Names of Igneous Rocks
• Texture + Composition = name
• Set up diagrams (many ternary ones again, you
remember how these work?) to represent
composition changes for rocks of a certain
texture
• Composition can be related to specific minerals,
or even physical characteristics of mineral grains
• Modal Composition - % of minerals
comprising a rock
Visual Estimation of Modal Abundance
Classification based on Modal Mineralogy
• Felsic rocks: mnemonic based on feldspar and
silica. Also applies to rocks containing abundant
feldspathoids, such as nepheline. GRANITE
• Mafic rocks: mnemonic based on magnesium and
ferrous/ferric. Synonymous with ferromagnesian,
which refers to biotite, amphibole, pyroxene,
olivine, and Fe-Ti oxides. BASALT
• Ultramafic rocks: very rich in Mg and Fe.
Generally have little feldspar. PERIDOTITE
• Silicic rocks: dominated by quartz and alkali fsp.
Sometimes refered to as sialic (Si + Al).
granite
granodiorite
Classification of Phaneritic
Igneous Rocks
Q
Quartzolite
90
90
Quartz-rich
Granitoid
60
60
The rock must contain a total of
at least 10% of the minerals below.
Renormalize to 100%
Q=quartz
A=Alkali fledspars
(An0-An5)
P=Plagioclase feldspars
(An5-An100)
F=Feldspathoid
Granodiorite
Granite
Alkali Fs.
Quartz Syenite
Alkali Fs.
Syenite
20
20
Quartz
Monzonite
Quartz
Syenite
5
10
A
Syenite
(Foid)-bearing
Syenite
35
Monzonite
(Foid)-bearing
Monzonite
Quartz
Monzodiorite
65
Monzodiorite
(Foid)-bearing
Monzodiorite
10
(Foid)-bearing
Alkali Fs. Syenite
(Foid)
Monzosyenite
Figure 2-2. A classification of the phaneritic igneous rocks.
a. Phaneritic rocks with more than 10% (quartz + feldspar
+ feldspathoids). After IUGS.
(Foid)
Monzodiorite
60
60
(Foid)olites
F
Qtz. Diorite/
Qtz. Gabbro
5 Diorite/Gabbro/
90
Anorthosite
P
10 (Foid)-bearing
Diorite/Gabbro
Classification of Igneous Rocks
Figure 2-4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al.
(1986) J. Petrol., 27, 745-750. Oxford University Press.
Aphanitic rocks
basalt
rhyolite
Q
Classification of aphanitic
Igneous Rocks
60
60
Rhyolite
Dacite
20
20
Trachyte
Latite
35
A
10
(foid)-bearing
Trachyte
Andesite/Basalt
65
(foid)-bearing
Latite
Phonolite
(foid)-bearing
Andesite/Basalt
10
Tephrite
Figure 2-3. A classification and nomenclature of
volcanic rocks. After IUGS.
60
60
(Foid)ites
F
P
Classification of Igneous Rocks
Figure 2-5. Classification of the pyroclastic rocks. a. Based on type of material. After Pettijohn
(1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b. Based on the
size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.
Classification of Igneous Rocks
Plagioclase
Feldspar
Plagioclase
Anorthosite
Figure 2-2. A classification of the phaneritic
igneous rocks. b. Gabbroic rocks. c. Ultramafic
rocks. After IUGS.
Olivine
gabbro
lite
cto
Tro
Ga
bb
ro
90
Olivine
Dunite
90
Peridotites
Plagioclase-bearing ultramafic rocks
Pyroxene
Pyroxene
(b)
Lherzolite
Olivine
Olivine
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
10
Orthopyroxene
Websterite
Clinopyroxenite
Clinopyroxene
reticulite
Pele’s tears
Pele’s Hair (the really thin strands among the rocks…)
Obsidian
Volcanic glass formed from more felsic magmas (more silica), typically rhyolitic and
usually black from small amount of nanocrystalline magnetite - Glass can get
molecularly thin pretty easily, making it very sharp and using through antiquity for
knives/spears, and still used today for scalpels (permits a much finer blade than does
steel)
Obsidian Cliffs, Yellowstone N.P.