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Petrology Lecture 2

Classification and Nomenclature of Igneous Rocks

GLY 4310 - Spring, 2012

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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.

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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

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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.

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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

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Chemical Composition Classification

• Felsic (acidic) > 66 wt. % SiO2

• Intermediate 52-66 wt. % SiO2

• Mafic (basic) 45-52 wt. % SiO2

• Ultramafic (ultrabasic) < 45 wt. % SiO2

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Alumina Saturation

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Color Index

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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

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Using IUGS Classification II

• 3. Determine Q + A + P or F + A + P

• 4. Determine if the rock is phaneritic (intrusive) or aphanitic (extrusive)

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IUGS Classification• Phaneritic

rocks

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IUGS Classification

• Aphanitic Rocks

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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

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IUGS Classification

• Gabbroic rocks

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IUGS Classification• Ultramafic Rocks

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IUGS Classification• Chemical classification of volcanic rocks

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IUGS Classification

• Pyroclastic rocks• Based on type of

fragmental material

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IUGS Classification

• Pyroclastic rocks• Based on fragment

size

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Size Classification

• 1. Ash < 2mm

• 2. Lapilli 2-64 mm

• 3. Blocks or bombs >64 mm