Field book of common rocks and minerals

by Frederic Brewster Loomis

Silicon, Silica and the Silicates

Silicon is one of the non-metallic elements, and does not occur as such in Nature. When isolated it is either a dark-brown powder, or steel-gray crystals. However silicon is next to oxygen in its importance in making the crust of the earth. Forty-seven per cent of the surface rocks are composed of oxygen, and 28% of silicon, the latter appearing in a host of minerals. The oxide of silicon is termed silica (SiO₂), its crystal form being quartz, the commonest of all minerals. In non-crystalline form silica is also widely distributed, as chalcedony and opal, even appearing in the tissues of animals and plants, as in the feathers of birds, the shells of certain Protozoa (Radiolaria), the spicules of sponges; and in plants, as the shells of diatoms, and in the stalks of grasses, especially cereals and bamboo. Silica in the form of sand is widely used in making glass, porcelain, china, etc., and in the various cements.

Then there are a considerable number of acids of silicon, which do not occur in Nature, but their salts do, and make a host of minerals, which are known as the silicates, such as mica, feldspar, hornblende, etc. Either as quartz, or as silicates, silicon is represented in most all the igneous and metamorphic rocks and in many of the sedimentary rocks.

[Quartz]
SiO₂
[Pl. 30]

Occurs as hexagonal crystals, or in grains or masses; hardness 7; specific gravity 2.65; colorless when pure; luster vitreous; transparent on thin edges.

Quartz is not hard to identify. Its hardness and the crystal-form separate it from most all other minerals. It is the most common mineral, making 12% of the earth’s crust. The usual crystal form is a hexagonal prism with the sides horizontally striated, and a six-sided pyramid on one or both ends. This six-sided pyramid is really two rhombohedrons, a right-handed one and a left-handed one, so that the alternate faces of the pyramid may show peculiarities, for instance three may be large and three small, as in Fig. B, [Plate 30], or the alternate ones may be duller or etched in some manner. The crystals are clear and when pure colorless, but there is a tendency for some slight impurity to color them almost any hue.

The most perfect double-ended crystals form only where growth is possible in all directions, as in clay. In cavities and caves there is an opportunity for the crystals to grow in toward the open spaces, and in such places, one finds fine large crystals; the Alps, Brazil, Japan, and Madagascar being especially famous localities. The largest quartz crystal on record is one 25 feet in circumference which came from Madagascar. In this country the caves at Little Rock, Ark., have furnished some very fine large crystals. Smaller, but very clear crystals, come from about Herkimer, N. Y. Some of these have been used as “Rhine-stones” and as cheap imitations of diamonds. Clear quartz is beautiful enough to be a gem, but it is too common to interest people as jewelry, however many objects of art have been carved from it. One of these took the form of crystal balls, which, through the Middle Ages particularly, developed into a form of mysticism. The gazing into the crystal ball was supposed to give some people supernatural vision. It seems to be a form of hypnotism, gazing at the bright reflecting surface tiring the eye, and making possible visions, which are subjective rather than anything external.

Silica is slightly soluble in water, especially when it is alkaline; so that most river-, lake-, and sea-waters have some silica in solution, and are carrying it from one place to another. The waters, which percolate through the rocks, carry even more, and when they come out into open spaces, they give up some of the silica, making crystals lining these openings, whether fissures or cavities. Not infrequently these silica-bearing waters dissolve out some other crystal, and then deposit in its place silica, thus making a crystal which has the form of what was dissolved, rather than that of quartz. Such a form is known as a pseudomorph.

When molten masses of igneous rock were cooling the quartz crystals had their faces interfered with as they grew, and we have resulting crystalline quartz, simply filling in the spaces between the other crystals, such as feldspar and mica, in the granite. Quartz is a large component in many igneous rocks, also in metamorphic rocks, and certain sedimentary rocks like sandstone are almost wholly made up of quartz grains. Quartz is also the gangue mineral in many veins. In this case it seems to have been deposited from hot water or vapors, as they rose from cooling magmas. With it are associated all sorts of metallic ores as has been suggested.

Quartz has been largely used to make imitations of other much rarer minerals, sometimes in its crystalline form to imitate the diamond, at other times ground and made into a “paste,” which is colored to imitate other gems. This paste is a mixture of about 4 parts of quartz, 5 parts of red lead and 1 part of potassium carbonate, melted and cooled slowly. It is clear and has a brilliant luster like the diamond. If some coloring matter is put into it it can be used for rubies, sapphires, etc. When there is any reason to think that this is being used, it is easily detected by being so much softer than any of the true gems, and even than true quartz. Quartz will scratch glass readily, but this imitation has only the hardness of very soft glass, or about 5.