The classification of minerals. The classification of some important minerals by chemical composition

The most convenient and widely used way of classifying minerals is on the basis of their chemical composition. Figure 5.9 lists the principal mineral groups with some common examples of the minerals in each. Within each group the minerals have generally similar physical properties : many sulphides are grey or yellow with a metallic lustre ; oxides are hard, dense and generally dark coloured, while hydroxides occur commonly as earthy-looking, non-crystalline (amorphous) masses of low density ; and most chlorides, sulphates and carbonates are translucent or glassy (vitreous) and quite soft (easily scratched).

5.9: The classification of some important minerals by chemical composition (with symbols or formulas).

Note. The chemical compositions of many minerals, notably the silicates are given in simplified form. Hydroxides are defined as those minerals in which the principal anion component is the hydroxyl ion, OH^-. Some other mineral groups, especially the silicates, have hydroxyl ions as a component of complex polyatomic anions, and these are sometimes also called hydrous minerals.

This community of physical properties can include similarity of crystal form and cleavage. When crystalline substances with analogous chemical formulas exhibit similar crystal forms they are said to be isomorphous. The carbonates of iron, magnesium and calcium (and manganese), for example, all crystallize in the trigonal system and have rhombohedral cleavage. Similarity of chemical composition is not always accompanied by isomorphism, however. The minerals sylvite (KC1) and halite (NaCl), for instance, crystallize in the orthorhombic and cubic systems respectively.

Conversely some minerals have quite different crystal forms and symmetries, but identical compositions ; they are polymorphous. Probably the best-known examples are the polymorphs of carbon (diamond and graphite) and of calcium carbonate (calcite and aragonite). As a general rule the denser polymorphs will form under conditions of higher pressure and/or lower temperature than the less dense polymorphs. Thus diamond can only form at pressures equivalent to depths of around 1 50 km below the Earth's surface, which accounts for the scarcity of natural diamonds : they are brought to the surface by volcanic activity that has to originate at considerable depths and is therefore rather exceptional.

Diamond is metastable at the Earth's surface, but I fortunately for industry and jewellers), its rate of inversion to the low-density polymorph, graphite, is infinitesimal. Aragonite. however, the dense polymorph of calcium carbonate, inverts to calcite over measurable periods of geological time ; aragonite is rare in rocks older than about fifty million years.

Most of the mineral groups listed in Figure 5.9 can be considered as products of simple chemical reactions, and that is how mineralogists thought of them until well into the present century. Oxides and hydroxides are easily understood in these terms, and the chemical formulas of many of the other minerals look like those of salts resulting from acid-base reactions (eg NaCl—halite, and CaS0₄—anhydrite). However, the variety of compositions within and between the silicate subgroups is too complex for them to be satisfactorily categorized as the products of reactions with recognizable silicic acids. It required the development of X-ray crystallography before a proper understanding of the silicate minerals could be achieved.