Igneous rocks are formed through the colling and solidification of magma or lava. The magma can be derived form partial melts of existing rocks neither a planets mantle or curst .

1. Introduction To Geochemistry of
Igneous Rocks

2. Igneous Rocks
• Igneous rocks are formed through the colling
and solidification of magma or lava.
• The magma can be derived form partial melts
of existing rocks neither a planets mantle or
curst .

3. Why rocks are melts?
• Typically the meltin is caused by one or more
of the three processes.
• An increase in temperature,
• A decrease in pressure,
• A change in composition,

4. Solidification into rocks occurs
either below the surface as
intrusive rocks.
If on the surface ,
extrusive rocks

5. Chemistry of igneous rocks:
• The chemical composition of magma
determines which minerals and how much of
each will crystallize when an igneous rocks
forms.
• Magma contains :
• Silicon
• Oxygen

6. Igneous rocks are rocks composed By
• Primarily by silicate minerals as;
Quartz, plagioclase ,feldspar, potassium feldspar,
amphibole, pyroxene, biotite and olivine.
A magma that is rich in silica, aluminium,
potassium, and sodium will crystallize that
contain those elements (feldspar and quartz)

7. magma rich in
iron and magnesium and calcium will contain
• Dark colour fero-magneium minerals,
• pyroxene
• Amphibole
• olivine and biotite.

8. Chemical analysis by weight % of
oxides
• e.g Si, O, Mg, Na
• Where siO2 is most abundent elements.
Based on amount of silica. Igneous rocks are
classified into four groups .
Which are in order of decreasing silica content.

9. Felsic
intermediate
mafic
ultramafic

10. FELSIC ROCKS
• Rcks with a silica content of 65% or more (by
weight) are cconsidered to be silica rich.
• The remaicning 25% to 35% of these rocks id
mostly ;
• Aluminium oxide (Al2O3)
• Sodium oxide (Na2O)
• Potasium (K2O)

11. Small amount of
calcium (CaO)
Magnesium (Mgo)
Iron (Feo and Fe2O3)

12. These are called felsic rocks silica
rich igneous rocks with a relatively
hig content of potassium and
sodium

13. FEL----SIC
Fel feldspar
Sic silica
Feldspar crystallize from the K , Na ,Al
Silica from silicon oxides
‘

14. INTERMEDIATE
ROCKS
• Rocks with a silica content of between
55% to 65% are classified as intermediate
rocks

15. Chemistry of intermediate Igneous rocks
• Rocks contain significant amounts
(30 to 50%)
Of dark ferromagnesium minerals
Like
pyroxene
amphibole
Light colored plagioclase feldspar and small
amount of quartz.

16. Diorites
• The course grained intermediate rocks.about
half of the mineral grains are dark in colour
and the other half are white.

17. Andesite
• Fine grained and medium grey or greenish
grey .

18. Rhyolite
• Is a fine grained extrusive felsic rocks

19. Granite
• Is the coarse grained intrusive equivalent of
rhyolite

20. Mafic rocks
• Rocks with a silica content of between 45%
and 55% by weight are considered as silica-
poor.
• The remaining is composed mainly of the
oxides of aluminum, calcium, magnesium and
iron.

21. • Rocks in this group are called mafic
silica-deficient igneous rocks with a
relatively high content of
magnesium, iron and calcium.
• The mafic term comes from
magnesium and ferum the latin
word for iron.

22. Chemistry of Mafic rocks
• Mafic rocks are made up of ;
• Grey plagioclase
• Feldspar
And the ferromagnesium minerals
• Pyroxene and
• olivine
Tend to be dark color.

23. Gabbro
• Mafic magma that cool slowly beneth the
surface form the coarse grained intrusive rock
called as gabbro

24. Basalt
• If magma erupts on the surfaces, it forms the
dark fine grained extrusive rocks called as
basalt.

25. Ultramafic Rocks
• Ans ultra-mafic rocks is one that contain less
than 45% silica and rich in iron, magnesium
and clacium.

26. Chemistry of ultramafic Rocks
• Ultramafic rocks are composed entirely of the
ferromagnesium minerals :
• Olivine and pyroxene
• No feldspar and quartz are present.
• Most ultramafic rocks comes form mantle
rather than form the earth’s crust .

27. Komatite
• The volcanic ultramafic rocks is ver
rare, ultramafic extrusive rocks are
mostly resdricted to the very early
history of the earth.

28. Peridote
• The coarse graine intyusive rocks is composed
of olivine and pyoxene and is the most
abundant ultramafic rocks.

29. INTRUSIVE IGNEOUS ROCKS
• Intusive igneous rocks crystallize below earth’s
surface and the slow cooling that occur there
allows large crustals to form
• Examples of intrusice ingeous rocks
• Diorite
• Gabbro
• Granite pegmatite
• peridotite