Silicification

“ In some polished slices of siliceous nodules, the transition from flint to agate, chalcedony, and crystallized quartz, is beautifully shown. The curious fact that the shells of Echinites In chalk are almost invariably filled with flint, while their crustaceous shells are changed into calc-spar, is probably in many instances to be attributed to the animal matter having undergone silicification; for the most organized parts are those which appear to have been most susceptible of this transmutation. ”

“ Silicic acid, see Silex.
— ether, Ebelman, xlix, 192.
Silicification of fossils, proceeding from thermal springs in Florida, C. U. Shepard, xxv, 165.
— of wood, xxxiii, 108.
—, remarks on, J. D. Dana, xlviii, 88, 397; xlix, 227.
Silicified wood, see Fossil.
Silicon, how obtained, ix, 377.
—, a process for obtaining, R. Hare, xxii, 189; xxiv, 247f; xl, 28.
—, production of, from paracyanogen, S. M. Brown, xli, 208.
—, on the alleged conversion of carbon into, xlii, 193.
Silk, on the manufacture of, in America, xviii, 382; xix, 175.
—, review of F. Pascalis on the culture of, xvii, 202 ”

“ Silex, or SILICA, abounds in flint, sand, sandstone, agate, jasper, &c.; it forms the basis of many precious stones, and particularly of those which strike fire with steel. It is rough to the touch, scratches and wears away metals; it is acted upon by no acid but the fluoric, and is not soluble in water by any known process; but nature certainly dissolves it by means with which we are unacquainted, and thus produces a variety of siliceous crystals, and amongst these rock crystal, 52 which is the purest specimen of this earth. ”

“ 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 compound which is in excess in all granite rocks is silica, the oxide of the element silicon. The varieties are formed chiefly by small percentages, more or less, of the oxides, alumina, and magnesia. This silica, or quartz, as well as the other components of the igneous rocks, is what has been termed "burnt material." It is the product of a most complete and tremendous conflagration; for the oxidation of silicon is as much and as powerful a combustion as the oxidation or burning of coal. ”

“ When required, the analysis is best made on the ignited residue, and separately reported as "analysis of the insoluble portion."
Silicon only occurs in nature in the oxidised state; but the oxide generally known as silica (SiO2) is common, being represented by the abundant minerals—quartz, flint, &c. Silica, combined with alumina, lime, oxide of iron, magnesia and the alkalies, forms a large number of rock-forming minerals. Most rock masses, other than limestones, contain over 50 per cent. of silica. The following are analyses of some of the commoner silicates ”

“ Silica is the gritty part of sand—it is also the basis of all fibrous vegetable matter—a familiar example of this is the ash which shows in packing, which has been burnt by the heat in steam; by a peculiar chemical treatment silica has been made into soluble glass—a liquid. 65 per cent. of the earth’s crust is composed of silica—it is the principal part of rock—pure white sand is silica itself—it is composed of an element called silicum combined with the oxygen of the air. ”

“ Silicon in the pure state is a dark brown powder; if ignited at a very high temperature it assumes a chocolate colour, which is supposed to be the allotropic condition, because it no longer burns when heated moderately in oxygen or air, and is not attacked by hydrofluoric acid. [Pg 162] The most interesting combination of silicon is the teroxide called silicic acid, silica (SiO3) . Silicon is next to oxygen so far as regards its plentifulness, and is found in the state of silica in nearly every mineral, but especially in rock crystal, quartz, flint, sand, jasper, agate, and tripoli. ”

“ Silica is composed of 21.3 parts of silicon with 24 of oxygen, it is that earth which constitutes a great part of almost every soil, and forms sand, sandstone, and flint, it enters largely also into the formation of granite and the other primitive rocks ”

“ Silica is the earth to which we have referred, as being susceptible of fusion when mixed with the oxide of iron. Silica, also, when mixed with the other earths, renders them more fusible than is its own mixture with oxide of iron. ”

“ Silica, sil′i-ka, n. silicon dioxide, or silicic anhydride, a white or colourless substance, the most abundant solid constituent of our globe, existing both in the crystalline and in the amorphous form, the best examples of the former being rock-crystal, quartz, chalcedony, flint, sandstone, and quartzose sand ”

“ Silicon never occurs in the free state, but combined with oxygen it forms silica, which constitutes more than one-half of the earth’s crust. The oxid of silicon occurs in the very common form of quartz, and likewise, as silicate of alumina, lime or magnesia. Silicon forms an essential part of many minerals, such as the feldspars, amphiboles, pyroxenes, and the micas, besides being an essential ingredient of many other minerals. ”

“ It would be hard to find any one less informed about such phenomena than myself; but of course I had heard of mineral springs so saturated with silica that the leaves and twigs which fell into them were turned to stone after a time. I dimly comprehended the process, how the silica replaced the vegetable matter, atom by atom, and the result was a duplicate of the object in stone. ”

“ If the vapor of silicon follows the general rule—that the density of gases is in proportion to their atomic weights—then it was but a fraction heavier than oxygen, and therefore not far below it in the atmospheric strata. The unceasing commotion of the elements would soon have brought this first cloud-mist of silicon into contact with oxygen, to which it has a strong affinity under high heat. Oxidized, and in molten drops of silica, or crystals of quartz, this new-formed material commenced its descent toward the centre of gravity—the first creation from the primordial elements. ”

“ They went bouncing by across Xanthus just as they'd bounced by us back in the Mare Chronium. I've a hunch how they work, too—this is for your information, Leroy. I think the crystal shell of silica is no more than a protective covering, like an eggshell, and that the active principle is the smell inside. It's some sort of gas that attacks silicon, and if the shell is broken near a supply of that element, some reaction starts that ultimately develops into a beast like that one. ”

“ We are dealing, as in the case of so large a part of the rocks and minerals of the earth’s surface, with certain silicates of the alkalis and alkaline earths, with silicates of alumina above all. All natural clays used for fictile purposes consist essentially of silicates of various bases, such as alumina, lime, iron, potash, and soda, more or less intimately combined with water, and with the addition, generally, of some free silica. If the clay be good in working quality and colour, the next point the potter has to look to is the question of its fusibility. ”

“ The solution of silica obtained by this method becomes gelatinous on standing, on being heated, or on evaporation under the receiver of an air-pump, &c. The hydrosol is transformed into the hydrogel, the soluble hydrate into the gelatinous.
Thus in addition to the gelatinous form of the silicic acid, there exists also a variety of this substance, soluble in water, as is the case with alumina. Such variation in properties and exactly the same relations with regard to water characterise an immense series of other substances having a great significance in nature. ”

“ Silica, or silicium combined with oxygen, is found abundantly in perhaps every igneous rock, and very commonly Is combined in definite atomic proportions with lime, alumina, &c., so as to form a peculiar class of compounds, called silicates, bi silicates, and trisilicates, according to the atomic proportion of silica in the mineral. ”

“ The nodules of silex, especially those found in the chalk, are not always homogeneous and solid. Sometimes there is remarked an organic form towards their centre, as a madrepore or a shell, which seems to have served as their nucleus; occasionally the centre is hollow, and its sides are studded over with crystals of quartz, carbonate of iron, pyrites, concretionary silex or calcedony, filled with pulverulent silica nearly pure, or silex mixed with sulphur; a very singular circumstance.
Flints are observed to be generally humid when broken immediately after being dug out of the ground ”

“ It might surprise you to hear that one of the commonest substances that is found in solution in the waters of many hot springs is silica; for silica is practically sand, and sand does not easily dissolve in water as does sugar. The very hot water, however, which comes from the hot spring, whose temperature below the earth's surface is very much higher than it is when it comes out of the spring, possesses the power of readily dissolving silica from the rocks over which it flows. ”

“ Again, cane sugar, which is composed of charcoal, oxygen, and hydrogen, crystallizes in hard transparent four-sided and irregular six-sided prisms, but is not called a salt. Silica or sand is found crystallized most perfectly in nature in six-sided pyramids, but is not a salt; it is an acid termed silicic-acid. Sand has no acid taste, because it is insoluble in water, but when melted in a crucible with an alkali, such as potash, it forms a salt called silicate of potash. ”

“ It constitutes the principal portion of most of the hard stones and minerals which compose the crust of the globe; occurring nearly pure in rock crystal, quartz, agate, calcedony, flint, &c. Silica or silicic acid may be obtained perfectly pure, and also in the finest state of comminution, by taking the precipitate formed by passing silicated fluoric gas through water, filtering, washing, and igniting it, to expel the last traces of the fluoride of silicon. ”

“ Nevertheless, it has been clearly demonstrated by observation, that silica, where it occurs in certain combinations with other mineral substances, may be dissolved readily enough: for instance, in the decomposition of felspar, and of all rocks in which felspar is an ingredient, silica is carried off in a state of solution.37 And since these rocks are very numerous, and distributed over every part of the earth, we may fairly conclude that a solution of silica exists very abundantly in nature. ”

“ Silica and Insoluble Silicates.—Weigh up another 2 grams of the dried sample, and place them in a platinum dish; moisten with water, and cover with 20 c.c. of sulphuric acid. Evaporate and heat gently to drive off the greater portion of the free acid. Allow to cool; and repeat the operation. Extract by boiling with dilute hydrochloric acid, filter, wash, dry, ignite, and weigh. The quantity of insoluble silicates is determined by dissolving out the separated silica with a strong boiling solution of sodium carbonate. The residue (washed, dried, and ignited) is weighed, and reported as "sand." ”

“ The silica, which is the principal ingredient of sand, as well as of nearly all the earthy minerals, is known as “rock crystal” in its naturally crystallized form. Colored of a delicate purple, these crystals are what we call “amethysts.” Silica is also met with in the “carnelian” and we find it constituting jasper, agate, cat’s-eye, onyx and opals. In the latter it is combined with water. Many natural waters present us with silica in a dissolved state, although it is not soluble in pure water. ”

“ The silicates are decomposed by this treatment and the solution is evaporated to dryness on the water-bath, and when dry slightly heated. The silicic acid (silica) is by this treatment rendered insoluble. It is collected on a filter, washed, ignited, and weighed. The resulting compound should be mixed with ammonium fluorid and sulfuric acid, and after the disappearance of the silica the residue should be dried and weighed. The loss in weight represents the true silica. The loss in weight should be corrected by calculating the sulfates of the alkalies back to oxids. ”

“ When the organic matter has disappeared, there remains only a frail and elegant lace-work. True, nothing is there except silica, but what is interesting is the form this silica has taken, and we could not understand it if we did not know the living sponge which has given it precisely this form. ”

“ These, which have the same affinity with their respective bases as rust has to iron, are silex, lime, barytes, and alumine. Until, however, some further light be thrown upon their nature and constitution, they must continue to hold their former situation of simple earths.
33. Silex, or Siliceous Earth, is the basis of all substances known by the name of quartz and silex (76) . In a state of nature it has never been found pure; but, in combination with other substances, it abounds in almost every country of the globe. ”

“ There can be no doubt that they were crystals of silica, however difficult we may find it to form them at pleasure, and the rarity of the occurrence only serves to prove that there are properties and relations of this substance with which we are as yet unacquainted. ”