1 Petrology Lecture 2 Classification and Nomenclature of Igneous Rocks GLY 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. % SiO 2 Intermediate52-66 wt. % SiO 2 Mafic (basic)45-52 wt. % SiO 2 Ultramafic (ultrabasic) < 45 wt. % SiO 2

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, An  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