Petrology - Florida Atlantic University

Petrology - Florida Atlantic University

Petrology Lecture 2 Classification and Nomenclature of Igneous Rocks GLY 4310 - Spring, 2019 1 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. 2 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 earths surface Groundmass - Small crystals or glass, formed by more rapid cooling 3

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 or other amphiboles, and biotite. The term can be used to modify either a mineral or a rock name. 4 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 5 Chemical Composition Classification

Felsic (acidic) Intermediate > 66 wt. % SiO2 52-66 wt. % SiO2 Mafic (basic) 45-52 wt. % SiO2 Ultramafic (ultrabasic) < 45 wt. % SiO2

6 Alumina Saturation 7 Color Index

8 Using IUGS Classification 1. Determine the mode by determining 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 9

Using IUGS Classification II 2. Determine Q + A + P or F + A + P 3. Determine if the rock is phaneritic (intrusive) or aphanitic (extrusive) 10 IUGS

Classification Phaneritic rocks 11 IUGS Classification Aphanitic Rocks

12 Using IUGS Classification III 4. Determine the name of the rock from the diagram 5. If P + M (gabbroic) >90% or M > 90% (ultramafic), different classification schemes are used

13 IUGS Classification Gabbroic rocks 14

IUGS Classification Ultramafic Rocks 15 IUGS Classification Phaneritic rocks

16 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 P Apex Classification Again, 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 IUGS Classification Chemical classification of volcanic rocks 19

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 IUGS Classification Pyroclastic rocks Based on type of fragmental material

21 IUGS Classification Pyroclastic rocks Based on fragment size Ash < 2mm Lapilli 2-64 mm

Blocks or bombs >64 mm 22 Web Resource The preceding was a very short outline of igneous rock classification for those students wanting more information, the following is a good

resource: http://www.sepmstrata.org/page.aspx?pageid=593 Includes flow charts designed to aid in igneous rock classification 23

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