1. 1 Petrology Lecture 2 Classification and Nomenclature of Igneous Rocks GLY 4310 - Spring, 2016

2. 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. 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. 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. 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. 6 Chemical Composition Classification Felsic (acidic) > 66 wt. % SiO 2 Intermediate52-66 wt. % SiO 2 Mafic (basic)45-52 wt. % SiO 2 Ultramafic (ultrabasic) < 45 wt. % SiO 2

7. 7 Alumina Saturation

8. 8 Color Index

9. 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. 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. 11 IUGS Classification Phaneritic rocks

12. 12 IUGS Classification Aphanitic Rocks

13. 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. 14 IUGS Classification Gabbroic rocks

15. 15 IUGS Classification Ultramafic Rocks

16. 16 IUGS Classification Phaneritic rocks

17. Aphanetic Rocks In principle, we follow the same steps for volcanic rocks as we do for intrusive rocks However, the nature of the aphanitic rocks makes determination of the mode difficult, especially in hand specimen Even in thin section, groundmass material may be too small to recognize, or may be amorphous Phenotypes – based on phenocrysts only 17

18. “P” Apex Classification Again, rocks near the “P” apex are troublesome.  Andesite may be defined as a plagioclase-rich rock with SiO 2 > 52%, or a color index < 35%  Basalt has SiO 2 35%. 18

19. 19 IUGS Classification Chemical classification of volcanic rocks

20. Problems with Pyroclastics These 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 approach 20

21. 21 IUGS Classification Pyroclastic rocks Based on type of fragmental material

22. 22 IUGS Classification Pyroclastic rocks Based on fragment size  Ash < 2mm  Lapilli 2-64 mm  Blocks or bombs >64 mm