The inspiration for this article came a couple of weeks ago when collecting minerals in the famous Erzgebirge Mountains in Germany. Looking around it was very clear that this was a land where igneous, metamorphic and sedimentary rocks all occur closely together. This set us thinking about the relationship between these different types of rock and how they are all related to one another. Taking our thoughts further, the inescapable conclusion is that igneous rocks ultimately give rise to all other types of rock. Thus sedimentary rocks are made of igneous rocks ground up and redeposited elsewhere, and metamorphic rocks are sedimantary or igneous rocks altered from their initial state by heating or pressure.

Since igneous rocks seem to be the precursors of every other kind of rock, what is the origin and composition of these rocks?

The answer to the first question is magma. When the magma rises from the earth`s interior it cools and solidifies. No one knows what magma looks like, nor indeed where it comes from, because it either solidifies into rock underground, or spills out through cracks in the surface, by which time it has altered to become lava. Consequently all that we know about its nature has been deduced from the rocks that have been formed by its cooling and solidifying. One thing is certain, that magma, and consequently the rocks that it forms contain silica. On cooling, the silica can bond with itself in a number of ways, and more importantly, enter into combination with metallic elements which happen to be present in the original mix.

This gives an enormous variety of possible combinations, which are reflected in the large number of different kinds of igneous rocks. Originally, before the understanding of their chemical and physical structure any small difference was enough to encourage the creation of a new name. All that was needed was a limited knowledge of Greek and Latin, to come up with linguistic disasters like Katzenbuckelite. Nowadays, as understanding has grown, the trend has been to cull individual names, replacing them with families of igneous rocks based on their chemical composition and texture, which is now known to reflect their cooling history.

A hundred years ago, Norman Bowen achieved this classification by bridging between geology and physical chemistry. His pioneering work on the crystalisation of silicate melts is still accepted as the general model for the formation of igneous rocks. There are some exceptions, but the rule has stood the test of time for almost 100 years. Bowen discovered that very specific minerals form at very specific temperatures as a silicate melt cools. At higher temperatures, associated with melts containing relatively high amounts of the heavier elements the general sequence of minerals can be separated into two branches.

The Continuous Branch describes the solid-solution series of the Plagioplase Feldspars as they evolve from being calcium-rich to sodium-rich. The general formula for the series ranges from NaAlSi3O8 in Albite to CaAl2Si2O8 in Anorthite. All are Framework, or Tectosilicates, have a triclinic structure and are the most important rock-forming minerals.

The Discontinuous Branch describes the sequential formation of olivine, pyroxene, amphibole, and biotite. Bowen found that at a certain temperature the melt might produce Olivine, but if that same melt was allowed to cool further, the Olivine would "react" with the residual magma, and change to the next mineral in the series, Pyroxene. Further cooling, would in turn give rise to Amphibole, and eventually to Biotite.

At the head of the group is Olivine, which is actually another solid-solution series of minerals ranging from Forsterite, with the formula Mg2SiO4, to Fayalite, Fe2SiO4. The end minerals, together with intermediates, are independent tetrahedral, or nesosilicates. Forsterite is the most common member of the Olivine series, but they all look very similar and are characterised by the olive-green colour that gives the group`s name. All form orthorhombic crystals, usually of prisms or biprisms. The crystals often group to form sugary grains. Olivine is a very important rock-forming mineral found in igneous rocks such as Peridotite and Basalt. A gem variety of Olivine is known as Peridot.

Pyroxine, is similarly, one of a group of very closely related minerals which resemble the later Amphiboles, but have an entirely different cleavage angle very close to 90 degrees. The minerals of the group have a mostly monoclinic structure and the internal structure is characterised by single chains of silica ions with a general chemical formula of XYSi2O6, where X can be Calcium, Magnesium, Iron, Manganese or Lithium, and Y can be any of those plus Aluminium, Chromium and Titanium. The most common members of the group include Augite, being an attractive dark green, Enstatite, Diopside, Aegerine Jadeite and Spodumene. Their well formed, large crystals mean that the Pyroxines are commonly found as major components of Pegmatites.

Not to be outdone, the Amphiboles are also a group of closely related minerals which form at progressively lower temperatures in conditions where OH is available. The general formula for the minerals is X2Y5Si8O22-(OH), where W is Sodium or Magnesium, X is Calcium, Sodium, Iron or Manganese, and Y is Manganese, Iron, Aluminium or Titanium. The Amphiboles resemble the Pyroxines in many respects, including being chain siliactes, except the chains line up in pairs. Also, unlike Pyroxenes they contain OH and the cleavage direction is different, falling at 56 or 124 degrees. The latter feature is the easiest way of telling them apart. The crystals are monoclinic, commonly forming long columnar aggregates. The minerals in the group include Hornblende, the Tremolite-Actinolite series and Glaucophane. Many occur in metamorphic rocks, mostly in asbestos-like forms.

The minerals of the two branches of the Bowen model, especially those in the Discontinuous group, generally give rise to dense, dark rocks, known as Mafic, due to their content of heavy metals including Magnesium and Iron. The most common Mafic rocks are Basalt and Gabbro.

Below a certain temperature both branches merge to produce minerals common to the Felsic rocks, Orthoclase, Muscovite, and Quartz. Orthoclase, and another closely related mineral, Microcline, are members of a group of minerals known as Potassium, or K, Feldspars. Both have identical chemical composition KAlSi3O8 but differ in internal structure. Orthoclase is monoclinic, whereas Microcline is triclinic. Both are framework silicates and are major components of rocks like Granites and Syenites.

The last important rock-forming minerals are the Micas. They form under the coolest conditions and are noted for forming extended flexible sheets. The Bowen series members are Muscovite and Biotite. Both have complex chemical compositions and belong to the family phylosilicates. Micas are major components of Pegmatites.