Unit 2: Mineralogy Practicals

Part A: Megascopic Mineralogy (Hand Specimen Identification)

This is the identification of minerals using the naked eye and simple tools. The goal is to systematically test a mineral's physical properties to determine its identity.

Your Practical Toolkit

• Hand Lens (10x): To see small grains and features.
• Mohs Hardness Kit (or common items): Fingernail (2.5), Copper Coin (3.5), Steel Knife/Nail (5.5), Glass Plate (5.5), Streak Plate (6.5).
• Streak Plate: An unglazed white porcelain tile.
• Dilute HCl (Hydrochloric Acid) Bottle: To test for carbonates.
• Magnet: To test for magnetism.

The Diagnostic Checklist (Physical Properties)

1. Lustre: How the mineral reflects light.
   • Metallic: Looks like polished metal (e.g., Pyrite, Galena).
   • Non-Metallic:
     • Vitreous: Glassy (e.g., Quartz).
     • Pearly: Iridescent, like a pearl (e.g., Muscovite on cleavage).
     • Resinous: Looks like resin (e.g., Sphalerite).
     • Dull/Earthy: No shine (e.g., Hematite-oolitic).
2. Colour: The obvious colour. Often unreliable (e.g., Quartz can be white, pink, purple).
3. Streak: The colour of the mineral's powder when rubbed on a streak plate. Very reliable. (e.g., Hematite is always cherry-red; Pyrite is greenish-black).
4. Hardness: Resistance to scratching. Tested using the Mohs scale.

   Mohs Scale: 1. Talc, 2. Gypsum, 3. Calcite, 4. Fluorite, 5. Apatite, 6. Orthoclase (K-Feldspar), 7. Quartz, 8. Topaz, 9. Corundum, 10. Diamond.

5. Cleavage: The tendency to break along planes of atomic weakness, creating flat, shiny surfaces.
   • 1 Direction (Basal): Micas (Muscovite, Biotite).
   • 2 Directions at ~90°: Pyroxene, K-Feldspar.
   • 2 Directions at 56°/124°: Amphibole (Hornblende).
   • 3 Directions at 90° (Cubic): Halite, Galena.
   • 3 Directions not at 90° (Rhombohedral): Calcite.
6. Fracture: How a mineral breaks *other* than cleavage.
   • Conchoidal: Smooth, curved surfaces (e.g., Quartz, Obsidian).
   • Uneven/Irregular: Rough, irregular surface.
7. Form/Habit: The characteristic shape of the crystal (e.g., Prismatic, Tabular, Acicular (needle-like), Botryoidal (grape-like)).
8. Specific Gravity: How "heavy" it feels for its size (its density). Galena is very heavy; Talc is light.
9. Special Tests:
   • Acid Test: Calcite fizzes strongly with HCl.
   • Magnetism: Magnetite is strongly magnetic.
   • Taste: Halite tastes salty (use with caution).

Part B: Microscopic Mineralogy (Optical Identification)

This involves using a Petrological Microscope to identify minerals in a thin section (a 0.03 mm thick slice of rock on a glass slide). We observe properties under two light conditions.

1. Properties in Plane Polarized Light (PPL)

(Analyzer *out*. Light comes from the bottom polarizer only.)

• Colour: The mineral's transmitted colour (e.g., Biotite is brown; Hornblende is green/brown). Many minerals are colourless (Quartz, Feldspar, Calcite).
• Pleochroism: The colour changes as the stage is rotated. Highly diagnostic for minerals like Hornblende and Biotite.
• Relief: How much the mineral "stands out" from its surroundings. Caused by a difference in Refractive Index (R.I.).
  • High Relief: Thick, dark borders (e.g., Garnet, Olivine).
  • Low Relief: Hard to see the grain outline (e.g., Quartz in Feldspar).
• Cleavage: Visible as sharp, parallel lines in the crystal. The number of sets and their angles are key (e.g., Hornblende vs. Augite).
• Form:
  • Euhedral: Perfect crystal shape.
  • Subhedral: Partially formed.
  • Anhedral: No regular shape, filled in gaps.

2. Properties in Cross Polarized Light (XPL)

(Analyzer *in*. Two polarizers are crossed, so the field of view is black.)

• Isotropic vs. Anisotropic:
  • Isotropic: Mineral stays black (extinct) at all angles. It has only one R.I. (e.g., Garnet, and all cubic minerals).
  • Anisotropic: Mineral lights up and shows interference colours. It goes extinct (black) 4 times in a 360° rotation.
• Interference Colours (Birefringence): The colours seen in XPL. The "order" of the colour (1st, 2nd, 3rd) is diagnostic.
  • 1st Order (Low): Grey, White, Yellow (e.g., Quartz, Feldspar).
  • 2nd-3rd Order (High): Bright blues, greens, pinks (e.g., Calcite, Olivine, Micas).
• Extinction Angle: The angle between the mineral's cleavage (or long axis) and the position where it goes extinct (black).
  • Parallel Extinction: Extinct when cleavage is parallel to crosshairs.
  • Inclined Extinction: Extinct at some angle to the crosshairs.
• Twinning: Diagnostic patterns in crystals.
  • Albite/Polysynthetic Twinning: "Zebra stripes" in Plagioclase Feldspar.
  • Carlsbad Twinning: Simple twin dividing the crystal in two (common in K-Feldspar, Plagioclase).
  • Cross-Hatch/Tartan Twinning: Grid-like pattern in Microcline (K-Feldspar).

Common Mineral Identification Tables

Hand Specimen ID Table (Common Rock-Forming Minerals)