1.3.4 Classifying igneous rocks

To classify (i.e. to name) igneous rocks, geologists use three pieces of information in combination – the grain size and the identity and proportions of the minerals present. The identification of minerals in a rock relies on recognising their particular distinguishing features. Such features include colour, lustre (the way in which light is reflected from the mineral's surface) and shape. The way in which certain minerals break apart along preferred planes, a property known as cleavage, can also be useful in identifying minerals because this property depends on the way that the mineral's constituent atoms are arranged. In some minerals the atoms are bound more strongly in some directions than in others, in which case there are natural planes of weakness present in the crystal. The mineral tends to break preferentially along these planes.

One mineral that shows this feature clearly is mica, a silicate mineral containing potassium (K), iron (Fe), magnesium (Mg) and aluminium (Al), together with hydroxyl (OH) groups. These compositional details needn't distract us, but the important point is that the potassium ions occur in layers, which separate sheets of more tightly bound silicon, oxygen and other atoms. Mica therefore splits apart parallel to the sheets, so has just one set of cleavage surfaces parallel to each other (Figure 5a). This is why mica forms platy or flake-like crystals, rather like the pages of a book. Other minerals can have two or three sets of cleavages (which intersect at characteristic angles) whereas others, notably quartz, have no cleavage and break irregularly.

Figure 5 Some examples of cleavage shown by different minerals. Note that in each case the number of cleavage traces with different directions depends on which face of the crystal is being looked at. (a) Mica, one set of cleavages when viewed edge on. (b) Pyroxene, two sets of cleavages intersecting at about 90° when viewed end on. (c) Amphibole, two sets of cleavages intersecting at about 120° when viewed end on. Table 1 gives additional information about these minerals, including their true colours

The most common silicate minerals found in igneous rocks (most of which are also common in sedimentary and metamorphic rocks) are listed in Table 1: minerals rich in silicon appear towards the top, and those poorer in silicon but richer in magnesium and iron appear towards the bottom of Table 1.

The variation in the mineral content of igneous rocks is shown in Figure 6, and this diagram provides the means to classify igneous rocks. The percentage of each mineral present is represented on the vertical scale, and the range of rock type (effectively the chemical composition) is given on the horizontal scale. Coarsegrained rocks (typical grain size greater than 2 mm) are named separately from finegrained rocks (typical grain size less than 0.25 mm). Notice that the proportion of pale-coloured minerals (felsic minerals) increases from left to right, reflecting the increase in the concentration of silicon in the chemical composition of the rocks. According to the diagram, granite is defined as a coarse-grained igneous rock containing quartz, feldspar (both alkali and plagioclase feldspar), mica and sometimes amphibole. The relative proportions of these different minerals are given by the width of the appropriate band on the diagram. For instance the quartz content of granite can range from about 10% to 35%, and the mica content from about 5% to 15%.

Figure 6 Classification of common coarse- and fine-grained igneous rocks according to their mineral content. (Medium-grained rocks are omitted for simplicity.) Dark-coloured minerals are known as mafic minerals (they are rich in magnesium and iron (Fe)). Pale minerals are known as felsic minerals (fromfeldspar and silicon)