Geology

Next to oxygen, silicon is by far the most abundant constituent of the earth’s crust, though never occurring alone. It is united with oxygen to form silica (Si0₂) or enters into the formation of more complex compounds. The oxide, silica, is the commonest mode of occurrence and forms the most abundant of all the minerals.

Quartz (Si0₂)

Quartz is anhydrous silica in a crystalline state; it belongs in the hexagonal system, and crystallizes in hexagonal prisms capped by six-sided pyramids, or in double six-sided pyramids, or in modifications of these forms. It is insoluble in any acid except hydrofluoric, and only very slowly soluble in boiling caustic alkalis. When dissolved, as may be done by somewhat complicated processes, silica shows a distinct acid reaction.

Quartz has no cleavage and is very hard (H = 7), scratching glass readily, while it cannot be scratched with a knife; the specific gravity (sp. gr.) is 2.6.

When pure and symmetrically crystallized, quartz is transparent, colorless, and lustrous (rock crystal), but it more commonly is found in dull masses. Different varieties are colored by metallic oxides: thus, amethyst is quartz stained purple by’ the oxide of manganese; smoky quartz, or cairngorm, owes its brownish or yellowish color to the oxide of iron; and there are many other kinds.

In its various forms quartz is the most abundant of minerals, and plays the most important part in the formation of the different classes of rocks.

Chalcedony

Chalcedony occurs in spheroidal or stalactitic masses, composed of more or less concentric shells. The structure is crystalline, and displays radiating fibers, which are perpendicular to the shells. The chemical composition and behavior of this mineral are the same as in quartz, but the specific gravity is somewhat lower (2.59-2.64), and the optical properties different. Chalcedony has a waxy appearance, and is translucent or semi-opaque, and of various pale colors.

Opal, Hyalite

Hydrated silica (Si02, xH20). These minerals are amorphous and have no crystalline form. Opal is either translucent or opaque and of various colors. Precious opal (a gem) and common opal differ in color, and in the fact that the former is iridescent, the latter not. Hyalite is colorless and transparent. Hydrated silica is lighter than the anhydrous (sp. gr. = 2.2) and more readily soluble in hot alkaline waters. These minerals are of much less importance as constituents of rock than the forms of quartz.

Agate

Agate is a banded mineral, composed of layers of amorphous and crystalline silica, chalcedony, jasper, amethyst, rock crystal, etc.

Flint and Chert

Flint and Chert are also believed to be mixtures of hydrated and anhydrous silica. They occur in amorphous masses of neutral or dark colors, and are opaque, or somewhat translucent in thin pieces.