Hence we see that granite consists of a mixture of three substances, called quartz, felspar, and mica, the felspar being in greatest quantity. Each of these substances possesses properties more or less peculiar to itself, such as hardness, solubility in acids, specific gravity, crystalline form, way of splitting, etc. Hence, each of these substances has a definite chemical composition and constant physical properties which define them as minerals.
This definition may be understood to include such substances as coal and chalk, which are the mineralized remains of plants and animals respectively. Even water and gases of the atmosphere may be said to belong to the mineral kingdom of nature, as plants and their parts are said to belong to the vegetable kingdom, and animals and their parts to the animal kingdom.
CHIEF ROCK-FORMING
MINERALS
The total number of rock-forming minerals is very large, but many of them are very rare, and form but a very small part of the earth’s crust.
The most abundant materials or earths of which rocks are composed are silica, lime and aluminum. Silica or flint is very universally diffused. It is found almost pure in quartz, opal, chalcedony, rock crystal, and the flinty sand of the sea-shore. Lime is also a very generally distributed earth, and is usually found in the form of carbonate. Under the several names of marl, limestone, oolite, and chalk it constitutes mountains, and even ranges of mountains. Aluminum is likewise very abundant, and of great importance to mankind. It enters largely into the clayey or argillaceous earths, and forms part of various kinds of rock which possess the property of not permitting water to pass through its substance—a property which renders it of inestimable value both for natural and artificial reservoirs of water.
CHIEF CHEMICAL ELEMENTS WHICH
FORM MINERALS
The larger number of elements play so small a part in the constitution of the earth that they may be neglected by the geologist. The following list includes the elements of which ninety-nine per cent of the earth’s crust, as known to us, is composed, with their relative proportions, as indicated by Clarke’s laborious analyses of a very large number of typical rocks:
| Element | Chemical Symbol | Percentage of Earth’s Crust Which It Forms |
|---|---|---|
| Oxygen | O | 47.02 |
| Silicon | Si | 28.06 |
| Aluminum | Al | 8.16 |
| Iron | Fe | 4.64 |
| Calcium | Ca | 3.50 |
| Magnesium | Mg | 2.62 |
| Sodium | Na | 2.63 |
| Potassium | K | 2.32 |
| Hydrogen | H | 0.17 |
| Carbon | C | 0.12 |
| 99.24 |
The ten elements given above form 99.24 of the earth’s solid crust.