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Classification of Igneous Rocks
The most abundant elements in the crust are oxygen, silicon, aluminum, iron, magnesium,calcium, sodium and potassium. These eight elements account for 99 per cent of the crust.
Since oxygen is by far the dominant anion, rock compositions are usually reported as oxides
rather than as separate elements. Most minerals can be written as combinations of the oxides.
For example, K-feldspar (KAlSi3O8) can be written as 1/2(K2O + Al2O3 + 3SiO2)
The most abundant oxide by far is SiO2, so the first question petrologists ask in classifying
igneous rocks ishow much silica is present? Is there so much that after all other minerals are
accounted for, silica is left over to form quartz? Or is there so little that silica deficient minerals
like olivine, leucite or nepheline are present?
The most abundant oxide by far is SiO2, so the first question petrologists ask in classifying
igneous rocks ishow much silica is present? Is there so much that after all other minerals are
accounted for, silica is left over to form quartz? Or is there so little that silica deficient minerals
like olivine, leucite or nepheline are present?
That leaves Mg, Fe and minor constituents. The third question petrologists ask in classifying
igneous rocks is what other minerals are present?
As a student, I was puzzled and frustrated that the igneous rocks were classified on the basis ofminerals that looked so much alike in thin section. However, to a practiced petrologist, quartz,
K-feldspar and plagioclase are quite distinct and easily recognizable.
Apart from accounting for most of the major elements, there's another reason we define
igneous rocks in terms of quartz and the feldspars. Relatively minor influences like water
content can drastically change the ferromagnesian minerals in a rock. A very anhydrous
magma might form the ferromagnesian pyroxene hypersthene. With a bit more water, the
otherwise identical magma might form the amphibole actinolite or biotite mica.
Important Classes of Igneous Rocks
The silica and aluminum contents of igneous rocks can be placed in broad classes:
Silica content
Oversaturated rocks are those with quartz.
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Undersaturated rocks are those with silica-deficient minerals that are incompatible withquartz. These minerals include corundum, olivine, leucite and nepheline.
Aluminum content
Peraluminous rocks are those with an excess of aluminum, so that after the feldspars form,excess aluminum remains to form aluminum-rich minerals like corundum, andalusite, kyanite,
sillimanite, or garnet.
Peralkaline rocks are those with so little aluminum that sodium or potassium are left overafter the feldspars form. The most common indications of peralkaline rocks are the sodium
pyroxene aegerine (acmite) and the sodium amphibole riebeckite.
IUGS Igneous rock names
Oversaturated rocks can be plotted on a triangle diagram with its vertices occupied by quartz,
alkali feldspar and plagioclase.
Undersaturated rocks can contain alkali feldspar and plagioclase, but not quartz. Instead they
contain minerals like leucite or nepheline. These minerals were once called feldspathoids, a
name that aptly describes their "ecological niche" since they perform the role of feldspars but
form instead because of insufficient silica. In modern petrological classification, these minerals
are termed "foids", a meaningless name that describes nothing. Undersaturated rocks can alsobe plotted on a triangle diagram with vertices occupied by foids, alkali feldspar and
plagioclase.
Oversaturated rocks can be plotted on a triangle diagram with its vertices occupied by quartz, alkali
feldspar and plagioclase.
Undersaturated rocks can contain alkali feldspar and plagioclase, but not quartz. Instead they contain
minerals like leucite or nepheline. These minerals were once called feldspathoids, a name that aptly
describes their "ecological niche" since they perform the role of feldspars but form instead because ofinsufficient silica. In modern petrological classification, these minerals are termed "foids", a meaningless
name that describes nothing. Undersaturated rocks can also be plotted on a triangle diagram with vertices
occupied by foids, alkali feldspar and plagioclase.
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Since the two triangles have alkali feldspar
and plagioclase in common, it is customary
to join the two base to base with alkali
feldspar and plagioclase along the common
edge and quartz and foids at the top and
bottom vertices. The two triangles are
mutually exclusive.
In the diagram here, broad families of rocks (granitic rocks, syenite, gabbro) are shown by common colors.
The term "alkali feldspar" refers to K-feldspar or albite (less than 10% anorthite). These feldspars form a
fairly complete solid solution series. Any plagioclase richer than 10% anorthite is considered plagioclase.
Gabbroic Rocks
Rocks with mostly plagioclase are termed gabbro or diorite. There are several subcategories of
these rocks.
Rocks with less than 5 per cent ferromagnesian minerals (i.e. mostly made of plagioclase) are
termedanorthosite. Rocks with over 40 % ferromagnesian minerals are generally
termed gabbro. Rocks with 5-40 percent ferromagnesian minerals are termed diorite if their
feldspar consists of less than 50 percent anorthite, or leucogabbro (leuco- is a Greek prefix
meaning light or white) if their feldspar consists of more than 50 percent anorthite.
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Ultramafic Rocks
Rocks containing more than 90 per cent ferromagnesian minerals are classified on the basis of their dark
minerals. If the ferromagnesian minerals consist only of olivine and pyroxene, they are classified on the
basis of their contents of olivine, orthopyroxene (usually enstatite or hypersthene) and clinopyroxene
(usually augite). Of these rocks, three are especially important. Dunite virtually never forms directly from a
dunite melt, but almost always as the result of magmatic segregation. Harzburgite and lherzolite are the
dominant rock types of the uppermost mantle.
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Igneous Rock Classification
Igneous rocks: very diverse in chemistry and texture, yetthey have very gradational boundaries (Table 3-7). Wemust pick a rational basis for classifying them. The
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classification system used, will depend on how much weknow about the rock being examined.
Basis for Classification
1) Field and hand specimen examination: texture, colour etc.2) Chemical Data: rock chemistry.
3) Petrographic examination: mineral identification
Examine these classification systems in more detail.
1) Field and hand specimen examination
The most primitive classifications are based onrock characteristics such as:
a) Extrusive or Intrusive (grain size)
Extrusive Volcanic rocks are formed near the earths
surface. They are fine grained to glassy except for coarsergrained pheoncrysts (which formed at depth beforeeruption). Eg volcanic flows or ashes.
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Igneous Rock Classification cont
Intrusive Hypabyssal rocks are formed at shallowdepths (less than 1 km). They are fine grained, maycontain phenocrysts. Eg tabular dykes or sills. (Oftenlumped with volcanics because of similarity).
Intrusive Plutonic rocks form at depth greater than 1km. They are medium to coarse grained. Eg granite
diorite etc. (also often used for regional metamorphicrocks formed at depth such as granite gneiss).
b) Colour index
(% of dark minerals)
c) Other features visible to the naked eye. Egphenocrysts, vesicles, flow banding, cumulate textures
etc.2) Chemical Classification
As technology improves, the use of chemicalclassification has become more common, easier andcheaper. Eg 30 years ago 10 major elements cost about$100. Now you get the same analysis, REE and some
minor elements for $10.Geologists use an informalclassification of major elements and minor elements:
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Igneous Rock Classification cont
a) Major Elements: make up the bulk of the rock. Eg Si,Al, Fe2+, Fe3+, Mn. Mg, Ca, K, Na, P, Ti, H2O.
b) Minor elements: present in ppm quantities. Eg Cr, Ni,Zr, Rb, Sr, REEs.
Chemistry is most useful when dealing with altered andvery fine grained rocks. In general, if you test a suite of
rocks, the boundary between rock types becomes lessarbitrary.
Chemistry of igneous rocks is reported in % oxides (Table3-7). Note the ranges for most rocks.
SiO235-75% (basalts 45-50%, granites 70%, Ultramafic 30-40%
Al2O3 5-20%TiO20-5%CO20-5%
MgO 1-40%Na20.5-5%MnO 0-0.5%CaO 1-20%K2O 0-5%P2O50-0.5%
Fetot1-15%H2O 0.2-5%
Now we can apply one of a number of classifications:
A] Classification based on Silica Percentage
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Igneous Rock Classification cont
This can be combined with Table 3-3 with leucocraticbeing applied to felsic rocks, mesocratic being applied tointermediate rocks and melanocratic being applied tomafic and ultramafic rocks.
Problems arise with this classification system becauseyou are comparing a chemical system (SiO2%) with a
system based on % of dark minerals. You sometimes runinto problems: nepheline syenite is considered a felsicrock yet it does not contain >66% SiO2.
B] Silica Saturation
As SiO2is so abundant, a classification can also be basedon the presence or absence of various mineral phaseswhich reflect the SiO2content in relation to the other
chemical components.Typical saturated minerals that can occur with freequartz include feldspar, Al & Ti poor pyroxene,amphibole, mica, almandine garnet. Typicalundersaturated minerals that are not stable in the
presence of free SiO2include leucite, nephelene,
sodalite, olivine, melanite garnet, corundum, Al & Tirich clinopyroxene.
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Igneous Rock Classification cont
Classification
Oversaturatedrocks -have quartz and tridimitein abundanceSaturatedrocks -have no free quartz and noundersaturated minerals
Undersaturatedrocks -have no quartz and have
undersaturated minerals.This system is therefore based primarily on relationshipsof silica content to the rest of the rock.
C] Alumina SaturationBased on Al2O3similar to the SiO2classificationsystem
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Peraluminous: molecular proportion of Al2O3exceedsthe sum of CaO, Na2O and K2O. For plagioclase +alkali feldspar, this ratio is about 1:1. Any Al2O3that isleft over goes in to forming corundum. These rocks
tend to be mica rich.
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Igneous Rock Classification cont
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Metaluminous: molecular proportion of Al2O3exceeds thesum of Na2O and K2O, but is less that the sum of Na2O,K2O and CaO. These rocks tend to be rich in anorthite andusually also contain hornblende, epidote, biotite and
pyroxene.Subaluminous: molecular proportion of Al2O3isapproximately equal to the sum of Na2O and K2O. Theserocks tend to form alkali feldspar and a little Ca
plagioclase and usually contain olivine and pyroxenes.Peralkaline : molecular proportion of Al2O3is less thanthe sum of Na2O and K2O. There is insufficient alumina
to use all the Na2O and K2O by making feldspar. Thefree alkalis become incorporated into alkali richferromagnesium minerals such as aegerine or reibeckite.
D] Alkali-Lime Index
This system tells us about the alkalinity of the rocks.
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Igneous Rock Classification cont
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E] Common Chemical X-Y and Ternary Plots
Typically, for X-Y plots you plot oxides against acommon or stable or highly variable component. Whichcomponents to plot depends on experience and what you
wish to know.
Tholeiitic basalts -ophiolites,ocean floor, greenstone belts.
Alkali basalts -crustal melts,Hawaii
orFigure 3-6 plots CaO vs SiO2and Na2O+K2O vs SiO2. Since
CaO usually decreases as Na2O+K2O increases with respect toSiO2, therefore the curves cross. The SiO2content, at the point atwhich the curves cross, indicates the alkalinity of therocksuite.or Figure 6-16 -normalized REE
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Igneous Rock Classification cont
71Common Ternary Plots -3 component systemsa) A(B)FMDiagram(J.B.Thompson1957)A=Al2O3B=K2OF=FeOM=MgOb) ACF Diagram(Eskolaearly 1900s)A=Al2O3+Fe2O3-(Na2O+K2O)C=CaOF=MgO+FeO+MnOc) AKFDiagram(Eskola, early 1900s)A=Al2O3-
(CaO+Na2O+K2O)K=K2OF=FeO+MgO+MnO
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Igneous Rock Classificationcont
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These plots are used to easily and clearly distinguishdifferent rock types. They are particularly useful forfine grained or altered rocks where identification can
be difficult.Some of these ternary plots (Figures 6-20 and 6-16)are specific for a particular rock type: Ti-Zr-Y(Sr)Diagram for basalts. Field A+B are low K tholeitic,field B are ocean floor basalts, field B+C are calc-alkali basalts and field D are oceanic island orcontinental basalts.
These are all relatively immobile trace elements.These diagrams are useful if the original environmentis scrambled. Eg: ocean floor basalts thrust onto thecontinent; basalts within the plate (oceanic orcontinental) VS plate margin (ocean ridge to ocean
floor).
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Igneous Rock Classificationcont
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3) Classification based on Petrographic Examination
Thin sections of rocks are relatively easy to make andidentification of rocks based on the mineralogy observed is
possible.Rules:
a) Make sure the thin section is representative of the rock.
b) Identify the major components of mineralogy andestimate their relative proportions.c) Use proportions to classify the rock according to a scheme.Any scheme is somewhat arbitrary. See handout andStreckeisen.
Criteria which are important:
1) Proportion of mafic to felsic components
2) Composition of the plagioclase
3) Proportion of alkali feldspar to plagioclase
4) Presence or absence of quartz
5) Presence or absence of feldspathoid minerals6) Grain size or texture (extrusive or intrusive)
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Igneous Rock Classification cont
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Discussion -In general
a) These methods are time consuming but relatively straightforward for coarse grained rocks.
c) Volcanic rocks are harder to identify mineralogy. Grains
are small and difficult to identify petrographically.c) Glassy rocks -often impossible to identify mineralogy
petrographically.d) Altered rocks -Bad news, the system can break down.
Some problems related to some classification schemes:
i) No subdivisions of granites or rhyolites. All just felsic richacidic rocks.
ii) No subdivisions of basalts and andesites. Need furtherrules.
iii) Lack of description for mafic rocks in general.
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Igneous Rock ClassificationStreckeisen Classification System
In 1967 Albert Streckeisen, with the cooperation of many
geologists in many countries, came up with a generally acceptedrock classification system.
The International Union of Geological Sciences (IUGS) modifiedand expanded his work to form what is an internationally accepted
igneous rock classification system.In order to use this system, you must be able to determine thepercentage of five minerals (or mineral groups): quartz, plagioclase,alkali feldspars, ferromagnesian minerals and feldspathoids (such asnepheline or leucite).
The Q (or F) , A and P mineral percentage is recalculated to add to100% and is plotted on the triangular plot.
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Igneous RockClassificationStreckeisen ClassificationSystem
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gneous RockClassification
treckeisen Classificationystem
The plagioclase rich area of the
iagram has some additionalequirements for rock distinction.
For plutonic rocks: anorthosites a rock containing >90%lagioclase, gabbro containslagioclase more calcic than
An50 and usually contains >35%mafic minerals (augite,
ypersthene or olivine), Diorite
ontains plagioclase more sodichan An50 and usually contains
>35% mafic mineralshornblende or hypersthene ugite).
For volcanic rocks: the
istinction between basalt andndesite is bases on the silicaontent. A rock with >52%iO2is andesite while one with
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cation
Ultramafic RocksUltramafic rocks
(containing morethan 90% maficminerals) areclassified byalternativemethods. Some of
the most commontypes are definedas follows:Peridotite: a rockcontaining 40-100%olivine, with theremainder mainly
pyroxene and/orhornblende.Dunite: a rockcontaining 90-100% olivinewith theremainder mainly
pyroxene.
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There are a few rocks that dont fit the IUGS
classification system that are named on thebasis of texture, with mineral content being
of secondary consideration. Some of themore important of these are defined asfollows:Pegmatite: a very coarse grained (>1 cm)rock with interlocking grains. Usuallygranitic in composition.Obsidian: a black volcanic glass withconchoidal fracture, rhyolitic incomposition.Tuff: a compacted deposit of ash and dustcontaining up to 50% sedimentary
material.Ultramafic RockscontPyroxenite: a rock composed mainly of
pyroxene with the remainder olivine and/orhornblende.Hornblendite: a rock composedmainly of hornblende with the remaindermainly pyroxene and/or olivine .78
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Breccia: Similar to a tuff, but withlarge angular fragments in a fine
matrix.There are also few well recognizedigneous rocks that are found in a highlyaltered state. The alteration is related totheir method of origin. Some of the moreimportant of these are defined as follows:Spilite: an altered, usually vesicular
basalt exhibiting pillow structures.Feldspars have been altered to albite andis usually found with chlorite, calcite,epidote, chalcedony or prehnite.Serpentinite: a rock containing almostentirely serpentine (from the alteration of
olivine and pyroxene).Kimberlite: an altered porphyritic micaperidotite containing olivine (altered toserpentine or a carbonate mineral) and
phlogopite (commonly altered tochlorite). Some also contain diamonds.
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Igneous Rock Classification contThere are rock classification systems that
attempt to combine chemistry and mineralogy.In this case, you take the chemistry data andtransform it into theoretical mineralogy. This iscalled the CIPW NormativeClassification(Cross, Iddings, Pirsson andWashington). The norms are based on
molecules of ideal composition.Methodology
A] Convert % oxides into molecular proportions
wt% oxide formula wt = Molecular Proportion
Eg SiO272.67 60.09 = 1.211
B] Allocate molecular proportions to mineralsusing the following rules:
1) Apatite is one of the first minerals toprecipitate. All P is in apatite.2) Allocate Fe2O3, FeO to magnetite. Thelimiting factor is the total amount of Fe2O3.Molecular proportion of Fe2O3= Molecular
proportion of FeO.
3) Make pure Orthoclae, Albite and Anorthite.
Eg: Orthoclase 1K2O1Al2O36SiO24) Use remaining Al2O3makingCorundum
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Igneous Rock Classification cont5) Allocate remaining FeO, MgO to
hypersthene. Molecular proportion ofFeO+MgO = molecular proportion ofSiO2.
6) Allocate the remainingSiO2to quartz.
C] Once the molecular fraction has been
calculated for each mineral, multiply throughby the atomic weight of that mineral. This willgive you a proportion (5) of each mineralspecies.
Limitations: Often Severe
1) Can only calculate anhydrous species,therefore biotite and amphiboles are ignored.
2) normative mineralogy will not equal modal mineralogy3) Theoretical end members are usedwhich may not match actual members
present.
4) FeO/Fe2O3allocation can cause problems. It isassigned to magnetite but what about other ironminerals and iron in silicate structures?
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