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Classification of Igneous
Section 5.2
Mineral Composition
– Igneous intrusion
– Large crystals (macroscopic)
– Lava flow or flood basalts
– Small crystals (requires magnification)
Mineral Composition
Basaltic rocks
– Dark colored
– Lower silica content
– Contain mostly plagioclase and pyroxene
Granite rocks
– Light colored
– High silica content
– Contain mostly quartz, K+ & plagioclase
Mineral Compositions
Intermediate rocks have a composition
of minerals between baslatic and granite
 Ultrabasic rocks
– Contain only iron-rich minerals
– Always dark
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
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
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
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
Pegmatites – veins of extremely largegrained minerals
 Lithium and Beryllium ores form in
 Produce beautiful crystals
 Mount Rushmore in South Dakota
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
Construction Uses
Interlocking grain textures make igneous
rocks strong
 Resistant to weathering
 Granite is used in countertops, floors, and