Unit 3: Igneous Petrology Practicals

This unit applies your mineralogy skills (Unit 2) to identify igneous rocks. The practical involves a two-step process: first identify texture, then identify mineral composition.

Part A: Megascopic (Hand Specimen) Identification

This is the identification of igneous rocks using their visible texture and mineralogy.

Step 1: Identify the Texture

Texture tells you the cooling history (intrusive vs. extrusive).

• Phaneritic (Coarse-grained): Crystals are large enough to see with the naked eye. Rock cooled slowly (intrusive/plutonic).
• Aphanitic (Fine-grained): Crystals are too small to see. Rock cooled quickly (extrusive/volcanic).
• Porphyritic: A mixed texture with large crystals (phenocrysts) in a fine-grained matrix (groundmass). Indicates two-stage cooling.
• Glassy: Looks like glass, no crystals (e.g., Obsidian). Instant cooling.
• Vesicular: Contains bubbles (vesicles) from escaping gas (e.g., Pumice, Scoria/Basalt).

Step 2: Identify the Composition (Colour Index)

Composition tells you the magma type. Use the Colour Index (CI): the percentage of dark (mafic) minerals.

• Felsic: (CI = 0-15%) Light-coloured (pink, white, light grey). Rich in Quartz and K-Feldspar.
• Intermediate: (CI = 15-45%) "Salt and pepper" look (mix of light and dark). Rich in Plagioclase and Amphibole.
• Mafic: (CI = 45-85%) Dark-coloured (black, dark green). Rich in Pyroxene and Ca-Plagioclase.
• Ultramafic: (CI > 85%) Very dark, often green. Rich in Olivine and Pyroxene.

Step 3: Combine and Name the Rock

Use this table to combine your findings from Step 1 and 2.

Part B: Microscopic (Thin Section) Identification

Here, you confirm the hand specimen ID by looking at the rock's texture and mineralogy under the microscope.

Key Micro-Textures to Identify

• Phaneritic (Equigranular): Interlocking crystals of roughly equal size.
• Porphyritic: Large, euhedral phenocrysts in a fine-grained (microcrystalline) or glassy groundmass.
• Ophitic / Sub-ophitic: Laths (needles) of plagioclase are partially or fully enclosed by larger pyroxene crystals. Diagnostic for Gabbro and Dolerite/Diabase.
• Trachytic: Small plagioclase laths show a flow alignment. Diagnostic for Trachyte.

Lab Identification Guide (Microscopic)

• Granite:
  • Texture: Phaneritic.
  • Minerals: Dominated by Quartz (clear, low relief, undulose extinction), K-Feldspar (Carlsbad or cross-hatch twinning, often cloudy/altered), and Plagioclase (Albite twinning). Small amounts of Biotite or Muscovite.
• Rhyolite:
  • Texture: Aphanitic, often Porphyritic.
  • Minerals: Phenocrysts of Quartz (can be "eaten" or resorbed) and K-Feldspar in a glassy or microcrystalline groundmass of the same minerals.
• Diorite:
  • Texture: Phaneritic.
  • Minerals: Dominated by Plagioclase (strongly zoned, Albite twinning) and Hornblende (green/brown, pleochroic, 56/124 cleavage). Little to no Quartz.
• Andesite:
  • Texture: Aphanitic, Porphyritic.
  • Minerals: Phenocrysts of Plagioclase (often zoned) and Hornblende (pleochroic) in a microcrystalline groundmass of plagioclase.
• Gabbro:
  • Texture: Phaneritic.
  • Minerals: Dominated by Ca-rich Plagioclase (Albite twinning) and Pyroxene (Augite) (high relief, 90° cleavage, inclined extinction). May have Olivine (high relief, fractured). Often shows ophitic texture.
• Basalt:
  • Texture: Aphanitic, Porphyritic, sometimes Vesicular.
  • Minerals: Phenocrysts of Pyroxene or Olivine in a fine groundmass of Plagioclase laths and Pyroxene grains.
• Peridotite:
  • Texture: Phaneritic.
  • Minerals: Dominated by Olivine (clear, high relief, fractured, high birefringence) and Pyroxene. No plagioclase.

Part C: Classification Exercises (IUGS QAPF Diagram)

This is a formal method for classifying plutonic rocks based on their *actual* mineral percentages, which are determined by a method called point counting on a thin section.

The QAPF Diagram: A diamond-shaped plot based on the relative percentages of four mineral groups:

• Q = Quartz
• A = Alkali Feldspar (K-Feldspar + Albite)
• P = Plagioclase Feldspar
• F = Feldspathoids (e.g., Nepheline. These form when magma is silica-poor).

A rock can never have both Q and F. It's either on the top QAP triangle (silica-saturated) or the bottom FAP triangle (silica-undersaturated).

How to Use It (The Exercise)

1. Get Modal Data: From point-counting, you get percentages. (e.g., Quartz = 30%, K-Feldspar = 40%, Plagioclase = 25%, Biotite = 5%).
2. Ignore Mafic Minerals: The QAPF diagram only uses Q, A, P, and F. Ignore the Biotite (5%).
3. Recalculate to 100%:
   • Q = 30
   • A = 40
   • P = 25
   • Total (Q+A+P) = 30 + 40 + 25 = 95
   • Recalculated %Q = (30 / 95) * 100 = 31.6%
   • Recalculated %A = (40 / 95) * 100 = 42.1%
   • Recalculated %P = (25 / 95) * 100 = 26.3%
4. Plot the Point: Plot this position (31.6% Q, 42.1% A, 26.3% P) on the QAP triangle.
5. Find the Name: This point will fall squarely in the Granite field.