Igneous rocks can be classified according to many criteria, such as texture, crystal or grain size, colour,
mineralogy, chemical composition, mode of occurrence, and genesis. Since it must be possible to give a
rock a name in the field, a first order classification cannot be based on chemical composition since this
first requires chemical analysis. Genetic classifications should be avoided as far as possible, but this is
not always easy. For example, the main subdivision of rocks into sedimentary, igneous and metamorphic
is genetic but cannot be avoided.
Igneous rocks are classified by their content of essential minerals i.e. those that make up the bulk of the rock – this is known as the mode of the rock. Minor amounts of accessory minerals are not considered. As we will see later, this is fairly straightforward for coarse grained rocks but can be very dificult for very fine grained ones – and impossible for glassy rocks.
Igneous rocks are classified by their content of essential minerals i.e. those that make up the bulk of the rock – this is known as the mode of the rock. Minor amounts of accessory minerals are not considered. As we will see later, this is fairly straightforward for coarse grained rocks but can be very dificult for very fine grained ones – and impossible for glassy rocks.
Two terms used in connection with grain size are:
phaneritic: individual crystals can be distinguished with the naked eye.
aphanitic: most of the individual crystals cannot be distinguished with the naked eye.
Igneous rocks are divided into two main groups on the basis of their field relations or on their grain size.
PLUTONIC – crystallized at depth. Phaneritic. Average crystal or grain size > 5mm (coarse); 1–5mm (medium); 0.5–1mm ( fine grained).
VOLCANIC – extruded at the surface of the Earth. Aphanitic. Grain size < 0.5mm (very fine grained).
It is not unusual for igneous rocks to have large crystals of one or more mineral(s) in a finer grained groundmass (also called the matrix). In this case it is the grain size of the matrix that is used. For example, a rock with a black, fine grained matrix may contain cm-sized pyroxene crystals. The rock was extruded from a volcano as lava and is volcanic, despite the presence of some large crystals which crystallized in a magma chamber before eruption. Large crystals in a finer grained matrix are called phenocrysts. The presence of phenocrysts in a finer grained matrix is referred to as a porphyritic texture.
phaneritic: individual crystals can be distinguished with the naked eye.
aphanitic: most of the individual crystals cannot be distinguished with the naked eye.
Igneous rocks are divided into two main groups on the basis of their field relations or on their grain size.
PLUTONIC – crystallized at depth. Phaneritic. Average crystal or grain size > 5mm (coarse); 1–5mm (medium); 0.5–1mm ( fine grained).
VOLCANIC – extruded at the surface of the Earth. Aphanitic. Grain size < 0.5mm (very fine grained).
It is not unusual for igneous rocks to have large crystals of one or more mineral(s) in a finer grained groundmass (also called the matrix). In this case it is the grain size of the matrix that is used. For example, a rock with a black, fine grained matrix may contain cm-sized pyroxene crystals. The rock was extruded from a volcano as lava and is volcanic, despite the presence of some large crystals which crystallized in a magma chamber before eruption. Large crystals in a finer grained matrix are called phenocrysts. The presence of phenocrysts in a finer grained matrix is referred to as a porphyritic texture.
Plutonic rocks
Plutonic rocks are classified according to their modal mineral content i.e. actual mineral content in volume %. The classification (from 1976) is known as the “Streckeisen” system after the Austrian professor who was chairman of an international committee to systematise the naming of igneous rocks. Before this there was considerable international confusion and disagreement as to how igneous rocks should be named.
For the naming of plutonic rocks, five groups of minerals are used. These are:
Q quartz
A alkali feldspar – orthoclase, microcline, perthite, anorthoclase and albite (An00–05)
P plagioclase (An05–100)
F feldspathoid minerals or FOIDS – nepheline, leucite, sodalite etc.
M mafic minerals. “Mafic” stands for Mg-Fe minerals and includes olivine, pyroxenes, amphiboles, micas, garnets, oxide and sulphide minerals etc.
Q, A, P and F are light coloured minerals. M are dark minerals. The percentage of dark minerals (%M) is often referred to as the COLOUR INDEX of a rock. The light coloured minerals are referred to as felsic minerals from feldspar and silica. The word mafic is constructed from the magnesium – ferrous (= iron) nature of the relevant minerals.
Plutonic rocks are classified according to their modal mineral content i.e. actual mineral content in volume %. The classification (from 1976) is known as the “Streckeisen” system after the Austrian professor who was chairman of an international committee to systematise the naming of igneous rocks. Before this there was considerable international confusion and disagreement as to how igneous rocks should be named.
For the naming of plutonic rocks, five groups of minerals are used. These are:
Q quartz
A alkali feldspar – orthoclase, microcline, perthite, anorthoclase and albite (An00–05)
P plagioclase (An05–100)
F feldspathoid minerals or FOIDS – nepheline, leucite, sodalite etc.
M mafic minerals. “Mafic” stands for Mg-Fe minerals and includes olivine, pyroxenes, amphiboles, micas, garnets, oxide and sulphide minerals etc.
Q, A, P and F are light coloured minerals. M are dark minerals. The percentage of dark minerals (%M) is often referred to as the COLOUR INDEX of a rock. The light coloured minerals are referred to as felsic minerals from feldspar and silica. The word mafic is constructed from the magnesium – ferrous (= iron) nature of the relevant minerals.
The primary division of plutonic rocks is based on the amount of mafic minerals. When dark minerals
make up more than 90% (i.e. M > 90%) the rocks are ULTRAMAFIC and are classified according to the
mafic minerals present. But the majority of igneous rocks have M < 90% and we will start with these.
M < 90% (QAPF double triangle)
The light coloured (felsic) minerals Q + A + P + F are calculated to 100%, ignoring the dark minerals.
Since quartz and feldspathoid minerals cannot coexist, the light minerals will in fact be:
Q + A + P = 100 or F + A + P = 100
These are then plotted in the QAPF double triangle. The QAPF double triangle is divided into 15 fields with different rock names. The fact that the QAPF double triangle is used for all plutonic rocks with M < 90% means that even if a rock consists of, for example, 89% olivine and 11% plagioclase, it is named according to the 11% plagioclase.
Q + A + P = 100 or F + A + P = 100
These are then plotted in the QAPF double triangle. The QAPF double triangle is divided into 15 fields with different rock names. The fact that the QAPF double triangle is used for all plutonic rocks with M < 90% means that even if a rock consists of, for example, 89% olivine and 11% plagioclase, it is named according to the 11% plagioclase.
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Igneous rocks with > 60% quartz amongst the light minerals are very rare. Rocks with
> 90% are quartzolites. With 60–90% quartz the term quartz-rich is used as a prefix to the appropriate name from eld 2, 3, 4 or 5. e.g. quartz-rich granodiorite.
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Alkali feldspar granites are, as the name implies, granitic rocks which are rich in alkali
feldspar and poor in plagioclase.
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The granite field is the largest one in the QAP triangle and granites are very common in the
continental crust. Note that granites can contain 20–60% quartz (ignoring dark minerals)
and that the ratio of alkali feldspar to plagioclase can vary from 9:1 to ~1:2 (actually 35:65).
Fields 2, 3 and 4 are all “granitic rocks”. The term “granite” sensu strictu is exclusively used
for rocks that lie in eld 3.
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Granodiorite is the name given to rocks that are more plagioclase-rich than true granites.
ey can be considered as having a composition essentially intermediate between granites
( field 3) and diorites ( field 10)
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Rocks dominated by quartz and plagioclase are tonalites.
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Rocks that are dominantly composed of alkali feldspar are alkali feldspar syenites. If there is
5–20% quartz the term quartz alkali feldspar syenite is used. With up to 10% feldspathoid
mineral (e.g. nepheline) the rock is e.g. a nepheline-bearing alkali-feldspar syenite.
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Syenite (see point 6 for quartz syenite and nepheline-bearing syenite).
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Monzonite (see point 6 for quartz and nepheline-bearing types).
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Monzodiorite or monzogabbro (see point 6 for quartz and nepheline-bearing types). For
distinction between monzodiorite and monzogabbro see point 10.
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is eld is used for three rock types – diorite, gabbro and anorthosite. Anorthosites consist
of > 90% plagioclase. Distinction between diorite and gabbro is usually based on the composition of the plagioclase in the rock. Gabbro contains plagioclase with An>50; diorite has An<50 span="">. is is, of course, di cult to determine in hand specimen! Diorites are usually more felsic than gabbros. Diorites typically contain andesine (plagioclase in the range An30–50) + hornblende ± biotite ± clinopyroxene. Gabbros contain, for example, labradorite (plagioclase An50–70) + clinopyroxene ± orthopyroxene ± olivine. Gabbroic rocks are further classi ed according to their dark minerals (section 5.1.1.1.1).
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Rocks in this eld are foid syenites. If the foid (= feldspathoid) mineral is, for example,
nepheline, the rock is called a nepheline syenite.
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Foid monzosyenite e.g. nepheline monzosyenite.
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Foid monzodiorite or foid monzogabbro. e.g. nepheline monzodiorite.
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Foid gabbro or foid diorite e.g. nepheline gabbro.
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Rocks with > 60% foid minerals (amongst the light minerals) are rare and are called
fodolites (e.g. nephelinolite). e term nephelinite seems more obvious but is used for equivalent volcanic rocks.
