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

Figure 2-2. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS.

The rock must contain a total ofat least 10% of the minerals below.Renormalize to 100%

Quartz-richGranitoid

9090

6060

2020Alkali Fs.Quartz Syenite Quartz

SyeniteQuartz

MonzoniteQuartz

Monzodiorite

Syenite Monzonite Monzodiorite(Foid)-bearing

Syenite

5

10 35 65

(Foid)-bearingMonzonite

(Foid)-bearingMonzodiorite

90

Alkali Fs.Syenite

(Foid)-bearingAlkali Fs. Syenite

10

(Foid)Monzosyenite

(Foid) Syenite

(Foid)Monzodiorite

(Foi

d) G

abbr

o

Qtz. Diorite/Qtz. Gabbro

5

10

Diorite/Gabbro/Anorthosite

(Foid)-bearingDiorite/Gabbro

60

(Foid)olites

Quartzolite

Granite Grano-diorite

Tonalite

Alka

li Feld

spar

Gra

nite

Q

A P

F

60

Q=quartzA=Alkali fledspars (An0-An5)P=Plagioclase feldspars (An5-An100)F=Feldspathoid

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

Classification of aphanitic Igneous Rocks

Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS.

(foid)-bearing Trachyte

(foid)-bearing Latite

(foid)-bearing Andesite/Basalt

(Foid)ites

10

60 60

35 65

10

20 20

60 60

F

A P

Q

Rhyolite Dacite

Trachyte Latite Andesite/Basalt

Phonolite Tephrite

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 RocksFigure 2-2. A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS.

Plagioclase

OlivinePyroxene

Olivine gabbro

Plagioclase-bearing ultramafic rocks

90

(b)

Anorthosite

Olivine

ClinopyroxeneOrthopyroxene

LherzoliteHa

rzbu

rgite

Wehrlite

Websterite

Orthopyroxenite

Clinopyroxenite

Olivine Websterite

Peridotites

Pyroxenites

90

40

10

10

Dunite

Pyroxene Olivine

Plagioclase Feldspar

reticulite

Pele’s tearsPele’s Hair (the really thin strands among the rocks…)

ObsidianVolcanic 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.