Unit 1: Ore Microscope

Ore Microscope: Parts and Functions

An Ore Microscope (also known as a polarizing reflected-light microscope) is a specialized tool used to study opaque minerals, such as metallic ores (e.g., Pyrite, Galena, Magnetite).

Unlike a standard petrological microscope that shines light *through* a thin section (transmitted light), an ore microscope shines light *down* onto a highly polished sample and examines the light that is reflected off its surface.

Key Parts and Functions

• Light Source: An intense, external light source (lamp).
• Vertical Illuminator: This is the most important component. It is a system of prisms and reflectors located *above* the objective lens. It takes the horizontal light beam from the lamp and directs it *down* through the objective lens onto the surface of the sample.
• Reflector Plate: A piece of glass or a prism inside the vertical illuminator that reflects the light downwards. A half-mirror is often used so that the reflected light from the sample can pass back up through it to the eyepiece.
• Objective Lens: In an ore microscope, the objective acts as both a condenser (focusing light onto the sample) and a magnifier (collecting the reflected light).
• Polarizer: Just like in a petrological microscope, it creates plane-polarized light.
• Analyzer: A second polarizer set at 90° to the first. Used to check for anisotropy.
• Sample Stage: A stage designed to hold heavy, polished ore mounts or sections, often with leveling screws to ensure the polished surface is perfectly perpendicular to the light path.

Transmitted vs. Reflected Light Microscopy

Properties of Ore Minerals (Under Ore Microscope)

Because we are using reflected light, the properties we observe are different from those in transmitted light.

Properties in Plane Polarized Light (PPL - Analyzer Out)

• Reflectivity (or Reflectance):
  • What it is: The percentage of light that is reflected from the mineral's surface. It is the equivalent of "brightness."
  • Example: Native Silver has very high reflectivity (bright white), while Magnetite has moderate reflectivity (grey).
• Colour:
  • What it is: The actual colour of the mineral in reflected light.
  • Example: Pyrite is a distinct pale yellow; Gold is a rich golden yellow; Covellite is a deep indigo blue.
• Bireflectance (or Reflection Pleochroism):
  • What it is: The change in colour or brightness (reflectivity) as the stage is rotated 360°.
  • Example: Graphite shows strong bireflectance, changing from dark grey to light grey.

Properties in Cross Polarized Light (XPL - Analyzer In)

• Anisotropism (or Anisotropy):
  • What it is: When an anisotropic mineral is rotated under XPL, it changes brightness and/or shows vivid interference colours. It will go extinct (black) 4 times, just like in transmitted light. Isotropic minerals (cubic system) will stay black.
  • Example: Covellite shows brilliant orange-red anisotropic colours. Galena (cubic) is isotropic and stays black.
• Internal Reflections:
  • What it is: In some minerals that are not perfectly opaque (semi-transparent), light will penetrate the grain, reflect off internal cleavage planes or grain boundaries, and come back up.
  • Result: You see deep, rich colours (e.g., deep reds in Sphalerite or ruby-red in Cinnabar). These are best seen under XPL.

Techniques of Polishing Ore

The goal is to create a perfectly flat, scratch-free, mirror-like surface. Any scratches will scatter light and ruin the image. This is a multi-step process of grinding and polishing.

The Polishing Process:

1. Sample Cutting: A small, representative piece of the ore is cut from the hand specimen using a rock saw.
2. Mounting: The sample is placed in a cylindrical mold, and a resin (like epoxy) is poured around it. This creates a solid "puck" or "mount" that is easy to hold and fits in the microscope holder.
3. Grinding (Lapping): This stage is for making the surface perfectly flat.
   • The puck is held against a rotating metal wheel (a "lap") covered with abrasive silicon carbide (Carborundum) grit.
   • The process starts with a coarse grit (e.g., 220 grit) to quickly flatten the sample.
   • It is then repeated with progressively finer grits (e.g., 400 grit, then 600 grit, then 1000 grit). The sample must be washed thoroughly between each step to remove all coarser grains.
4. Polishing: This stage is for removing the fine scratches left by grinding to get a mirror finish.
   • The puck is held against a rotating lap covered with a soft polishing cloth (like felt or nylon).
   • A polishing agent (a very fine abrasive suspended in water) is dripped onto the cloth. Diamond paste or Alumina (Aluminum Oxide) powder of varying particle sizes (e.g., 5 micron, 1 micron, 0.3 micron) is used.
   • This is done in several stages, moving from coarser to finer polishing agent, until a perfect, scratch-free surface is achieved.
5. Cleaning: The final sample is cleaned (often in an ultrasonic bath) to remove all polishing residue and oils.