Igneous Overview

7/29/2019 Igneous Overview

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Overview of IgneousRocks


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Igneous rocks crystallize from magmasan most magmas are s cate magmas

(containing 40 to 75 wt.% SiO2)

Felsic (silicic or sialic) magmas are high in

Intermediate magmas are between Felsic

Mafic magmas contain less than 50 wt.%

2 an g er g , e an e2 3 Ultramafic magmas are even more SiO2

poor and MgO/FeO rich


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Igneous Rocks

Based on Felsic Mineral Content or rec e sen agram

For fine-grained volcanic rocks, naming isase on c em ca compos t on p otNa2O+K2O vs. SiO2)


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Streckeisen

(QAPF)

PlutonicIgneous Rock


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IUGS Volcanic Classification is Graphical

Alkaline


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How to plot on the Streckeisen Diagram

Note that several fields

+A+P

,

which do you use?

Q/ or e as . . an

Na-rich plagioclase with

An35 andCa-rich plagioclase with

>+A+P Anorthosite has >90%

plagioclase (


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Shows order in which minerals crystallizerom a yp ca ma c or asa c magma

Left side is called Discontinuous Side Mafic minerals change abruptly

Right side is called Continuous Side Plagioclase changes composition gradually


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Bowens Reaction Series

olivine Ca plagioclase

T

orthopyroxeneclinopyroxene NaCa plagioclase

amphibole (Hb)erature

biotite Na plagioclase

coolingtem

muscoviteresidual

phases

low T


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,

An Isolated Tetrahedral Silicate

complete solid solution with two end-mem ers:

2 4 Fe SiO Fa alite Fa


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Vitreous luster-

No ood cleava es H=6.5 G=3.2

Optical: high relief, moderately


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Olivine in thin section


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Pyroxenes Common in mafic and intermediate igneous rocks,

typically crystallize from basalt after Olivine (BRS)

Single Chain Silicates General Formula: Ca2+ M 2+ Fe2+ Si O

Two kinds:

Ortho roxenes O x M Fe Si O are orthorhombicand have perpendicular axes, low birefringence, // ext.

Enstatite

Clinopyroxenes (Cpx) Ca(Mg,Fe)Si2O6 are monoclinicand have one inclined axis, higher biref., inclined ext.

Augite and Diopside


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PyroxeneRelief, Color

PPL


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Orthopyroxene Properties

Orthopyroxenes (enstatite, hypersthene,

Color gray, bronze, brown Colorless to tan in thin section

High relief

Low birefringence


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Clinopyroxene Properties

Diopside (more likely in metamorphic rocks)

a~1 g~1 2 6 Color light green

Colorless in thin section

Moderate birefrin ence inclined extinction Augite Ca~1(Mg,Fe)~1Si2O6 (typical igneous Cpx)

Tan, light brown or green in thin section

Moderate birefringence, inclined extinction


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Augite (moderate birefringence, twinningand cleavage)


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Hornblende (Hb)

is the main amphibole in igneous rocks

from mafic magmas, and at lowertemperature

is most common in intermediatecomposition igneous rocks

ou e-c a n s ca e


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General Amphibole Formula:

(Na,K)0-1(Ca,Na,Fe,Mg)2(Mg,Fe,Al)5(Si,Al)8O22(OH)2 large medium small tetrahedral cations

Simple Amphibole (Tremolite):

_ a2 g5 8 22 2

Hornblende Formula:

(Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2 Substitution of (Alvi, Aliv) for (Mgvi, Siiv) of tremolite


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Black or dark green

Vitreous luster, translucent = - = -. .

two perfect prismatic cleavages at about 60o and ,

commonly in prisms with flattened hexagonal

cross-sect ons


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Various shades of brown, red-brown, green,

ue-green or tan n t n sect on

Moderate to strong pleochroism is typical

Moderate birefringence, commonly

obscured by its color Two good cleavages at ~60o and 120o toeachother

You have to be looking down the prism tosee 2 crossing sets of cleavages, in manyv ews you w see on y one se o c eavages


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Igneous Amphiboles

B g Horn en e Crysta s n t s ne-gra ne

volcanic rock are called Phenocrysts


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Hb cleavage, color & pleochroism


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Kaersutite Na Ti-rich i neous am hibole

w t strong re - rown p eoc ro sm


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Biotite is the lowest Mafic Mineral on

scon nuous e o owen s er es

t crysta zes very ate rom ract onate ma cmagmas and is only common in intermediate toe s c magmas, 2 must e present or ot te

to crystallize Biotite is a trioctahedral, t-o-t (or 2:1) sheetsilicate

Biotites properties: its strongly pleochroic,has cleava e arallel extinction birds-e eextinction and pleochroic haloes around zircon


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H2O in the liquid

biotite, except for color and pleochroism

ommon n ran tes an egmat tes


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Felsic Minerals on BRS

olivine Ca plagioclase

T

orthopyroxene

clinopyroxene NaCa plagioclase

amphibole (Hb)

e

rature

biotite Na plagioclase

coolingtem

muscovite residualphases

low T


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= =, .

Generally clear and glassy, may have avar e y o co ors c ear, smo y, rown, rose;its allochromatic)

Conc o a racture, no c eavage

Habit: hexagonal (6-sided prisms) or

massive O tical: low relief and low birefrin ence


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Quartz in Granite Thin Section

From Atlas of Rocks & Minerals in Thin Section


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The Feldspar Ternarya 2 2 8

AnorthiteAll naturale spars

No feldspars

NaAlSi3O8 KAlSi3O8

r oc ase

alkali feldspars


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.Properties

H=6, G=2.56 ,

or flesh-colored

Habit: stubby prisms, simple twins common

Optical: low relief and low birefringence Commonl Perthitic micro and macro


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Alkali Felds ars have Perthites

What do Perthites look like?

Thinsection

in XPL


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Alkali Feldspars have Perthites

What do Perthites look like?

1mm


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What causes Perthites?

Caused by un-mixing, exsolution orse aration of Na+ diameter~1.1 and K+

(diameter~1.6) as the feldspar cools

At low temperatures, there is a miscibility

3 8 3 8


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plaid twinning

1mm


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= - = - = =. , . . , .

Luster pearly, vitreous/translucent Co or w te to gray

One perfect, one good cleavage Optical: low relief and low birefringence

Not Perthitic!

Common y zone

Pl i l F ld h


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Plagioclase Feldspars have

po ysyn e c, ame ar e w ns

- blein

n

han

y

visi

L

)

siblei

usual

n(XP

bev

i

imen

,

secti

M

a

s

pe

thin


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Large crystals ofQuartz,

Muscovite(which are low on Bowens


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Elements and Volatiles

Incompatible Elements those elements

minerals (Li, B, Be)

o at es t ose e ements t at ave astrong preference for the gaseous phase

2 , 2, , ,


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Phase Diagrams tell MUCH moreabout Crystallization and Melting

What follows is just a quick preview of

some of the things phase diagrams can beused for

H d thi di t t ll


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How do we use this diagram to tell

Diopside-richqu

80% CaMgSi2O6 (Di)20% CaAl Si O An

X

Draw isopleth=

line of constantcomposition

What happens as

liquid cools?Anorthite-rich li uid Y65% CaAl2Si2O8 (An)

35% CaMgSi2O6 (Di)


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Textures of rocks in Di-An System

2 component system with


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2-component system with

Crystallization ofPlagioclase inBasalts (realgneous p ag oc ase

is not pure An like-

system)

P C iti d th


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Pyroxene Compositions and the

yroxene ua r atera

CaFeSi2O6CaMgSi2O6

ops e e en erg e

Augite

Ortho roxene

Mg2Si2O6 Fe2Si2O6

FerrosiliteEnstatite orthopyroxenes

Date post:	14-Apr-2018
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Igneous Overview

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