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Petrology Lecture 2

Petrology Lecture 2. Classification and Nomenclature of Igneous Rocks GLY 4310 - Spring, 2012. Igneous Textures. Phaneritic: Crystals are readily visible with the unaided eye. Aphanitic: Crystals, if present, are too small to be seen with the unaided eye.

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Petrology Lecture 2

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  1. Petrology Lecture 2 Classification and Nomenclature of Igneous Rocks GLY 4310 - Spring, 2012

  2. Igneous Textures • Phaneritic: Crystals are readily visible with the unaided eye. • Aphanitic: Crystals, if present, are too small to be seen with the unaided eye. • Fragmental: Composed of pieces of pre-existing, mostly igneous, rock, crystal fragments, and/or glass. These pieces were deposited and later amalgamated into a rock.

  3. Porphyritic Size Distribution • Porphyritic - bimodal size distribution, with large grains surrounded by numerous small grains or glass • Phenocrysts -Large crystals formed by relatively slow cooling below the earth’s surface • Groundmass - Small crystals or glass, formed by more rapid cooling

  4. Composition • Felsic: Rocks composed primarily of silica-rich minerals, generally feldspars and silica, from which the term is derived. Common minerals include plagioclase feldspar, alkali feldspar, quartz, and muscovite. The term can be used to modify either a mineral or a rock name. • Mafic: Rocks rich in magnesium and iron, and hence with lower silica contents. Common minerals include olivine, pyroxene, hornblende, and biotite. The term can be used to modify either a mineral or a rock name.

  5. Types of Minerals • Essential – Minerals which must be present in order for a rock to be classified with a certain name • Accessory – Minerals need not be present in a rock, but which may be present in small amounts

  6. Chemical Composition Classification • Felsic (acidic) > 66 wt. % SiO2 • Intermediate 52-66 wt. % SiO2 • Mafic (basic) 45-52 wt. % SiO2 • Ultramafic (ultrabasic) < 45 wt. % SiO2

  7. Alumina Saturation

  8. Color Index

  9. Using IUGS Classification • 1. Determine the mode • 2. Determine the volume percent of several classes of minerals: • a. Q = % quartz (rarely, other silica phases) • b. P = % plagioclase, An05-100 • c. A = % alkali feldspar (K-spar and An00-05) • d. F = % feldspathoids (often called foids) • e. M = % mafics and accessories

  10. Using IUGS Classification II • 3. Determine Q + A + P or F + A + P • 4. Determine if the rock is phaneritic (intrusive) or aphanitic (extrusive)

  11. IUGS Classification • Phaneritic rocks

  12. IUGS Classification • Aphanitic Rocks

  13. Using IUGS Classification III • 5. Determine the name of the rock from the diagram • 6. If P + M (gabbroic) >90% or M > 90% (ultramafic), different classification schemes are used

  14. IUGS Classification • Gabbroic rocks

  15. IUGS Classification • Ultramafic Rocks

  16. IUGS Classification • Chemical classification of volcanic rocks

  17. IUGS Classification • Pyroclastic rocks • Based on type of fragmental material

  18. IUGS Classification • Pyroclastic rocks • Based on fragment size

  19. Size Classification • 1. Ash < 2mm • 2. Lapilli 2-64 mm • 3. Blocks or bombs >64 mm

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