Igneous Rocks
Classification of Igneous Rocks
• Most Abundant Elements: O, Si, Al, Fe,
Ca, Mg, K, Na
• Calculate Elements as Oxides (Account
for O)
• How Much SiO2? (Account for Si)
• What Feldspars are Present? (Account for
Al, Ca, Na, K)
• What Else is Present? (Account for Mg,
Fe)
Silica Content
• Oversaturated: Excess of Silica
– Quartz Present
• Saturated: Just enough silica to combine
with other ions
• Undersaturated: Silica-deficient Minerals
Present
– Olivine, Nepheline, Corundum, etc.
– Can’t coexist with quartz
Feldspars
• Plagioclase vs. K-Spar (Ca and Na vs. K)
• Relative Aluminum Content
– Peraluminous: Al left over after Feldspars
form
• Sillimanite, garnet, corundum may be present
– Peralkaline: Al insufficient to form Feldspars
• Riebeckite, Aegerine, may be present
Other Ingredients
• Ferromagnesian minerals heavily
influenced by characteristics like water
– The only difference between rocks with
biotite, amphibole or pyroxene may be water
content
• Basis for classification of ultramafic rocks.
“Mainstream” Igneous Rocks
• Ultramafic
– Plutonic: Dunite
• Mafic
<40% SiO2
Volcanic: Komatiite
40-50% SiO2
– Plutonic: Gabbro Volcanic: Basalt
• Intermediate
– Plutonic: Diorite
• Felsic
– Plutonic: Granite
50-60% SiO2
Volcanic: Andesite
>60% SiO2
Volcanic: Rhyolite
The Feldspars
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Potassium Feldspars
– T dependent
– Microcline, Orthoclase, Sanidine
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Plagioclase
– Classic Example of Solid Solution
– Ca vs. Na content
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Perthite: exsolution texture
Anorthoclase: K, Ca, Na mixture
Potassium Feldspars
• Microcline
– Lowest Temperature variety
– Plutonic rocks
– Almost always perthitic
• Orthoclase
– Medium Temperatures
– Volcanic and Plutonic Rocks
• Sanidine
– Highest Temperature
– Volcanic Rocks
– May Have Appreciable Na
• More a function of cooling rate and pressure
than temperature?
Plagioclase Feldspars
• Albite (0-10% Ca): Where Na goes in
metamorphic rocks, metasomatism
• Oligoclase (10-30% Ca): Granitic rocks
• Andesine (30-50% Ca): Intermediate rocks
• Labradorite (50-70% Ca): Mafic rocks
• Bytownite (70-90% Ca): Rare - too sodic
for marble, too calcic for magmas
• Anorthite (90-100% Ca): Impure
metamorphosed limestones
Perthite and Anorthoclase
• Ionic Radii (nm)
– K:
– Ca
– Na
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0.133
0.099
0.097
Ca and Na substitute freely
K can fit in lattice at high T
Na can fit in K-spar lattice but not Ca
Perthite: K-spar and plagioclase separate
during cooling (Exsolution)
• Anorthoclase: Na-K mix, 10-40% K-spar
The
Feldspars
Overview of
the IUGS
classification
of igneous
rocks
Silica-Saturated Rocks
Foids (Feldspathoids)
• Fill the “ecological niche” of feldspars
when insufficient silica is available
• Major Minerals:
– Nepheline (Na,K)AlSiO4
– Leucite KAlSi2O6
Silica-Deficient Rocks
Volcanic and Plutonic Equivalents
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Granite
Granodiorite
Tonalite
Syenite
Monzonite
Diorite
Gabbro
Foid Syenite
Foid Monzonite
Foid Gabbro
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Rhyolite
Dacite
Dacite
Trachyte
Latite
Andesite
Basalt
Phonolite
Tephrite
Basanite
Olivine
• Like Plagioclase, a solid solution
– Forsterite (Mg2SiO4) and Fayalite (Fe2SiO4)
• Becomes More Fe-Rich as Magma Cools
• Forsterite
– Can be nearly pure in metamorphic rocks
– Cannot coexist with quartz
• Fayalite
– Rarely found pure
– Can coexist with quartz
Ortho- and Clinopyroxene
• Orthopyroxene
– Orthorhombic
– Mixture of Enstatite (Mg2Si2O6) and Ferrosilite
(Fe2Si2O6). The generic mixture is termed
Hypersthene ((Mg,Fe)2Si2O6)
• Clinopyroxene
– Monoclinic
– Mixture of Diopside (CaMgSi2O6) and
Hedenbergite (CaFeSi2O6) The generic
mixture is termed Augite ((Ca,Mg,Fe)2Si2O6)
Ultramafic Rocks
Mode and Norm
• Mode: What is actually present
• Norm: Ideal mineral composition
– Ignores water
– Assumes minor components used predictably
– Assumes major minerals form in predictable
sequence
– Purpose is to visualize rock from chemical
data
CIPW Norm
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Cross, Iddings, Pirrson and Washington
All Cations treated as oxides
Anions (S, F, Cl) treated as elements
Convert wt% to molecular proportions
(Wt%/Mol Wt)
• Allocate oxides to mineral phases
Allocate minor elements
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Ba, Sr  Ca; MnO  FeO
CO2  Calcite (with CaO)
P2O5  Apatite (with CaO)
S  Pyrite (with FeO)
TiO2  Ilmenite (with FeO)
F  Fluorite (with CaO)
Cr2O3  Chromite (with FeO)
Cl  Halite (With Na2O)
Start Forming Silicates
• ZrO2  Zircon (with SiO2)
• Form provisional Feldspars
– Na2O  Albite
– K2O  K-Spar
– CaO  Anorthite
– With SiO2 and Al2O3
– May need to convert to foids if SiO2 runs out
Allocate FeO, MgO and CaO
• Fe2O3  Acmite (With Na2O and SiO2)
and Magnetite (With FeO)
• FeO and MgO  Hypersthene
(provisional)
• CaO + Hy  Diopside
• Excess SiO2  Quartz
If Silica Runs Out
• Hypersthene  Olivine
• Albite  Nepheline
• K-Spar  Leucite
Example
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SiO2
TiO2
Al2O3
Fe2O3
FeO
MgO
CaO
83
2
16
2
10
17
17
• Na2O
• K2O
5
1
Let the Games Begin
• Ilmenite: TiO2 0; FeO  10 - 2 = 8
• K-Spar: K2O  0; Al2O3 16 – 1 = 15;
SiO2  83 – 6K2O = 77
• Albite: Na2O  0; Al2O3 15 – 5 = 10;
SiO2  77 – 6Na2O = 47
• Anorthite: CaO  0; Al2O3 10 – 17 = -7!
– Excess CaO
– CaO  17-10 = 7; Al2O3  0; SiO2  47 –
2CaO = 27
Final Allocations
• Magnetite: Fe2O3 0; FeO  10-2 = 8
• FeO + MgO = 8 + 17 = 25
• Diopside: CaO  0; FeO + MgO = 25 – 7
= 18; SiO2  SiO2 – 2CaO = 27-14 = 13
• Hypersthene: FeO + MgO  0; SiO2  13
– 18 = -5 (Call this -D)
• Olivine: Ol = D = 5
• Hypersthene: Hy – 2D = 18 – 10 = 8
Final Result
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Ilmenite: 2
K-Spar: 1
Albite: 5
Anorthite: 10
• These are molecular
proportions
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Magnetite: 2
Diopside: 7
Olivine: 5
Hypersthene: 8
• Multiply by Mol. Wt.
and normalize for
Wt%