSiO₃ according to the schemes

M₂SiO₃ + H₂O ⇔ 2M^• + 2OH' + colloidal silicic acid
MHSiO₃ + H₂O ⇔ M^• + OH' + colloidal silicic acid

is practically complete at a dilution of 48 litres, whilst silicates of the type M₂Si₅O₁₁ need to be diluted to 128 litres for complete hydrolysis. It may be concluded from this that in natural waters containing alkali silicate in very dilute solution the silicic acid is always present in the free, colloidal state.

Very weak acids form ammonium salts with difficulty, and silicic acid shares with stannic and a few other acids the inability to form an ammonium salt. Hence when an ammonium salt is added to sodium silicate solution gelatinous silicic acid is at once precipitated.

The heats of reaction of silicic acid with different amounts of sodium hydroxide have been measured by Thomsen. These are very small, as is consistent with the weakness of the acid and its very imperfect reaction with the base. The following is one of the values obtained by Thomsen at 18°-20° C., the dilution being with 200 molecules of water:

H₂SiO₃ + 2NaOH = Na₂SiO₃ + 2H₂O + 5230 calories.

The equilibrium between carbon. dioxide and silica when competing for the same base as in the reaction

M₂SiO₃ + CO₂ M₂CO₃ + SiO₂

has been studied by von Wittorf, who heated alkali carbonates with silica to temperatures ranging between 800° C. and 1300° C. in an atmosphere of carbon dioxide, the pressure of which was regulated between 0.07 and 1.0 atmosphere. It was found that the power possessed by silica to expel carbon dioxide increased with temperature, but that the direction of the reaction depended upon whether the pressure of the superincumbent carbon dioxide was increasing or decreasing. The proportion of carbonate present under similar conditions of temperature and pressure increased with rise of atomic weight in the series Li₂CO₃, Na₂CO₃, K₂CO₃, Rb₂CO₃, Cs₂CO₃.

These conclusions are in accord with the principles of chemical dynamics; CO₂, considered as a molecule, no doubt has a greater affinity for bases than SiO₂, but the great difference in physical properties between these two substances modifies the influence of chemical affinity. Thus, whilst silica displaces carbon dioxide from a carbonate at high temperature, especially if the expelled carbon dioxide is allowed to escape freely, because the latter is volatile whilst the former is not, carbonic acid displaces silicic acid from aqueous solutions of its salts, not only on account of superior affinity, but because of the insolubility of the latter. These considerations have a terrestrial significance. Thus the igneous rocks contain silicates rather than carbonates; for if atmospheric carbon dioxide was present when they were formed, it could not successfully compete with silica at the temperature of their formation. Since, however, the cooling of these rocks and the advent of water on our globe, atmospheric carbon dioxide has been perpetually disintegrating natural silicates, setting free silica, and forming carbonates instead.