Classification of Igneous Rock Section 5 2 Mineral

Classification of Igneous Rock Section 5. 2

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

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

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

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

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

Thin Sections n. A slice of rock usually 2 cm X 4 cm and only 0. 03 mm thick n Light is able to pass through it n A petrographic microscope is used to view thin sections

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

Pegmatites n Pegmatites – veins of extremely largegrained minerals n Lithium and Beryllium ores form in pegmatites n Produce beautiful crystals n Mount Rushmore in South Dakota

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

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