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Understanding Igneous Rocks: Composition and Texture

Explore the world of igneous rocks, from magma to volcanic rocks. Learn about different textures like aphanitic and phaneritic, and distinguish between mafic and silicic compositions. Discover common types such as granite, diorite, gabbro, rhyolite, and basalt. The classification based on composition and texture is demystified, and key minerals like orthoclase, biotite, plagioclase, quartz, and hornblende are explained. Unravel the mysteries of porphyritic and glassy textures in various igneous rock formations.

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Understanding Igneous Rocks: Composition and Texture

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  1. Igneous Rocks

  2. Igneous Rocks • Magma- molten rock material • Lava- magma that reaches the surface of the Earth • Volcanic Rocks- from form lava at the surface • Cool quickly so crystals are small • Plutonic Rocks- cool beneath the surface • Cool slowly so fewer and larger crystals

  3. Igneous Rocks • Textures: • Aphanitic- crystals too small to see by eye • Phaneritic- can see the constituent minerals • Fine grained- < 1 mm diameter (< sugar) • Medium grained- 1-5 mm diameter (< pea) • Coarse grained- 5-50 mm diameter • Pegmatitic- > 50 mm diameter • Porphyritic- bimodal grain size distribution • Glassy- no crystals formed • Vesicular- contains voids (once gas bubbles) • Fragmental- contains pieces of rock and mineral expelled from an explosive eruption

  4. Classification based on composition (Mafic –Silicic) vs. texture

  5. Classification based on composition (Mafic –Silicic) vs. texture

  6. Igneous Rocks Orthoclase (pink) Biotite (shiny black flakes) Plagioclase (white) Quartz (gray, transparent, glassy-looking) Hornblende (black laths) Granite- Coarse-grained (plutonic) silicic

  7. Granite in hand sample and thin section

  8. Igneous Rocks Plagioclase (white) Hornblende (black) Diorite- phaneritic intermediate “salt-and-pepper”

  9. Igneous Rocks Plagioclase (gray) Pryoxene (blackish green) Gabbro- phaneritic mafic (dark)

  10. Igneous Rocks Rhyolite- aphanitic silicic = light-colored (pink, tan, lt gray)

  11. Igneous Rocks Porphyritic Andesite Andesite- aphanitic intermediate (green, gray, red)

  12. Basaltic Scoria- aphanitic dark-colored with numerous vesicles

  13. Igneous Rocks Basalt in thin-section under the polarizing microscope

  14. Igneous Rocks Porphyritic Gabbro- coarse plagioclase phenocrysts in a fine-grained phaneritic groundmass

  15. Igneous Rocks Glassy texture: obsidian (really silicic rhyolite although dark-colored)

  16. Igneous Rocks Rhyolite Pumice- frothy vesicular, light color

  17. Fragmental texture of andesitic volcanic breccia

  18. Igneous Rocks Mafic xenoliths and silicic dike in granite

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