Mineral Guide > Quartz and the Silicates

Quartz and the Silicates

Comparatively few persons associate the gem opal, with its brilliant internal colored reflections, with that material forming so large a part of the soil, sand. Yet the two are almost identical in composition. The mineral constituent of sand and of opal is quartz, though the latter often contains in addition some water.

Quartz is composed of the two elements occurring the most abundantly in the earth's crust, silicon and oxygen, both non-metals. As already indicated, the most common representative of the mineral substance is the sand of the soil. The sand grains are generally so eroded by the atmosphere and surface waters as to show little of the true quartz structure. As studied by means of the rock crystal, quartz is remarkable for its transparency, its regular crystal form, and its great degree of hardness. Its transparency is such that printing may be read through the crystal. Its crystalline form affords an unfailing means to the mineralogist of recognizing the substance as quartz.

Quartz has an economic value directly in glass sand, and, of course, as a soil constituent. In the latter capacity it is taken up by many plants, and is the silica that studs the saw edges of the blades of sedges and grasses. The precious stones, agate, amethyst, and jasper, are varieties of quartz.

The silicon that is so important a constituent of quartz composes with aluminum a large part of various minerals comprised under the name feldspar. This substance is slightly less hard than quartz and has many variations in color, but, unlike quartz, shows regular cleavage faces. Feldspar is always crystalline, but good crystals are not common. It is very difficultly soluble, yet readily yields to the influence of weathering.

Asbestus picture
Cats Eye Quartz Polished picture
Feldspar picture
Garnet picture
Opal picture
Quartz Crystal picture
Rutilated Quartz Polished picture
Small Garnets in Rock picture
Smoky Quartz picture