Ge 114 - Optical Mineralogy -Laboratory Exercise 1
INTRODUCTION TO THE POLARIZING MICROSCOPE
Reading:Phillips (1971) Ch. 2. or Nesse (1991) Ch. 2
Phillips (1971) Ch. 3 or Nesse (1991) Ch. 3
Nikon Polarizing Microscope LABOPHOT-POL instructions
Or Leica Microscope instructions
The optical microscope is one of the first tools which is used in the serious examination of rocks and minerals. These microscopes are designed to study materials with properties which vary with direction. The features which distinguish them from "standard" biological microscopes are the use of linearly polarized light which defines the direction of vibration of the light incident upon the sample, rotating stages which allow the sample to be freely positioned with respect to the direction of vibrations of the light, the use of special strain-free objectives which do not distort the polarization properties of the light, and a variety of special accessories which are used in conjunction with the polarized light.
The following instructions will guide you through the steps required to familiarize yourself with the use of the microscope. All questions or procedures in bold type should be answered and recorded in the lab report.
A) Introduction to the Petrographic Microscope
1. Identify the parts of the microscope and accessory equipment for the microscope, using p.1-14 in the Nikon manual and Ch. 2 in Nesse. This should include standard oculars (one with cross hairs), four objectives (mounted on turret nosepiece) and a combined 1/4 and tint (530nm=1) plate. Check and report any deficiencies or indication of damage to any of this equipment.
2. Set up the microscope and adjust its position for maximum comfort in operation (p. 8 Nikon manual). The light sources should already be factory-adjusted for optimum illumination.
3. Practice focusing on a glass slide. Start with the lowest (5x) objective. Check the coarse and fine adjustments of the microscope focusing mechanism. Read p.21-22 Nesse concerning the proper technique for focusing with various objectives to avoid running an objective lens into a thin section! Change objectives by gently turning the rotating nosepiece. Do NOT grasp the objective lenses- you might misalign and damage them.
4. If necessary, carefully center all objectives (p. 8 Nikon manual; p. 42 Phillips, 1971). Learn to center the stage if all objectives are misaligned.
5. Calibrate the ocular micrometer scale: Using a stage micrometer, determine for each objective the length represented by 1 scale division of a micrometer ocular. Make a table with the magnification of each objective lens and the corresponding length per division on the ocular scale. Also determine and include in this table the length represented by the diameter of the field of view for each ocular. This information will help later for quick estimates of grain size in mineral powders and in thin sections.
6. Check the centering of the substage diaphragms (including the light source diaphragm) of your microscope (p. 9 Nikon manual). For each objective, note what effect is produced by closing down each diaphragm. Make the tests with condenser assembly at the top, and then at the bottom of its travel, for the low- and medium- power objectives. For the high-power objective, carry out these tests both with and without the movable substage condenser in the optic train. Realize that in general it is not possible to have all elements perfectly centered under all possible combinations.
7. Verify that the vibration direction of the analyzer (upper polarizer) is set normal to that of the polarizer (see p.19 Nesse). To do this, insert the upper polarizer. When the vibration direction of the upper polarizer is perpendicular to that of the lower polarizer, the field of view is dark: this is called crossed polars. When the upper polar is parallel to the lower polar, light is transmitted and the field is light. The upper polarizer may be rotated by loosening a screw on the left side of the polar and rotating the numbered ring. The default setting is with the “0”’s aligned. You may rotate the lower polar slightly to maximize extinction in the field of view.
8. Check the (lower) polarizer setting using a slide of biotite. In biotite the greatest absorption occurs when the trace of the basal cleavage is parallel to the vibration direction of the polarizer. Why? (see p.44 Phillips, 1971). Record whether the polarization direction is NS (north/south=up to down) or EW (east/west= left to right). Compare your result to Fig. 20 on page 13 of the Nikon manual.
B.Reliefand Becke Lines
References: Phillips (1971) p.48-50 or Nesse (1991) pp. 25-28
When grains of a mineral are mounted in a medium of a different refractive index, the apparent contrast between the grain and the mounting medium depends upon the difference between their indices of refraction. This apparent contrast, commonly called "relief", comes largely from irregularities in the surface of the mineral grain. When the relief is high, the mineral grain appears to have a "bumpy" surface. We want to become familiar with the appearance of minerals of different indices of refraction in a medium of fixed index.
To familiarize yourself with the Becke line which is commonly used to compare the index of refraction of adjacent mineral grains, first examine microscope slide Ge114 2-2 (see sketch) which contains grains of two different minerals, one with an index of refraction lower than the mounting medium (fluorite) and the other with a higher index (garnet). Make sure you place this and all other slides on the microsope stage with the cover slip side up. Under intermediate magnification, examine the grains with the lower index (near the black ink label). Lower the stage of the microscope to bring the sample out of focus and observe the Becke line move out of the grain into the mounting medium. Now raise the stage to bring the grain in focus and observe the Becke line come back into the grain.
Repeat this sequence with the grains of the mineral with the higher index. You should see the Becke line move into the sample as the sample stage is lowered. Read about Becke lines in the references above and write a short explanation of this phenomena.
Examine one of the special slides illustrating the relief scale. The slides contain examples of the range of relief effects observable for mineral grains imbedded in Canada balsam (n=1.54), from moderately negative (fluorite) to very high positive (garnet). Try to quantitatively calibrate your sense of mineral refractive index in relation to the relief shown. Make a table of the behavior of the Becke line and the index of refraction for each mineral in the slide.
October 19, 20181