2. Igneous Minerals
Quartz, Feldspars (plagioclase and alkaline),
Olivines, Pyroxenes, Amphiboles
Accessory Minerals – mostly in small quantities
or in ‘special’ rocks
Magnetite (Fe3O4)
Ilmenite (FeTiO3)
Apatite (Ca5(PO4)3(OH,F,Cl)
Zircon (ZrSiO4)
Titanite (CaTiSiO5)
Pyrite (FeS2)
Fluorite (CaF2)
3.
4. Minerals which form are thus a
function of melt composition and
how fast it cools (re-equilibration?)
governed by the stability of
those minerals and how quickly
they may or may not react with the
melt during crystallization
General Compositions Silicic
(Si-rich), Sialic (Si and Al rich),
Intermediate, Mafic (Mg and Fe-
rich), Ultramafic
Also ID’d based on alkalic (K and
Na) or alkaline (Ca-rich)
Liquid hot
MAGMA
Ca2+ Na+
Mg2+
Fe2+
Si4+
Si4+
Si4+
O2-
O2-
O2-
O2-
O2- O2-
O2-
O2-
O2-
O2-
rock
cooling
Mg2+
Fe2+
Mg2+
5. Composition
From Magma we saw how a crystal’s
composition can change on crystallization
different elemental composition from melt on
partial crystallization
6. Silica and Aluminum Content
Silica
Oversaturated if it contains Quartz
Undersaturated if it has silica-deficient minerals
(like the feldspathoids, ex: nepheline)
Aluminum
Peraluminous if it has a great excess of aluminum
after feldspars form, more Al left over for Al-rich
phases like corumndum, garnet, kyanite, etc.
Peralkaline – So little Al left after feldspars form,
only Al-deficient minerals like aegerine (type of
pyroxene) and riebekite (sodic amphibole)
7. 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.
9. Igneous Textures
Figure 3-1. Idealized rates of crystal
nucleation and growth as a function
of temperature below the melting
point. Slow cooling results in only
minor undercooling (Ta), so that
rapid growth and slow nucleation
produce fewer coarse-grained
crystals. Rapid cooling permits more
undercooling (Tb), so that slower
growth and rapid nucleation produce
many fine-grained crystals. Very
rapid cooling involves little if any
nucleation or growth (Tc) producing
a glass.
11. Textures I
Aphanitic - fine grain size (< 1 mm); result of
quick cooling
Rhyolite, Basalt, Rhyolite, Andesite
Phaneritic - coarse grain size; visible grains (1-
10 mm); result of slow cooling
Granite, Diorite, Gabbro
Pegmatitic - very large crystals (many over 2
cm)
Granite pegmatite or pegmatitic granite
12. Porphyritic- Mixture of grain sizes caused by mixed cooling history;
slow cooling first, followed by a period of somewhat faster cooling.
Terms for the textural components:
Phenocrysts - the large crystals
Groundmass or matrix - the finer crystals surrounding the large
crystals. The groundmass may be either aphanitic or phaneritic.
Types of porphyritic textures:
Porphyritic-aphanitic
Porphyritic-phaneritic
Origin: mixed grain sizes and hence cooling rates, imply upward
movement of magma from a deeper (hotter) location of extremely slow
cooling, to either:
a much shallower (cooler) location with fast cooling (porphyritic-
aphanitic), or
a somewhat shallower (slightly cooler) location with continued
fairly slow cooling (porphyritic-phaneritic).
13. Typically Volcanic Textures
Glassy - instantaneous cooling
Obsidian = volcanic glass
Vesicular - contains tiny holes called vesicles which formed due to
gas bubbles in the lava or magma. Very porous. May resemble a
sponge. Commonly low density; may float on water.
Vesicular Basalt, Pumice, Scoria
Pyroclastic or Fragmental - pieces of rock and ash come out of a
volcano and get welded together by heat. May resemble rhyolite or
andesite, but close examination shows pieces of fine-grained rock
fragments in it. May also resemble a sedimentary conglomerate or
breccia, except that rock fragments are all fine-grained igneous or
vesicular.
Tuff - made of volcanic ash
Volcanic breccia - contains fragments of fine-grained igneous rocks
that are larger than ash.
14. Classification based on Field Relations
Extrusive or volcanic rocks: typically aphanitic or glassy.
Many varieties are porphyritic and some have fragmental
(volcaniclastic) fabric. High-T disordered fsp is common
(e.g. sanadine). Also see leucite, tridymite, and
cristobalite.
Intrusive or plutonic rocks: typically phaneritic.
Monomineralic rocks of plagioclase, olivine, or pyroxene
are well known but rare. Amphiboles and biotites are
commonly altered to chlorite. Muscovite found in some
granites, but rarely in volcanic rocks. Perthitic fsp,
reflecting slow cooling and exsolution is widespread.
15. 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
17. 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).
22. Classification of
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
23. 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.
24. Classification of Igneous Rocks
Figure 2-2. A classification of the phaneritic
igneous rocks. b. Gabbroic rocks. c. Ultramafic
rocks. After IUGS.
Plagioclase
Olivine
Pyroxene
G
a
b
b
r
o
T
r
o
c
t
o
l
i
t
e
Olivine
gabbro
Plagioclase-bearing ultramafic rocks
90
(b)
Anorthosite
Olivine
Clinopyroxene
Orthopyroxene
Lherzolite
Websterite
Orthopyroxenite
Clinopyroxenite
Olivine Websterite
Peridotites
Pyroxenites
90
40
10
10
Dunite
(c)
Pyroxene Olivine
Plagioclase Feldspar
Anorthosite