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Classification of Igneous Rock

Classification of Igneous Rock. Section 5.2. Mineral Composition. Intrusive Igneous intrusion Large crystals (macroscopic) Extrusive Lava flow or flood basalts Small crystals (requires magnification). Mineral Composition. Basaltic rocks Dark colored Lower silica content

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Classification of Igneous Rock

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  1. Classification of Igneous Rock Section 5.2

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

  3. 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 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

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