Unit 5: Metamorphic Petrology Practicals
Table of Contents
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 |