Composition and constituents of igneous rocks_Md. Yousuf Gazi

1. Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

2. Common Elements in Magma
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

3. Common Oxides in magma
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

4. Pyrogenetic minerals
➢ The minerals formed from igneous magmas are termed
pyrogenetic .
➢ Since Oxygen and Silicon are the most abundant
elements in magma, silicates and silica forms the chief
igneous minerals.
➢ Hence, most of the igneous rock minerals are silicates.
➢ A few other oxides occur besides silica.
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

5. Common silicates in magma
Pyrogenetic silicates fall into three groups:
1. Orthosilicates
Example: Olivine - 2(MgFe)O,SiO2
2. Metasilicates
Example: Enstatite – (MgFe)O,SiO2
3. Polysilicates
Example: Orthoclase-K2O.Al2O3,6SiO2
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

6. Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

7. ❑ Sodium(Na) and Potassium(K) are form more active bases
(elements) in magma. Calcium(Ca) is less and Magnesium(mg) and
Iron(Fe) are relatively weakest.
❑ Na and K have high affinity with Si and O2. So they instantly form
polysilicates. If there is a deficiency of Si and O2. then they form
metasilicates and if the quantity are decreases orthosilicates are
formed.
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

8. Minerals of Low Silication
If there is a considerably deficiency of silica in magma, elements may not be
able to combine with sufficient silica to form minerals. This type of minerals
are called minerals of low silication. Example: K and Na form Leucite and
Nephetine instead of orthoclase and albite.
Minerals of High Silication
In magma, when there is an adequate amount of silica, elements form the
minerals which are rich in silica contents. These minerals are termed as
minerals of high silication.
Mineral of Low silication+silica Mineral of High silication
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

9. Minerals of Low Silication Minerals of High Silication
Leucite Orthoclase
Nepheline Albite
Analcite Anorthoclase
Olivine Orthorhombic pyroxene
Biotite Augite
Aegirine
Hornblende
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

10. Quartz(SiO2) doesn’t exist with minerlas of low silication
Any excess of silica which may be left over after the bases are fully satisfied
crystallizes out as quartz. The inter relationship of minerals of low and high silication
shows that minerals of low silication and SiO2 combinedly forms minerals of high
silication. Hence, quartz can’t co-exists with minerals of low silication.
NaAl(SiO4) + 2(SiO2) = NaAl.Si3O8
Nepheline Quartz Albite
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

11. Essential Igneous rock forming minerals
Felsic/Silicic/Light colored
minerals
➢ Quartz
➢ Plagioclase Feldspar
➢ Potash Feldsper
➢ Mica(muscovite)
Mafic/Ultramafic/Dark
colored minerals
➢ Olivine
➢ Pyroxene
➢ Amphiboles
➢ Biotite
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)