Classification of Igneous Rock

7/27/2019 Classification of Igneous Rock

1/32

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.


2/32

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.


3/32

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.


4/32

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.


5/32


6/32

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


7/32

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.

64


8/32

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:

65


9/32


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

66


10/32


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.

67


11/32


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

68

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.


12/32


69

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.


13/32


70

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


14/32


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


15/32


16/32

Igneous Rock Classificationcont

72


17/32

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).


18/32


19/32

Igneous Rock Classificationcont

73


20/32

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)


21/32


74

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.


22/32

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.

75


23/32

Igneous RockClassificationStreckeisen ClassificationSystem

76


24/32

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


25/32

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.

7


26/32


27/32

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


28/32

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.

79


29/32

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


30/32

80


31/32

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?

81


32/32

Documents

Classification of Igneous Rock