Interference Figures of Uniaxial Minerals

Goal: To find a centered, an off-centered, and a flash interference figure of our unknown uniaxial mineral and sketch it. To determine the ω and ε directions using an off-centered interference figure and to determine the optic sign of the mineral by using a gypsum plate.

Materials:

• Microscope with power source
• Mineral samples
• Oils with previously determined ω and ε
• Acetone
• Detergent
• Kim Wipes
• Paper towels
• Marker
• Scoop
• Slide
• Cover slips
• Gypsum plate

Procedure:

• Set up the microscope.
  • Take the microscope out of the case by holding the base and the arm. Plug in the microscope and turn the light on.
  • Rotate the condensing lens out of the way.
  • Rotate the Bertrand lens out of the way
  • Make sure the polarizers are not crossed by moving the analyzer into the same direction as the lower polarizer (make sure light is visible).
  • Make sure the microscope is on its lowest magnification.
• Set up the slide.
  • Clean the slide.
    • Put a couple of squirts of detergent on the slide and gently rub with fingers, then rinse.
    • Be sure to only touch the slide on the edges from now on.
    • Dry the slide with a paper towel.
    • Spray a small amount of acetone on the slide and wipe it dry with a kim wipe. Place the kim wipe in the toxic waste bucket.
  • Clean three cover slips the same way that you clean the slide.
  • Choose the two oils that you previously determined to be the values of ω and ε.
  • Use a marker to write on the slide the three n values as far apart from each other as possible.
  • Next to the written n values, use the eye dropper in each oil bottle to put a very small amount of oil on the slide. First, rub the dropper along the inner edge of the bottle to remove excess oil, then put the drop on the slide without touching the slide. Do not touch the slide with the dropper because it could contaminate the oil when you put the dropper back in the bottle.
  • Take the scoop and clean it with the acetone and kim wipe. Put it in the mineral grain sample bottle and pick up a few grains. Tap your hand that is holding the scoop over an oil drop so that a few grains fall into the oil. Repeat for each oil sample and then put the cover slips on the oil samples. Then close the mineral grain sample bottle.
• Find the interference figures.
  • Put the slide on the stage and find a suitable grain (explained lower down for each interference figure). Change the objective to high power, raise the substage, swing the condensing lens into place, and turn on the analyzer and the Bertrand lens. You should see a smaller field of view, isochromes, and isogyres. The ω direction is always tangential to the isochromes and the ε direction is perpendicular to the ω direction. Sketch the contents of the field of view: the isogyres, the isochromes (including what colors they are), and the melatope. Do the same after rotating the stage.
  • Types of Interference Figures:
    • Centered Figure
      • A centered figure will be a mineral that is extinct for an entire 360° stage rotation. The optic-axis is perpendicular to the plane of the stage and is represented by the melatope.
      • The melatope will be at the center of the crosshairs and the isogyres will be bisected by the N-S and E-W crosshairs. When the stage is rotated, there should be no change in the appearance of the isogyres or melatope. The isogyres and isochromes have the melatope as their origin.
    • Off-Centered Figure
      • An off-centered figure will be a mineral that goes extinct four times, every 90°, for an entire 360° stage rotation. The optic axis is at an oblique angle to the plane of the stage and is represented by the melatope.
      • The melatope will not be located at the center of the crosshairs, it will be located at some angle, ν, from the center. During a stage rotation, the melatope will rotate in the same direction around the center of the crosshairs. The isogyres will not rotate around the melatope; they will keep their positions parallel to the crosshairs. The isogyres and isochromes have the melatope as their origin.
    • Flash Figure
      • A flash figure will be a mineral that has maximum birefringence (the difference between the highest and lowest indices of refraction). The optic axis is parallel to the plane of the stage.
      • The vibration direction of the rays emerging from the mineral are almost all parallel to each other, so once the optic axis is rotated until it is parallel to a nicol privileged direction, almost the entire field of view is filled with a thick, black cross. At the extreme NE, SE, SW, and NW corners of the view there is no extinction indicating the few non-parallel rays. A 1° rotation of the stage results in the large cross resolving into two hyperbolic isogyres in the quadrants containing the optic axis.
• Compare with previous ω and ε values.
  • Find ω.
    • While on the off-centered interference figure, orient the crystal as if you are trying to determine ω while in the ω oil.
    • Change the objective to medium power and make sure the analyzer is off.
    • If the Becke line is not visible, then the crystal was in the correct orientation when you previously obtained ω.
    • If the Becke line moves inwards, toward the grain, then the oil’s refractive index is lower than the grain’s refractive index and you should have used an oil with a higher index of refraction.
    • If the Becke line moves outwards, toward the oil, then the oil’s refractive index is higher than the grain’s refractive index and you should have used an oil with a lower index of refraction.
  • Find ε.
    • While on the off-centered interference figure, orient the crystal as if you are trying to determine ε while in the ε oil.
    • Change the objective to medium power and make sure the analyzer is off.
    • If the Becke line is not visible, then the crystal was in the correct orientation when you previously obtained ε.
    • If the Becke line moves inwards, toward the grain, then the oil’s refractive index is lower than the grain’s refractive index and you should have used an oil with a higher index of refraction.
    • If the Becke line moves outwards, toward the oil, then the oil’s refractive index is higher than the grain’s refractive index and you should have used an oil with a lower index of refraction.
• Determine the Optic Sign.
  • Find an optically centered interference figure or a slightly off-centered interference figure that has a melatope in the field of view.
  • Note the N direction on the gypsum plate and then insert it.
  • In a centered optic axis interference figure, ω vibrates in the direction tangential to the isochromes and ε vibrates in the direction perpendicular to the isochromes. The gypsum plate is vibrating in the NE-SW direction. In the NE (and SW) quadrant, the gypsum plate will be vibrating perpendicular to ω and parallel to ε. If the crystal is optically negative, the gypsum plate vibration is subtracted from the ω direction, making the difference between ω and ε smaller; ε is less than ω and the NE quadrant is a 1° yellow color. If the crystal is optically positive, the gypsum plate vibration is added to the ε direction, making the difference between ω and ε greater; ω is less than ε and the NE quadrant is 2° blue color. Sketch the field, including the colors.
  • If you are using an off-centered interference figure without the melatope in the field of view, rotate the stage counterclockwise and note whether you are seeing a horizontal isogyre or a vertical isogyre and in what direction it is moving. After you see a vertical isogyre move towards the left, then a horizontal isogyre move towards the bottom, you have moved into the NE quadrant. Note the color of the field as you would have if this was a centered interference figure.
• Put everything away.
  • Clean the slide and the cover slips again and place them back in their original spots. Put the microscope back under low power magnification, turn the light off, and unplug it. While holding the base and arm, place the microscope back in the case.

Results: The value that I previously determined for ω matched with what I determined in this lab. The value for ε was wrong. The Becke line went into the grain, meaning I should have used an oil with a higher index of refraction.
The flash figure, with the gypsum plate in, is almost entirely pink because most of the figure is extinct. When rotated a degree or two, the isogyres disappear and a hodge-podge of colors are left in the field.
The NE quadrant of the crystal is 1° yellow, so the crystal is optically negative.

Conclusion: Using interference figures to determine a mineral’s optic sign is easier and more effective than using oils to determine indices of refraction and optic sign. Since many minerals have similar indices of refraction, it is not a very useful piece of data for determining what an unknown mineral is, but knowing what its optic sign is greatly limits the possibilities.

Related Articles

• Becke Line Method Lab, Isotropic Minerals
• Becke Line Method Lab, Uniaxial Minerals