Unit 2: Optical Microscopy

Properties of Light and Optical Mineralogy

Optical Mineralogy is the study of minerals using a microscope, by observing how they interact with light. Light is an electromagnetic wave. In a vacuum, it travels at a constant speed, but when it enters a substance (like a mineral), it slows down.

Refractive Index (R.I.)

The Refractive Index (n) of a mineral is a measure of how much it slows down light. It is defined as:

n = (Speed of light in a vacuum) / (Speed of light in the mineral)

Because minerals (except glass) are crystalline, their R.I. often changes depending on the *direction* light travels through them.

• Isotropic Minerals: (Cubic system, glass) Have only one R.I. Light travels at the same speed in all directions.
• Anisotropic Minerals: (All other crystal systems) Have two or three different R.I.s. Light is split into two rays that travel at different speeds and vibrate in different directions. This splitting is called birefringence.

The Petrological Microscope

This is a specialized microscope with two polarizing filters, which are the key to optical mineralogy.

Key Parts:

• Light Source: Illuminates the sample from below.
• Lower Polarizer (or Polarizer): Located below the stage. It takes normal, unpolarized light and converts it into Plane Polarized Light (PPL), where all light waves vibrate in a single plane (e.g., East-West).
• Rotating Stage: A circular stage that holds the thin section (rock slice) and can be rotated 360°.
• Objective Lens: Magnifies the image (e.g., 4x, 10x, 40x).
• Upper Polarizer (or Analyzer): Located above the objective. It is another polarizing filter, permanently set to vibrate at 90° to the lower polarizer (e.g., North-South).
  • When the analyzer is OUT, we see the mineral in PPL.
  • When the analyzer is IN, we see the mineral in Cross Polarized Light (XPL).
• Accessory Slot: A slot to insert optical accessories.

Identification of Minerals in Plane Polarized Light (PPL)

(Analyzer is OUT). We observe the mineral's interaction with a single plane of light.

• Colour: The mineral's true 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. (e.g., changes from dark green to light green). This only happens in anisotropic, coloured minerals.
• Relief: How much the mineral "stands out" from its surroundings (usually the epoxy, n = 1.54). It is a measure of the difference in 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. Pyroxene).

Identification of Minerals in Cross Polarized Light (XPL)

(Analyzer is IN). The two polarizers are crossed, so the field of view is black. Only anisotropic minerals will "light up."

• Isotropic vs. Anisotropic:
  • Isotropic: Mineral stays black (extinct) at all angles of rotation. It cannot split light. (e.g., Garnet, glass, 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 (e.g., grey, white, yellow, red, blue). The "order" of the colour (1st, 2nd, 3rd) is a measure of the mineral's birefringence and is highly diagnostic.
  • 1st Order (Low): Grey, White, Yellow (e.g., Quartz, Feldspar).
  • 2nd-3rd Order (High): Bright blues, greens, pinks (e.g., Calcite, Olivine, Micas).
• Twinning: Diagnostic patterns in crystals.
  • 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).

Refractive Index and its Determination

We determine R.I. in PPL by comparing a mineral grain to a substance of known R.I. (usually the epoxy/cement, n ≈ 1.54). This test is called the Becke Line Test.

The Becke Line is a bright halo of light that appears at the boundary of two substances with different R.I.s.

Becke Line Rule: When you lower the stage (or increase focal distance), the Becke line will move into the substance with the HIGHER Refractive Index.

• If the Becke line moves *into* the mineral grain, the mineral has a higher R.I. than the epoxy.
• If the Becke line moves *out of* the mineral grain (into the epoxy), the mineral has a lower R.I. than the epoxy.

This tells you if the mineral has high or low relief.

Types of Extinction and Extinction Angle

Extinction is the black-out position for an anisotropic mineral in XPL. This happens when the mineral's vibration directions are aligned parallel to the N-S and E-W crosshairs of the microscope.

The Extinction Angle is the angle between a crystal's cleavage (or long axis) and its extinction position. This is a key diagnostic feature.

Types of Extinction:

1. Parallel Extinction: The mineral goes extinct when its cleavage or long axis is parallel to the crosshairs. (Extinction angle = 0°).
   • Example: Orthopyroxene, Muscovite.
2. Inclined Extinction: The mineral goes extinct when its cleavage or long axis is at an angle to the crosshairs.
   • Example: Clinopyroxene (Augite), Hornblende.
3. Symmetrical Extinction: The mineral goes extinct at an angle that bisects two cleavage directions.
   • Example: Amphibole (Hornblende) basal sections.

Optical Accessories

These are plates inserted into the accessory slot (above the objective) to help identify minerals, primarily by determining their "fast" and "slow" vibration directions.

Common Accessories:

• Gypsum Plate (or First-Order Red Plate):
  • Made of gypsum, it has a fixed retardation that produces a bright magenta/pink interference colour on its own.
  • When inserted, it adds or subtracts from the mineral's interference colour.
  • If colours go *up* (e.g., to blue), it's addition.
  • If colours go *down* (e.g., to yellow), it's subtraction. This is used to find the optic sign.
• Quartz Wedge:
  • A wedge of quartz with increasing thickness.
  • As it's inserted, it shows a "rainbow" of interference colours from 1st to 3rd/4th order.
  • Used to determine the *order* of a mineral's interference colour.
• Mica Plate:
  • Made of mica, it has a low (1st order white) retardation.
  • Used for minerals with very low birefringence (like quartz).