Classification of Igneous rocks

General

The large group of igneous rocks is initially divided into rocks that cool at the surface of the Earth (volcanic rocks) and those, that cool in the interior of the Earth (plutonic rocks).
They differ in their textures, which indicate the different cooling rates.

If a hot melt comes into contact with water at the ocean floor, it becomes literally shocked. This leads to the formation of glass, because the progress is too fast to allow for crystal growth.

^{Piece of volcanic glass (obsidian). https://en.wikipedia.org/wiki/Obsidian}

If the cooling rate is less extreme, fine-grained rocks, with grain sizes smaller than a millimeter, will develop. These grains are not detectable with the naked eye and their texture is described as aphanitic
If previously grown minerals have been swimming in the magma before it came into contact with the surface (which is often the case), they are preserved in the glassy or aphanitic groundmass. Together they form a porphyritic texture. If the large minerals have been formed from the same magma, they are called phenocrysts, if they have been picked up during the ascend of the magma or dropped into it, they are called xenocrysts. If a whole foreign piece of rock got into the melt, we call it a xenolith.

^{Aphanitic basalt. http://itc.gsw.edu/faculty/tweiland/igrx.htm}

^{Porphyritic basalt with plagioclase (white) and pyroxene (black). http://www.sandatlas.org/porphyry/}

If the melt cools in the interior of the Earth, the crystallizing processes are developing much slower. The smaller the temperature difference between the melt and the surrounding material, the slower is this process and the larger are the crystals. The overall texture is termed phaneritic
Plutonic rocks do not differ from their volcanic counterparts in chemical or mineralogical aspects, but purely in their grain size and texture. This results in a completely different appearance though.

<sup>Phaneritic gabbro. http://www.sandatlas.org/gabbro/

Phaneritic granite. http://www.sandatlas.org/granite/</sup>

If the rock crystallizes only a few hundred meters below the Earth’s surface, it lies texturally in between what we would usually call a volcanic or a plutonic rock. These rocks are called hypabyssal and are usually fine to medium grained.

A first approach to naming an igneous rock can be made using the diagram below. On it, the most common rock forming minerals for igneous rocks are displayed with their average modal abundances. It also provides relative amounts of certain elements as well as approximate melt temperatures. Although this diagram presents an excellent overview for igneous rocks, more accurate classifications have been developed.

^{Approximate classification of igneous rocks.
http://www.laniainakai.com/gg103/files/gg103_notes/z%20pages/ign_rock_chart.html}

QAPF-Diagram

Since the 1970s the most important magmatic rocks are classified by the IUGS (International Union of Geological Sciences) using the QAPF-diagram. The most important criteria, next to the texture, is the amount of minerals that are especially important for the genetic interpretation of the rock. These are: Quartz, Feldspars, and Foids. All other minerals, like pyroxenes, amphiboles, micas and even carbonates, are summarized by the term mafics.
The amount is determined as modal percentage. This means that the volume (approximated by the 2D view) of the minerals, but not their chemical composition or their weight is decisive.
At first one has to determine if the rock has more or less than 90 modal-% mafic minerals. If the rock contains less than 90% mafic minerals, it is classified using the QAPF-diagram. In it, the amount of quartz (Q), alkaline feldspar (A), plagioclase (P), and Foids (F, including nepheline, sodalite, leucite and others) represent the corners of the diagram. The four minerals (Q, A, P, F) together represent 100%. All other minerals are ignored at first, even if other minerals, like pyroxene for example, make up the majority of the rock.
Plutonic and volcanic rocks have each their own representative QAPF-diagram.
Quarz and feldspathoids exclude each other. This leads to the characteristic form of the double triangle. A rock therefore can only lie on the top half or the bottom half of the diagram.

@mirkon86 has taken a detailed look at the QAPF-diagram for plutonic rocks including great examples with beautiful pictures. You can find the article here.

^{IUGS classification of plutonic rocks (Left) and volcanic rocks (Right).
http://www.atlas-hornin.sk/en/article/7/classification-principles}

In certain cases the QAPF-nomenclature is not enough. This goes for rocks whose felsic phase is entirely made up of plagioclase (right corner of the QAPF-diagram), or for rocks whose grain size is simply too small to be identified. For both cases solutions have been developed, that additionally require chemical analysis.

Mafic and Ultramafic rocks

If the rock contains more than 90% mafic minerals, it is described as ultramafic and a special classification system has to be applied. Nearly all parts of the Earth’s mantle are of ultramafic composition.
Ultramafic rocks are usually composed of olivine and pyroxene. Pyroxene is divided into orthopyroxene and clinopyroxene. These three minerals form the corners of the central triangle shown in the diagram below and form the classification diagram for ultramafic rocks. Unmodified mantle occurs as Lherzolite (>40% olivine and both pyroxenes).
If plagioclase is the only felsic phase in the rock but is making up more than 10 modal-% it can be named using the three triangles on the sides of the diagram below. They all are represented in the QAPF-diagram by the P corner but a more detailed name can be given to it also by also including the mafic minerals present.

^{IUGS classification diagram for mafic and ultramafic rocks.
http://www.atlas-hornin.sk/en/article/7/classification-principles}

Sources

• Best, M. G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell
• Markl, G. (2008). Minerale und Gesteine, 2. Auflage. Spektrum Akademischer Verlag