GLY 4310 Spring 2020 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 ID: 1021032
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1. 1Petrology Lecture 2Classification and Nomenclature of Igneous Rocks GLY 4310 - Spring, 2020
2. 2Igneous TexturesPhaneritic: 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. 3Porphyritic Size DistributionPorphyritic - bimodal size distribution, with large grains surrounded by numerous small grains or glassPhenocrysts - Large crystals formed by relatively slow cooling below the earth’s surfaceGroundmass - Small crystals or glass, formed by more rapid cooling
4. 4CompositionFelsic: 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 or other amphiboles, and biotite. The term can be used to modify either a mineral or a rock name.
5. 5Types of MineralsEssential – Minerals which must be present in order for a rock to be classified with a certain nameAccessory – Minerals need not be present in a rock, but which may be present in small amounts
6. 6Chemical Composition ClassificationFelsic (acidic) > 66 wt. % SiO2Intermediate 52-66 wt. % SiO2Mafic (basic) 45-52 wt. % SiO2Ultramafic (ultrabasic) < 45 wt. % SiO2
7. 7Alumina Saturation
8. 8Color Index
9. 9Using IUGS Classification1. Determine the mode – by determining the volume percent of several classes of minerals:a. Q = % quartz (rarely, other silica phases)b. P = % plagioclase, An05-100c. A = % alkali feldspar (K-spar and An00-05)d. F = % feldspathoids (often called foids)e. M = % mafics and accessories
10. 10Using IUGS Classification II2. Determine Q + A + P or F + A + P3. Determine if the rock is phaneritic (intrusive) or aphanitic (extrusive)
11. 11IUGS ClassificationPhaneritic rocks
12. 12IUGS ClassificationAphanitic Rocks
13. 13Using IUGS Classification III4. Determine the name of the rock from the diagram5. If P + M (gabbroic) >90% or M > 90% (ultramafic), different classification schemes are used
14. 14IUGS ClassificationGabbroic rocks
15. 15IUGS ClassificationUltramafic Rocks
16. 16IUGS ClassificationPhaneritic rocks
17. Aphanetic RocksIn principle, we follow the same steps for volcanic rocks as we do for intrusive rocksHowever, the nature of the aphanitic rocks makes determination of the mode difficult, especially in hand specimenEven in thin section, groundmass material may be too small to recognize, or may be amorphous Phenotypes – based on phenocrysts only17
18. “P” Apex ClassificationAgain, rocks near the “P” apex are troublesome.Andesite may be defined as a plagioclase-rich rock with SiO2 > 52%, or a color index < 35%Basalt has SiO2 < 52%, or a color index > 35%. 18
19. 19IUGS ClassificationChemical classification of volcanic rocks
20. Problems with PyroclasticsThese rocks present special problems, because they often contain significant impurities (material blasted out by the eruption, or caught in the general updraft)In principle, they can be classified on the basis of a chemical analysis, but the presence of significant impurities argues against this approach20
21. 21IUGS ClassificationPyroclastic rocksBased on type of fragmental material
22. 22IUGS ClassificationPyroclastic rocksBased on fragment sizeAsh < 2mmLapilli 2-64 mmBlocks or bombs >64 mm
23. Web ResourceThe preceding was a very short outline of igneous rock classification – for those students wanting more information, the following are good resource:Igneous Rock Classification – from the Cascade Volcanic ObservatoryGeneral Classification of Igneous Rocks – from Prof. Stephen A. Nelson, Tulane University23