Minerals

Rocks are aggregates of substances called minerals, which as a rule are crystalline solids with fairly definite compositions and structures. Some rocks, for instance limestone, consist of a single mineral only, but the majority consist of several minerals in varying proportions. The different minerals in a coarse-grained rock like granite are apparent to the eye; in fine-grained rock, the separate minerals can be discerned with the help of a microscope.

What Minerals Are

It is not difficult to understand why certain substances occur as minerals and why others do not. We expect to find the more chemically inactive elements, such as gold, platinum, and sulfur, in the free state, whereas chemically active elements, such as sodium, calcium, and chlorine, are always found in combination as compounds. Compounds readily soluble in water, such as sodium chloride, sodium carbonate, and potassium nitrate, form deposits in desert regions but are rare elsewhere. Substances that tend to react with oxygen occur only well below the surface away from the oxygen of the atmosphere. Unstable compounds like phosphorus pentoxide are necessarily absent from the earth’s crust.

Silicates are by far the most abundant minerals; mica, feldspar, and topaz are familiar examples. Carbonates are another important class, its most conspicuous representative being the carbonate of calcium called calcite. Oxides and hydrated oxides include such common materials as hematite (ferric oxide), the chief ore of iron, and bauxite (hydrated aluminium oxide), the chief ore of aluminium. Various metals are obtained from deposits of sulfide minerals, such as galena (lead sulfide and sphalerite (zinc sulfide). Elements that occur free, or native, were mentioned above. Less frequent as minerals are sulfates, phosphates, and chlorides.

Unfortunately the study of minerals requires the learning of a special list of names, some of them apparently duplicates of other names. As an example, the mineral whose formula is CaCO₃ is given the name calcite instead of the chemical name calcium carbonate. For this seeming redundancy there are two reasons:

The formula CaCO₃ describes not only the composition of calcite but also that of aragonite, a less common mineral with a different crystal form, hardness, density, and so on; the chemical name calcium carbonate alone does not distinguish between calcite and aragonite.

Calcite often contains small quantities of MgCO₃ and FeCO₃ , and its composition is not precisely represented by the formula CaCO₃ because the iron and magnesium carbonates form an integral part of the calcite structure with Fe and Mg atoms replacing some of the Ca atoms in the crystal lattice.

Many other mineral formulas besides that of calcite apply to two or more distinct substances, and most minerals show a similar slight variability in composition. Hence chemical names are seldom really applicable, and the student of minerals finds necessary a new nomenclature.

Luckily, for present purposes we need only a few additions to our vocabulary. More than 2,000 different minerals are known, but most of these are rare. Even among the commoner minerals, the greater number occur abundantly only in occasional veins, pockets, and layers. The number of minerals that are important constituents of ordinary rocks is surprisingly small, so small that acquaintance with less than a dozen is adequate for an introduction to geology.