rock classification

7/23/2019 Rock Classification

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Igneous Rock Classification

Igneous rocks: very diverse in chemistry and texture, yet

they have very gradational boundaries (Table 3-7). We must

pick a rational basis for classifying them. The classificationsystem used, will depend on how much we know 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 on rock

characteristics such as:

a) Extrusive or Intrusive (grain size)

Extrusive Volcanic rocks are formed near the earth’s surface.

They are fine grained to glassy except for coarser grained

pheoncrysts (which formed at depth before eruption). Eg

volcanic flows or ashes.



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Igneous Rock Classification cont

Intrusive Hypabyssal rocks are formed at shallow depths

(less than 1 km). They are fine grained, may contain

phenocrysts. Eg tabular dykes or sills. (Often lumped withvolcanics because of similarity).

Intrusive Plutonic rocks form at depth greater than 1 km.

They are medium to coarse grained. Eg granite diorite etc.

(also often used for regional metamorphic rocks formed at

depth such as granite gneiss).

b) Colour index

(% of dark minerals)

c) Other features visible to the naked eye. Eg phenocrysts,

vesicles, flow banding, cumulate textures etc.

2) Chemical Classification

As technology improves, the use of chemical classificationhas become more common, easier and cheaper. 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 informal classification of major

elements and minor elements:



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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, REE’s.

Chemistry is most useful when dealing with altered and very

fine grained rocks. In general, if you test a suite of rocks, the

boundary between rock types becomes less arbitrary.

Chemistry of igneous rocks is reported in % oxides (Table 3-

7). Note the ranges for most rocks.

SiO2 35-75% (basalts 45-50%, granites 70%, Ultramafic 30-40%

Al2O3 5-20% TiO2 0-5% CO2 0-5%

MgO 1-40% Na2 0.5-5% MnO 0-0.5%

CaO 1-20% K 2O 0-5% P2O5 0-0.5%

Fetot 1-15% H2O 0.2-5%

Now we can apply one of a number of classifications:A] Classification based on Silica Percentage



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This can be combined with Table 3-3 with leucocratic being

applied to felsic rocks, mesocratic being applied to

intermediate rocks and melanocratic being applied to maficand ultramafic rocks.

Problems arise with this classification system because you

are comparing a chemical system (SiO2 %) with a system

based on % of dark minerals. You sometimes run into

problems: nepheline syenite is considered a felsic rock yet itdoes not contain >66% SiO2.

B] Silica Saturation

As SiO2 is so abundant, a classification can also be based on

the presence or absence of various mineral phases which

reflect the SiO2 content in relation to the other chemical

components.

Typical saturated minerals that can occur with free quartz

include feldspar, Al & Ti poor pyroxene, amphibole, mica,

almandine garnet. Typical undersaturated minerals that are

not stable in the presence of free SiO2 include leucite,nephelene, sodalite, olivine, melanite garnet, corundum, Al

& Ti rich clinopyroxene.



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Classification

Oversaturated rocks - have quartz and tridimite in

abundance

Saturated rocks - have no free quartz and no undersaturated

minerals

Undersaturated rocks - have no quartz and have

undersaturated minerals.

This system is therefore based primarily on relationships of

silica content to the rest of the rock.

C] Alumina Saturation

Based on Al2O3 similar to the SiO2 classification system

Peraluminous: molecular proportion of Al2O3 exceeds the

sum of CaO, Na2O and K 2O. For plagioclase + alkali

feldspar, this ratio is about 1:1. Any Al2O3 that is left over

goes in to forming corundum. These rocks tend to be mica

rich.



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Metaluminous: molecular proportion of Al2O3 exceeds the

sum of Na2O and K

2O, but is less that the sum of Na

2O, K

2O

and CaO. These rocks tend to be rich in anorthite and usually

also contain hornblende, epidote, biotite and pyroxene.

Subaluminous: molecular proportion of Al2O3 is

approximately equal to the sum of Na2O and K 2O. These

rocks tend to form alkali feldspar and a little Ca plagioclase

and usually contain olivine and pyroxenes.

Peralkaline : molecular proportion of Al2O3 is less than the

sum of Na2O and K 2O. There is insufficient alumina to use

all the Na2O and K 2O by making feldspar. The free alkalis

become incorporated into alkali rich ferromagnesium

minerals such as aegerine or reibeckite.

D] Alkali-Lime Index

This system tells us about the alkalinity of the rocks.



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Figure 3-6 plots CaO vs SiO2 and Na2O+K 2O vs SiO2. Since

CaO usually decreases as Na2O+K

2O increases with respect

to SiO2, therefore the curves cross. The SiO2 content, at the

point at which the curves cross, indicates the alkalinity of the

rock suite.

E] Common Chemical X-Y and Ternary Plots

Typically, for X-Y plots you plot oxides against a common

or stable or highly variable component. Which componentsto plot depends on experience and what you wish to know.

Tholeiitic basalts - ophiolites, ocean

floor, greenstone belts.

Alkali basalts - crustal melts, Hawaii

or

or Figure 6-16 - normalized REE



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Common Ternary Plots - 3 component systems

a) A(B)FM Diagram (J.B.Thompson 1957)A=Al2O3

B=K 2O

F=FeO

M=MgO

b) ACF Diagram (Eskola early 1900’s)

A=Al2O3+Fe2O3-(Na2O+K 2O)

C=CaO

F=MgO+FeO+MnO

c) AKF Diagram (Eskola, early 1900’s)

A=Al2O3-(CaO+Na2O+K 2O)

K=K 2O

F=FeO+MgO+MnO



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These plots are used to easily and clearly distinguish

different rock types. They are particularly useful for fine

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 or continental basalts.

These are all relatively immobile trace elements. These

diagrams are useful if the original environment is

scrambled. Eg: ocean floor basalts thrust onto the continent;

basalts within the plate (oceanic or continental) VS plate

margin (ocean ridge to ocean floor).



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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 and

estimate their relative proportions.

c) Use proportions to classify the rock according to a

scheme. Any scheme is somewhat arbitrary. See handout and

Streckeisen.

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 minerals

6) Grain size or texture (extrusive or intrusive)



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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 rich

acidic rocks.

ii) No subdivisions of basalts and andesites. Need further

rules.

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 accepted rock classification system.

• The International Union of Geological Sciences

(IUGS) modified and expanded his work to formwhat is an internationally accepted igneous rock

classification system.

• In order to use this system, you must be able to

determine the percentage of five minerals (or

mineral groups): quartz, plagioclase, alkalifeldspars, ferromagnesian minerals and

feldspathoids (such as nepheline or leucite).

• The Q (or F) , A and P mineral percentage is

recalculated to add to 100% and is plotted on the

triangular plot.



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• The plagioclase rich area of the diagram has some

additional requirements for rock distinction.

• For plutonic rocks: anorthosite is a rock containing

>90% plagioclase, gabbro contains plagioclase

more calcic than An50 and usually contains >35%

mafic minerals (augite, hypersthene or olivine),

Diorite contains plagioclase more sodic than An50

and usually contains >35% mafic minerals(hornblende or hypersthene ± augite).

• For volcanic rocks: the distinction between basalt

and andesite is bases on the silica content. A rock

with >52% SiO2 is andesite while one with <52%

SiO2 is basalt.

Rocks that don’t fit the IUGS Classification

Ultramafic Rocks

Ultramafic rocks (containing more than 90% maficminerals) are classified by alternative methods. Someof the most common types are defined as follows:

Peridotite: a rock containing 40-100% olivine, with theremainder mainly pyroxene and/or hornblende.

Dunite: a rock containing 90-100% olivine with theremainder mainly pyroxene.



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Ultramafic Rocks cont

Pyroxenite: a rock composed mainly of pyroxene with theremainder olivine and/or hornblende.

Hornblendite: a rock composed mainly of hornblende withthe remainder mainly pyroxene and/or olivine .

There are a few rocks that don’t fit the IUGS

classification system that are named on the basis of

texture, with mineral content being of secondary

consideration. Some of the more important of these are

defined as follows:

Pegmatite: a very coarse grained (>1 cm) rock withinterlocking grains. Usually granitic in composition.

Obsidian: a black volcanic glass with conchoidal

fracture, rhyolitic in composition.

Tuff: a compacted deposit of ash and dust containing up

to 50% sedimentary material.



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Breccia: Similar to a tuff, but with large angular

fragments in a fine matrix.

There are also few well recognized igneous rocks

that are found in a highly altered state. The

alteration is related to their method of origin.

Some of the more important of these are defined

as follows:

Spilite: an altered, usually vesicular basaltexhibiting pillow structures. Feldspars have been

altered to albite and is usually found with chlorite,

calcite, epidote, chalcedony or prehnite.

Serpentinite: a rock containing almost entirely

serpentine (from the alteration of olivine and

pyroxene).

Kimberlite: an altered porphyritic mica peridotite

containing olivine (altered to serpentine or a

carbonate mineral) and phlogopite (commonlyaltered to chlorite). Some also contain diamonds.



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There are rock classification systems that attempt to

combine chemistry and mineralogy. In this case, you take

the chemistry data and transform it into theoreticalmineralogy. This is called the CIPW Normative

Classification (Cross, Iddings, Pirsson and Washington).

The norms are based on molecules of ideal composition.

Methodology

A] Convert % oxides into molecular proportions

wt% oxide ÷ formula wt = Molecular Proportion

Eg SiO2 72.67 ÷ 60.09 = 1.211

B] Allocate molecular proportions to minerals using the

following rules:

1) Apatite is one of the first minerals to precipitate. All P

is in apatite.

2) Allocate Fe2O3, FeO to magnetite. The limiting factor is

the total amount of Fe2O3. Molecular proportion of Fe2O3

= Molecular proportion of FeO.

3) Make pure Orthoclae, Albite and Anorthite.

Eg: Orthoclase 1K 2O 1Al2O3 6SiO2

4) Use remaining Al2O3 making Corundum



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5) Allocate remaining FeO, MgO to hypersthene.

Molecular proportion of FeO+MgO = molecular

proportion of SiO2.

6) Allocate the remaining SiO2 to quartz.

C] Once the molecular fraction has been calculated for

each mineral, multiply through by the atomic weight of

that mineral. This will give you a proportion (5) of eachmineral species.

Limitations: Often Severe

1) Can only calculate anhydrous species, therefore biotite

and amphiboles are ignored.

2) normative mineralogy will not equal modal mineralogy

3) Theoretical end members are used which may not

match actual members present.

4) FeO/Fe2O3 allocation can cause problems. It is assigned

to magnetite but what about other iron minerals and iron in

silicate structures?

rock classification

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