Igneous rocks are further classified according to;

Describes the appearance of an igneous rock, based on the size, shape and arrangement of interlocking crystals.

1. Texture

2. Mineral Composition

Is the mineral makeup of an igneous rock based on the chemical composition of the magma.

Reference:

Pages 61 - 75

Classification of Igneous Rocks:

Classification of Igneous Rocks:

Igneous rocks are classified from

texture

mineral composition

depends on: chemical makeup of parent magma

depends on: how fast/slow magma cools

Igneous compositions

•mainly silicate minerals

•determined by composition of magma from

which it crystallized

•magma mainly 8 elements:

Si, O, Al, Ca, Na, K, Mg, Fe

Most abundant constituents

The minerals that are present in any igneous rock depends on the chemical composition and the environment (temperature of the magma) from which the molten rock crystallizes.

Bowen also noted that minerals will react with the magma to produce the next mineral in the reaction series, this is known as Bowen’s Reaction Series.

N. L. Bowen discovered that different minerals form at different temp.

Ca-Rich Plagioclase

Na-Rich Plagioclase

Mafic

Intermediate

Felsic

Igneous compositions

Olivine and Ca-rich plagioclase feldspar are the first to crystallize at very high temperatures and these minerals are often found in mafic igneous rocks.

Therefore, igneous rocks are classified according to their mineral makeup as Mafic, Intermediate, or Felsic.

Orthoclase feldspar and quartz crystallize at lower temperatures and are found in felsic igneous rocks

Ca-Rich Plagioclase

Na-Rich Plagioclase

Mafic

Intermediate

Felsic

Igneous compositions

Magma

cools

solidifies

forms silicate minerals

Two major silicate mineral groups:

DARK silicates LIGHT silicates

Igneous compositions

DARK

LIGHTLIGHT

Rich in

Fe and/or Mg

Si, K, Na, CaSi, K, Na, Ca

Poor in

Si

Fe and/or MgFe and/or Mg

Silicate group

olivine

pyroxene

amphibole

biotite

quartzquartz

muscovitemuscovite

feldsparsfeldsparsorthoclaseorthoclase

plagioclaseplagioclase

Examples

Igneous compositions

Igneous rocks types

mafic

felsic High in Si.

Lighter and Less Dense

Found in Continental Crust

High in Mg and Fe.

Dark and Dense

Found in Ocean Crust

magnesium + ferrum

feldspar + silica (quartz)

Igneous compositions

Mafic Composition

basaltbasalt

gabbrogabbro

m

a f

i c

m

a f

i c

zooming inzooming in

volc

an

icp

luto

nic

contain magnesium and iron rich minerals

form at high temperatures 1200C

found mainly in oceanic crust

form most volcanic rocks (Basalt)

low percentage of silica, very fluid magmas

forms dark colored minerals, thus rocks are dark color

minerals include; olivine, pyroxene, plagioclase . feldspar

Mafic Composition

andesiteandesite

dioritediorite

i n

t e

r m

e d

i a

t e

i n

t e

r m

e d

i a

t e

zooming inzooming in

volc

an

icp

luto

nic

Intermediate Composition

contains 25% dark silicate minerals (Amphibole, Pyroxene, . and Biotite Mica)

forms at moderate temperatures, approximately 800C

found associated with volcanic activity confined to the ….margins of the continents

form mainly volcanic intermediate rocks (Andesite)

high percentage of silica, very viscous (thick) magmas

contains both light and dark colored minerals

minerals include; Amphibole, Pyroxene, Biotite Mica and . Plagioclase Feldspar

Intermediate Composition

granitegranite

rhyoliterhyolite

zooming inzooming in

f

e l s

i c

f

e l s

i c

plu

ton

icvolc

an

ic

Felsic Composition

obsidianobsidian

pumicepumice

f

e l s

i c

f

e l s

i c

volc

an

icvolc

an

ic

Felsic Composition

contains orthoclase feldspar and quartz (silica)

forms at lower temperatures, approximately 600C

found mainly in continental crust

form mainly plutonic rocks (Granite)

high percentage of silica, very viscous (thick) magmas

forms light colored minerals, thus rocks are light colored

minerals include; orthoclase and plagioclase feldspars, . quartz, and muscovite mica

Felsic Composition

3 main types

basaltic

andesitic

granitic

“felsic”“felsic”

“mafic” magnesium + ferrumhigh in Mg, Fe. Dark, dense

feldspar + silicatehigh in Si, lighter, less dense

Igneous Magma Compositions

basaltic

andesitic

granitic

“felsic”

“mafic”

magnesium + ferrumhigh in Mg, Fe. Dark, dense

feldspar + silicatehigh in Si, lighter, less dense

“felsic”“felsic”

3 main typesIgneous Magma Compositions

Classification Chart

Mineral Composition

Plutonic

Volcanic

Sample Problem

Explain how mineral composition could be used to classify igneous rocks.Answer:

Mineral Composition:

If the igneous rock is light color and consist of the minerals quartz and orthoclase feldspar, it is classified as felsic.

If the igneous rock is dark color and consist of the minerals pyroxene and plagioclase feldspar, it is classified as mafic.