Unit 5: Metamorphic Petrology Practicals

Part A: Megascopic (Hand Specimen) Identification
Part B: Microscopic (Thin Section) Identification
Metamorphic Rock Identification Tables

This unit involves identifying metamorphic rocks. The single most important diagnostic feature is Foliation (the parallel alignment of minerals caused by directed pressure).

Part A: Megascopic (Hand Specimen) Identification

Your first step is to determine if the rock is foliated or non-foliated.

Category 1: Foliated Rocks

These are identified by the type and intensity of their foliation, which corresponds to metamorphic grade.

Slaty Cleavage:
- Rock: Slate
- Description: Very fine-grained, dense, dull lustre. Splits into perfect, thin, flat sheets. (Protolith: Shale).
Phyllitic Foliation:
- Rock: Phyllite
- Description: Fine-grained, but has a silky sheen or lustre (from microscopic micas). Often has a wavy or wrinkled surface. (Protolith: Slate).
Schistosity:
- Rock: Schist
- Description: Coarse-grained, visible, sparkly mica crystals (biotite/muscovite) are all aligned. May contain large porphyroblasts (e.g., Garnet). (Protolith: Phyllite).
Gneissic Banding:
- Rock: Gneiss
- Description: Coarse-grained, with compositional bands of light-coloured (quartz, feldspar) and dark-coloured (biotite, amphibole) minerals. (Protolith: Schist or Granite).

Category 2: Non-Foliated Rocks

These rocks have no mineral alignment, usually because they formed without directed stress (contact metamorphism) or from a single-mineral protolith.

Identify them based on their mineral composition.

Fizzes with HCl acid: → Marble
- Description: Crystalline, interlocking calcite grains. Soft (H=3). (Protolith: Limestone).
Does NOT fizz. Very Hard (H=7), scratches glass: → Quartzite
- Description: Crystalline, interlocking quartz grains. Sugary texture. (Protolith: Quartz Sandstone).
Does NOT fizz. Dark-coloured, crystalline: → Amphibolite
- Description: Dominated by black, needle-like or "splintery" Hornblende crystals. (Protolith: Basalt).
Does NOT fizz. Fine-grained, dense, "baked" look: → Hornfels
- Description: A dark, hard rock formed by contact metamorphism. Breaks with a conchoidal-like fracture.

Part B: Microscopic (Thin Section) Identification

Under the microscope, you confirm the hand specimen ID by observing the dominant minerals and textures.

Key Metamorphic Textures (Microscopic)

Lepidoblastic: Strong parallel alignment of platy (mica) minerals. (Diagnostic for Schist).
Nematoblastic: Strong parallel alignment of needle-like (amphibole) minerals. (Diagnostic for Amphibolite).
Granoblastic: A mosaic of interlocking, equidimensional, anhedral crystals. (Diagnostic for Quartzite and Marble).
Porphyroblastic: Large metamorphic crystals (porphyroblasts) like Garnet or Staurolite have grown within a finer-grained matrix (e.g., a Schist).
Slaty Cleavage: A very fine-grained, perfectly parallel alignment of micro-micas.

Metamorphic Rock Identification Tables

The practical exam will 100% have spots for Slate, Schist, Gneiss, Marble, and Quartzite. You must be able to identify them in both hand specimen and thin section.

Rock Name	Megascopic ID (Hand Specimen)	Microscopic ID (Thin Section)	Protolith
Slate	Very fine-grained, dull, perfect slaty cleavage, splits into flat sheets.	Slaty Cleavage: Microscopic micas and clays in perfect alignment.	Shale
Phyllite	Fine-grained, silky sheen, often wavy/crenulated foliation.	Fine-grained micas (sericite) in a wavy, aligned (phyllitic) texture.	Slate
Schist	Coarse-grained, sparkly micas, strong foliation (schistosity). Often has garnets.	Lepidoblastic Texture: Aligned muscovite and biotite. Often Porphyroblastic (e.g., Garnet).	Phyllite
Gneiss	Coarse-grained, compositional bands (light and dark layers).	Coarse-grained, banded. Light layers = Qz + Feldspar. Dark layers = Biotite + Hornblende.	Schist / Granite
Marble	Crystalline, soft (H=3), fizzes with HCl. White or coloured.	Granoblastic Texture: Interlocking mosaic of Calcite (high birefringence, twinning).	Limestone
Quartzite	Crystalline, very hard (H=7), scratches glass. Sugary texture.	Granoblastic Texture: Interlocking mosaic of Quartz (low birefringence, undulose extinction).	Quartz Sandstone
Amphibolite	Dark, crystalline, non-foliated or weakly foliated. Dominated by black hornblende.	Nematoblastic Texture: Aligned Hornblende (pleochroic, 56/124 cleavage) and Plagioclase.	Basalt / Gabbro