Unit 1: The Petrological Microscope

Microscope (Petrological): Parts and Function

What is a Petrological Microscope?

A petrological microscope (or polarizing microscope) is the most important tool used in petrology (the study of rocks). It is a standard biological microscope that has been modified with special filters to allow for the study of minerals using polarized light.

Key Parts and Their Functions

Polarization of Light

Normal light from the sun or a bulb is unpolarized. This means it is an electromagnetic wave that vibrates in all directions perpendicular to its direction of travel.

A polarizing filter (like a Polaroid sheet or a Nicol prism) acts like a set of vertical blinds. It only allows light vibrating in *one specific plane* to pass through.

Plane Polarized Light (PPL): Light that has been passed through a polarizing filter and is now vibrating in only one direction (e.g., East-West).

In the microscope, the Polarizer creates this PPL. This is the first step in all optical mineralogy.

Concept of Nicol Prisms

Before modern Polaroid filters, the Nicol prism was the primary device used to polarize light. Even though we don't use them much, the *concept* is important, and the term "Crossed Nicols" is still used today.

• Construction: It is made from a crystal of Calcite (specifically, the clear variety Iceland Spar).
• Principle: Double Refraction. Calcite has the unique property of splitting a single ray of unpolarized light into *two* rays (the O-ray and the E-ray), which are polarized at 90° to each other.
• Function:
  1. A calcite crystal is cut at a specific angle.
  2. It is then glued back together with a cement called Canada Balsam.
  3. When light enters, it splits into the O-ray and E-ray.
  4. The O-ray hits the Canada Balsam at an angle that causes total internal reflection. It is reflected to the side and absorbed by the prism's black casing.
  5. The E-ray (which is perfectly plane-polarized) passes straight through the prism and exits.

Observation of Thin Sections

What is a Thin Section?

A thin section is a sample of a rock that has been cut, glued to a glass slide, and ground down to a standard thickness of 30 micrometers (0.03 mm). At this thickness, most minerals (except for opaque ore minerals) become transparent to light.

Modes of Observation

There are two primary modes of viewing a thin section:

1. Plane Polarized Light (PPL)

• Setup: Analyzer is OUT. (Only the lower polarizer is in).
• What you see: The properties of the mineral in a single plane of polarized light.
• Properties observed in PPL:
  • Relief (apparent "bumpiness")
  • Pleochroism (change in color on rotation)
  • Colour (the mineral's natural color)
  • Habit/Form (the mineral's characteristic shape)
  • Cleavage (planes of weakness/cracks)

2. Crossed Polarized Light (XPL) or "Crossed Nicols"

• Setup: Analyzer is IN. (Both polarizer and analyzer are in, 90° to each other).
• What you see: How the mineral *interacts* with polarized light.
  How XPL Works: In this setup, the E-W light from the polarizer cannot pass through the N-S analyzer, so the field of view is black. However, if an anisotropic mineral is on the stage, it splits the PPL into two rays, which vibrate at different speeds. When these rays exit the crystal, they recombine. This "new" light ray now has a component that can pass through the N-S analyzer, producing a bright "interference color".
• Properties observed in XPL:
  • Isotropism vs. Anisotropism (Does it go black/extinct, or does it show color?)
  • Interference Colors (The bright colors seen in XPL, related to birefringence)
  • Twinning (Different parts of a crystal go extinct at different times)
  • Extinction Angle (The angle at which the mineral goes dark)

Relief

Relief is the apparent "topography" or "bumpiness" of a mineral as seen in PPL. It describes how much the mineral "stands out" from its surroundings (usually the mounting glue, Canada Balsam, or adjacent minerals).

• Cause: Relief is caused by a difference in Refractive Index (n) between the mineral and its surroundings. The mounting glue (Canada Balsam) has n ≈ 1.54.
• High Relief: The mineral has a very different R.I. from the glue (n >> 1.54 or n << 1.54). It will have thick, dark borders and look like it "stands out." Example: Garnet, Olivine.
• Low Relief: The mineral has an R.I. very similar to the glue (n ≈ 1.54). It will have thin, faint borders and be very difficult to see. Example: Quartz, Feldspar.

Pleochroism

Pleochroism: The property of some anisotropic minerals to show a change in color when the stage is rotated in Plane Polarized Light (PPL).

• Cause: The mineral absorbs different wavelengths (colors) of light in different crystallographic directions. As the mineral is rotated, we see the light vibrating along its different axes, resulting in a different perceived color.
• Who shows it? Only anisotropic minerals that are colored.
  • Isotropic minerals (like Garnet) do not show pleochroism (color is constant).
  • Anisotropic but colorless minerals (like Quartz) do not show pleochroism (they are always colorless).
• Examples:
  • Biotite: Rotates from light brown to dark brown.
  • Hornblende: Rotates from green to brown.
  • Tourmaline: Shows strong pleochroism, with its darkest color when the long axis is E-W.