1. Classification and Nomenclature of Igneous Rocks
Petrology Lecture 2
Classification and Nomenclature of Igneous Rocks
GLY Spring, 2018

2. Igneous Textures
Phaneritic: Crystals are readily visible with the unaided eye.
Aphanitic: Crystals, if present, are too small to be seen with the unaided eye.
Fragmental: Composed of pieces of pre-existing, mostly igneous, rock, crystal fragments, and/or glass. These pieces were deposited and later amalgamated into a rock.

3. Porphyritic Size Distribution
Porphyritic - bimodal size distribution, with large grains surrounded by numerous small grains or glass
Phenocrysts - Large crystals formed by relatively slow cooling below the earth’s surface
Groundmass - Small crystals or glass, formed by more rapid cooling

4. Composition
Felsic: Rocks composed primarily of silica-rich minerals, generally feldspars and silica, from which the term is derived. Common minerals include plagioclase feldspar, alkali feldspar, quartz, and muscovite. The term can be used to modify either a mineral or a rock name.
Mafic: Rocks rich in magnesium and iron, and hence with lower silica contents. Common minerals include olivine, pyroxene, hornblende, and biotite. The term can be used to modify either a mineral or a rock name.

5. Types of Minerals
Essential – Minerals which must be present in order for a rock to be classified with a certain name
Accessory – Minerals need not be present in a rock, but which may be present in small amounts

6. Chemical Composition Classification
Felsic (acidic) > 66 wt. % SiO2
Intermediate wt. % SiO2
Mafic (basic) wt. % SiO2
Ultramafic (ultrabasic) < 45 wt. % SiO2

7. Alumina Saturation

8. Color Index

9. Using IUGS Classification
1. Determine the mode
2. Determine the volume percent of several classes of minerals:
a. Q = % quartz (rarely, other silica phases)
b. P = % plagioclase, An05-100
c. A = % alkali feldspar (K-spar and An00-05)
d. F = % feldspathoids (often called foids)
e. M = % mafics and accessories

10. Using IUGS Classification II
3. Determine Q + A + P or F + A + P
4. Determine if the rock is phaneritic (intrusive) or aphanitic (extrusive)

11. IUGS Classification
Phaneritic rocks

12. IUGS Classification
Aphanitic Rocks

13. Using IUGS Classification III
5. Determine the name of the rock from the diagram
6. If P + M (gabbroic) >90% or M > 90% (ultramafic), different classification schemes are used

14. IUGS Classification
Gabbroic rocks

15. IUGS Classification
Ultramafic Rocks

16. IUGS Classification Phaneritic rocks
Rocks that plot near the “P” apex are difficult. Anorthosite, diorite, and gabbro all plot here. Anorthosite is composed of greater than 90% plagioclase (unnormalized). Diorite and gabbro are more difficult. For hand specimen, we use the criterion that gabbro has > 35% M, usually pyroxene or olivine. Diorite has < 35%M, usually hornblende, sometimes a pyroxene. In thin section, a different criterion is used. The plagioclase in gabbro is > An50, whereas in diorite the plagioclase is less than An50. Since it is possible to get two different answers, IUGS suggests that plagioclase composition prevail. A further complication occurs when the plagioclase is compositionally zoned.
IUGS allows modifiers for rock names. These may consist of mineralogical, textural, or chemical terms, or any combination thereof. Examples of textural terms are porphyritic or graphic, i.e. porphyritic orthoclase granite, or graphic granite. You can also use the color terms “leuco” or “mela” for rocks which are much lighter or much darker than is normal for the type. Mineral names used as modifiers of rock names should be listed in order of increasing modal composition, with the least to the left. Biotite hornblende granite has more hornblende than biotite. The most common chemical modifier is “alkali”, and others include calc-alkaline, aluminous, titaniferous, etc.

17. Aphanetic Rocks
In principle, we follow the same steps for volcanic rocks as we do for intrusive rocks
However, the nature of the aphanitic rocks makes determination of the mode difficult, especially in hand specimen
Even in thin section, groundmass material may be too small to recognize, or may be amorphous
Phenotypes – based on phenocrysts only
We then resort to using phenocrysts only. IUGS recommends that rocks identified based on phenocrysts alone be called phenotypes, and that we add the prefix “pheno” to the rock name (pheno-dacite). Since minerals crystallize in a sequence (essentially Bowen’s series) and the phenocrysts are often biased towards early-forming minerals, phenotype names are often biased in the direction of early forming minerals.
Again, rocks near the “P” apex are troublesome. Andesite may be defined as a plagioclase-rich rock with SiO2 > 52%, or a color index < 35%. Basalt has SiO2 < 52%, or a color index > 35%.

18. “P” Apex Classification
Again, rocks near the “P” apex are troublesome.
Andesite may be defined as a plagioclase-rich rock with SiO2 > 52%, or a color index < 35%
Basalt has SiO2 < 52%, or a color index > 35%.

19. IUGS Classification
Chemical classification of volcanic rocks

20. Problems with Pyroclastics
These rocks present special problems, because they often contain significant impurities (material blasted out by the eruption, or caught in the general updraft)
In principle, they can be classified on the basis of a chemical analysis, but the presence of significant impurities argues against this approach

21. IUGS Classification Pyroclastic rocks
Based on type of fragmental material

22. IUGS Classification Pyroclastic rocks Based on fragment size
Ash < 2mm
Lapilli 2-64 mm
Blocks or bombs >64 mm

23. Web Resource
The preceding was a very short outline of ligneous rock classification – for those students wanting more information, the following is a good resource:
A Web Browser Flow Chart for the Classification of Igneous Rocks -