According to
the British Geological Survey
(1999), a scheme for classification and nomenclature of igneous rocks should
be practical, logical, systematic, hierarchical and use clearly defined,
unambiguous rock names. Classification of Igneous Rocks
(Chapter 2)
last update:09/11/06
Goals of Classification
According to
the British Geological Survey
(1999), a scheme for classification and nomenclature of igneous rocks should
be practical, logical, systematic, hierarchical and use clearly defined,
unambiguous rock names.
It should also minimize the reliance on genetic names in favor of names based on descriptive attributes. This can be tough!
Main discriminating factors used in igneous classification
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Mineral grain or fragment size characteristics |
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Modal amounts of minerals |
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Chemical characteristics |
There are a tremendous number of names for igneous rocks, but there are fortunately much fewer that are common.
There are flow charts and diagrams that are of great assistance. The
Textural criteria is considered first - broad genetic implications
Phaneritic - crystals that compose the rock are readily visible with the naked eye
The genetic implication is that it cooled at depth and crystallized slowly = plutonic or intrusive rock
Aphanitic - crystals, if any, are too small to be readily visible with the naked eye
The genetic implication is that it cooled at the surface and crystallized rapidly = volcanic or extrusive rock
Fragmental - rock composed of disaggregated igneous material, deposited and later amalgamated.
The genetic implication is that it erupted at the surface, became disaggregated and rapidly deposited from air or water to form a rock = pyroclastic rock
Additional textural terms based on grain size distribution:
Equigranular - uniform grain size
Olivine and orthopyroxene
adcumulate from the Bushveld Complex, South Africa
(image source
http://web.uct.ac.za/depts/geolsci/dlr/bv_thin.html )
Porphyritic - two dominant grain size with the larger crystal termed phenocrysts and the finer crystals referred to as groundmass. This texture implies two distinct phases of cooling.
Laramide Porphyry from the Beartooth Mountains
(sample QC81-126b)
Compositional terms for igneous rocks
Most igneous rocks rocks composed of silicate minerals.
Some of these are Mg-Fe-bearing minerals that dark-colored and termed mafic
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olivine, pyroxene, hornblende, biotite, etc. |
Other minerals are light-colored and contain little Mg-Fe and are termed felsic.
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quartz, plagioclase, K-feldspar, muscovite, etc. |
Rocks, then, commonly pick up these terms such that a rock composed of predominantly felsic minerals is termed a felsic rock and rocks with mafic minerals are termed mafic rocks. In turn, if the rock contains >90% mafic minerals it is termed an ultramafic rock.
Rocks are also given terms based on their relative amounts of light and dark minerals such that a light-colored rock is termed leucocratic and a dark-colored rock is termed melanocratic.
The color is not a very definitive measure of the composition of the rock e.g. variations in the color of plagioclase
Chemical terms used to describe igneous rocks include:
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silicic - SiO2 content |
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acidic - wt% SiO2 > 66% |
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intermediate - wt% SiO2 = 52-66% |
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basic - wt% SiO2 = 45-52% |
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ultrabasic - wt% SiO2 < 45% |
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magnesian - MgO content |
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alkaline - (Na2O+K2O) content |
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aluminous - Al2O3 content |
To classify phaneritic or aphanitic igneous rocks according to the IUGS mineralgoical system, it is necessary to
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determine the mode (% of each mineral present. based on volume) - generally done with point counts |
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calculate the volume of Q' (% quartz), P' (% plagioclase of An5-100), A' (% alkali feldspar), F' (% feldspathoids) and M' (% mafics and accessories) |
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establish whether the rock is phaneritic or aphanitic |
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normalize the mineral modes, and determine the appropriate classification diagram |
The flow chart system of Wooley et al (1996) is excellent for classification. There is a slight modification of the IUGS classification scheme introduced by the British Geological Survey in 1999. There are elements of that scheme that we should also consider.
Example - slab of polished rock
Phaneritic Rocks
Q+A+P>10%
Classification gets tricky when the mode plots close to the P end because there are three common rocks that plot in that field: diorite, gabbro and anorthosite.
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Anorthosite is easy because it has >90% plagioclase in the un-normalized modes. |
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Gabbro vs. diorite is more difficult but is separate of the basis of plagioclase An content (>50% for gabbro) and % mafic minerals in the mode (>35% mafic). Therefore, gabbros are generally more melanocratic and diorites are salt-and-peppery. |
Modifying terms (chemical, mineralogical or textural):
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unusually light-colored - use leuco-modifier |
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unusually dark-colored - use mela-modifier |
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textural terms might include: porphyritic granite, Rapakivi granite, graphic granite |
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minerals that convey additional information: riebeckite granite or muscovite biotite granite (Note: if there are multiple mineral modifiers the order should be with increasing modes) |
Mafic (plots near P) and ultramafic (>90% mafics) igneous rocks
Aphanitic Rocks
Similar to the phaneritic mineralogical classification in that you determine the mode of minerals and then normalize. However, the mode can be difficult to accurately determine due to the fine-grained or glassy nature of some of the rocks. Sometimes modes are determined using phenocrysts and these rock names are termed phenotypes (more primitive name results).
For the rocks falling close to P there is a similar problem as with the phaneritic rocks - there are two rock types that plot there: basalt and andesite. IUGS recommends distinction based on color index or silica content with the lighter-colored rock (CI<35%) and silica-richer rock (>52% SiO2) being an andesite.
The best way is to analyze the rock and use a classification diagram such as the TAS diagram (below).
Some rocks such as hypabyssal (shallow intrusive) rocks (e.g. diabase) or carbonatites or lamproites or spilites or keratophyres are not covered here. Refer to the book: Le Maitre (2002) Igneous Rocks: A Classification and Glossary of Terms.
Fragmental (Pyroclastic) Rocks
Typically based on the type or size of clasts rather than the chemistry due to mixing problem.
