1. Classification of Igneous Rock
Section 5.2

2. Mineral Composition Intrusive Extrusive Igneous intrusion
Large crystals (macroscopic)
Extrusive
Lava flow or flood basalts
Small crystals (requires magnification)

3. Mineral Composition Basaltic rocks Granite rocks Dark colored
Lower silica content
Contain mostly plagioclase and pyroxene
Granite rocks
Light colored
High silica content
Contain mostly quartz, K+ & plagioclase feldspar

4. Mineral Compositions
Intermediate rocks have a composition of minerals between baslatic and granite rocks
Ultrabasic rocks
Contain only iron-rich minerals
Always dark

5. Texture
Texture refers to the size, shape, and distribution of the crystals or grains that make up a rock
Rhyolite – fine-grained
Granite – coarse-grained
How is crystal size affected by cooling rate?

6. Texture
Porphyritic texture – large well formed crystals surrounded by finer grained crystals of the same or different mineral
Forms when slowly cooling magma suddenly cooled rapidly
Vesicular texture – spongy appearance; Pumice is an example

7. Thin Sections
A slice of rock usually 2cm X 4 cm and only 0.03 mm thick
Light is able to pass through it
A petrographic microscope is used to view thin sections

8. Veins
Valuable ores occur within igneous intrusions; Ores also occur in the rocks surrounding intrusions called veins
Important metallic elements include gold, silver, lead, and copper

9. Pegmatites Pegmatites – veins of extremely large-grained minerals
Lithium and Beryllium ores form in pegmatites
Produce beautiful crystals
Mount Rushmore in South Dakota

10. Kimberlites Kimberlites are ultrabasic rocks
Form at depths of 150 to 300 km
Named for Kimberly, South Africa
Diamonds form in kimberlites
Where in the US have kimberlites been found?

11. Construction Uses
Interlocking grain textures make igneous rocks strong
Resistant to weathering
Granite is used in countertops, floors, and statues