Unit 2: Petrography of Common Igneous Rocks

Introduction to Petrography

Nomenclature is the system of naming rocks. Petrography is the branch of geology that deals with the description and systematic classification of rocks, especially using microscopic examination of thin sections.

For each rock, we will describe:

• Type: Intrusive (Plutonic) or Extrusive (Volcanic).
• Composition: Felsic, Intermediate, Mafic, or Ultramafic.
• Texture: e.g., Phaneritic, Aphanitic, Porphyritic.
• Essential Minerals: The minerals that must be present to define the rock.
• Accessory Minerals: Minerals present in small amounts that don't affect the classification.

Felsic Rocks (High Silica)

Granite

• Type: Intrusive (Plutonic)
• Composition: Felsic
• Texture: Phaneritic (coarse-grained), often equigranular.
• Essential Minerals: Quartz (>20% of felsic minerals) and Alkali Feldspar (K-Feldspar) > Plagioclase Feldspar.
• Accessory Minerals: Biotite, Muscovite, Hornblende.
• Description: Typically light-colored (pink, white, or gray). Forms massive batholiths and stocks, core of continents.

Pegmatite

• Type: Intrusive (Hypabyssal)
• Composition: Felsic (usually granitic)
• Texture: Very coarse-grained (Phaneritic), with crystals often several centimeters to meters long.
• Essential Minerals: Same as granite (Quartz, K-Feldspar, Plagioclase).
• Description: Forms from the last, water-rich portion of a cooling magma. Famous for containing rare minerals (e.g., beryl, tourmaline, lithium minerals).

Rhyolite

• Type: Extrusive (Volcanic)
• Composition: Felsic
• Texture: Aphanitic, often porphyritic (with Quartz or K-Feldspar phenocrysts) or glassy.
• Essential Minerals: Same as Granite, but too small to see (fine-grained groundmass of quartz and feldspar).
• Description: The volcanic equivalent of Granite. Associated with very viscous, explosive eruptions.

Intermediate Rocks

Diorite

• Type: Intrusive (Plutonic)
• Composition: Intermediate
• Texture: Phaneritic (coarse-grained).
• Essential Minerals: Plagioclase Feldspar (Na-rich) and Amphibole (Hornblende). Contains little to no Quartz.
• Accessory Minerals: Biotite, Pyroxene.
• Description: "Salt and pepper" appearance (white plagioclase and black hornblende).

Granodiorite

• Type: Intrusive (Plutonic)
• Composition: Intermediate (transitional between Granite and Diorite).
• Texture: Phaneritic (coarse-grained).
• Essential Minerals: Quartz, Plagioclase Feldspar > Alkali Feldspar.
• Accessory Minerals: Biotite, Hornblende.
• Description: Looks similar to granite but is typically darker due to more mafic minerals and plagioclase.

Andesite

• Type: Extrusive (Volcanic)
• Composition: Intermediate
• Texture: Aphanitic, commonly Porphyritic (with Plagioclase or Hornblende phenocrysts).
• Essential Minerals: Same as Diorite (fine-grained groundmass of plagioclase and amphibole).
• Description: The volcanic equivalent of Diorite. Typically gray or reddish-brown. Common in subduction zone volcanoes (e.g., the Andes Mountains).

Dacite

• Type: Extrusive (Volcanic)
• Composition: Intermediate (transitional)
• Texture: Aphanitic, commonly Porphyritic (with Quartz and Plagioclase phenocrysts).
• Essential Minerals: Same as Granodiorite (fine-grained groundmass).
• Description: The volcanic equivalent of Granodiorite.

Mafic Rocks (Low Silica)

Gabbro

• Type: Intrusive (Plutonic)
• Composition: Mafic
• Texture: Phaneritic (coarse-grained).
• Essential Minerals: Plagioclase Feldspar (Ca-rich) and Pyroxene (Clinopyroxene). Olivine may be present.
• Description: Dark-colored (black or dark green). Forms in magma chambers beneath mid-ocean ridges and in large mafic intrusions.

Dolerite (or Diabase)

• Type: Intrusive (Hypabyssal - shallow)
• Composition: Mafic
• Texture: Medium-grained (between Gabbro and Basalt). Often has a specific texture called ophitic or sub-ophitic where pyroxene crystals partially enclose plagioclase laths.
• Essential Minerals: Same as Gabbro (Ca-Plagioclase, Pyroxene).
• Description: Forms in sills and dikes.

Basalt

• Type: Extrusive (Volcanic)
• Composition: Mafic
• Texture: Aphanitic (fine-grained), often vesicular or porphyritic (with Olivine or Pyroxene phenocrysts).
• Essential Minerals: Same as Gabbro (fine-grained groundmass of Ca-Plagioclase and Pyroxene).
• Description: The volcanic equivalent of Gabbro. The most common volcanic rock on Earth, forming the ocean floor and large shield volcanoes (like Hawaii).

Alkaline Rocks (Quartz-Poor)

Syenite

• Type: Intrusive (Plutonic)
• Composition: Felsic to Intermediate (Alkaline)
• Texture: Phaneritic (coarse-grained).
• Essential Minerals: Alkali Feldspar (K-Feldspar). Contains very little or no Quartz (<5%).
• Accessory Minerals: Amphibole, Pyroxene, Biotite. (If it contains Feldspathoids, it's a Foid-Syenite).
• Description: Looks like a granite but lacks the glassy grey quartz.

Trachyte

• Type: Extrusive (Volcanic)
• Composition: Felsic to Intermediate (Alkaline)
• Texture: Aphanitic, Porphyritic (with K-Feldspar phenocrysts). Often shows flow banding.
• Has a characteristic trachytic texture where feldspar microlites are aligned in a sub-parallel fashion.
• Essential Minerals: Same as Syenite (fine-grained groundmass of K-Feldspar).
• Description: The volcanic equivalent of Syenite.

Ultramafic Rocks (Very Low Silica)

Dunite

• Type: Intrusive (Plutonic)
• Composition: Ultramafic
• Texture: Phaneritic (coarse-grained).
• Essential Minerals: > 90% Olivine.
• Accessory Minerals: Pyroxene, Chromite.
• Description: A type of peridotite. Typically green (from olivine) and weathers to a reddish-brown.

Peridotite

• Type: Intrusive (Plutonic)
• Composition: Ultramafic
• Texture: Phaneritic (coarse-grained).
• Essential Minerals: Primarily Olivine and Pyroxene. (Must have > 40% Olivine).
• Description: The primary rock of the Earth's upper mantle. Rarely found at the surface unless brought up by tectonic processes.

Summary of Igneous Rocks