This document discusses classifying igneous rocks based on their texture, grain size, color, density, and chemical composition. Igneous rocks can be classified as volcanic, hypabyssal, or plutonic based on grain size, and further classified as ultramafic, mafic, intermediate, or acid based on their chemistry and mineral content, with darker, denser rocks generally being richer in ferromagnesian minerals and lighter rocks richer in quartz and feldspar.

1. Igneous rock classification

2. Classifying igneous rocks
• Igneous rocks come in a range of different
textures and chemical compositions.
• We can easily classify hand-specimens by
colour and by grainsize.

3. Dark-coloured

4. Light-coloured

5. Fine-grained

6. Medium-grained

7. Coarse-grained

8. Simple classification
• As a rule of thumb, rocks rich in
ferromagnesian minerals tend to have a
darker colour and higher density than
rocks rich in quartz and alkali feldspar.
Obsidian is the obvious exception.
• Volcanic rocks tend to have very fine
grains. Grainsize generally increases with
the time taken for crystallisation.

9. Classifying igneous rocks
• Igneous rocks come in a range of different
textures and chemical compositions
Ultramafic Mafic Intermediate Acid
Volcanic Basalt Andesite Rhyolite
< 1mm
Hypabyssal Dolerite Micro-diorite Micro-granite
1-5mm
Plutonic Peridotite Gabbro Diorite Granite
>5mm

10. Classification by chemistry

11. Classification by mineralogy

12. Classification summary