Spectro 186-312B, Francis, 2013
ISOTROPIC is an optical property possessed by a medium that transmits light at a velocity that is independent of the direction of travel in the medium.
All isometric minerals are isotropic.
Materials lacking a crystalline structure such as glass, metamict minerals, etc. are also isotropic.
1) Place a few grains (5-10) of
(powder # 2-1, R.I. = 1.640) grains on a glass microscope slide.
2) Cover with a glass cover slip.
3) Touch a drop of oil (R.I. = 1.580) to edge of cover slip so that capillary action pulls it under and onto grains.
NEVER put any object into oil bottle, other than the dropper.
NEVER touch oil dropper to any object, other than inside of oil bottle.
FAILURE to follow the above will contaminate the oil and alter its refractive index.
4) Focus on the grains with the low power objective (5X). Insert the analyzer and observe while rotating the stage.
5) Save the slide of
and repeat steps 1-4 with powder # 2-2.
This mineral is anisotropic. How does its behaviour in cross-polarised light differ from that of the isotropic
?
The behaviour of sal ammoniac under crossed polars is a characteristic feature of isotropic minerals.
Remove the analyser and view the
slide with the 10X objective. Close the diaphragm partially. Turn the fine adjustment focus knob about one half turn, increasing the distance between specimen and objective.
Note that the illumination crowds towards the center of each grain ; this indicates that the index of the crystals (1.640) is higher than that of the liquid (1.580).
Prepare another mount of the same material, this time using a liquid of index about 1.700. Make the test again and note that the illumination of the grain spreads from the center . This indicates that the refractive index of the oil is higher than that of the mineral.
This test is best done by "racking" up and down through the focus position under very low lighting conditions, obtained by turning down the rheostat, and/or suitably adjusting of the diaphragm.
4

Spectro 186-312B, Francis, 2013
As the refractive index of the oil being used approaches that of the mineral, you will observe a definite bright line, which follows the periphery of the grain, that moves in or out in the same manner as central convergence. This is the
" Becke Line ", named after the Austrian mineralogist who first described it.
The Becke Line moves from the grain boundary into the material of higher refractive index, as the distance between the objective and the grain increases from
the focused position. The test reverses in the opposite direction.
Again this test is best performed by "racking" up and down through the focused position.
As you approach a close match between the refractive index of the oil and that of the mineral, you will rely more on the behaviour of the Becke line than on central convergence for the comparison of refractive index.
FIG. 2-1: The Becke line.
When thrown out of focus by raising microscope tube (or lowering stage), white line moves into medium of higher refractive index. (a) In focus. (b) n of grain > n of liquid.
(c) n of grain < n of liquid.
Use the slide mounts of the previous test. Insert the quartz accessory plate slowly and watch the grains adjacent to the leading edge of the accessory plate, adjusting the diaphragm as needed. For the mount having the liquid with a lower index ( mineral index greater than liquid index ) a shadow covers each grain on the side facing the advancing accessory plate. The other side of each grain remains bright. For the second mount ( mineral index lower than liquid index ) these relations are reversed. The test works well with either the
10X objective you are using, or the lowest power 5X objective, and is most useful for testing large numbers of grains at the same time. The Becky Line test works for the high-power objective as well, and is better for accurate measurements on single grains.
This oblique illumination test may be reversed for objectives on some microscopes - check the low and medium power objectives on your microscope with both the Becke line test and the oblique test to make sure they agree. It is important that you know the way the oblique illumination method works for your microscope.
5

Spectro 186-312B, Francis, 2013
Relief is a measure of the visual contrast between the mineral grains and the surrounding medium (the oil in this case). The higher the relief, the greater the difference between the refractive indices of the mineral and oil. As an example, prepare a mount of
(R.I. = 1.64) with an oil (R.I. =
1.63). Compare this slide with either of the previous
slides.
The grains are almost invisible in oil (R.I. 1.63), but quite distinct in oil (R.I. =
1.58 or 1.70). In the first case, the grains are described as having low relief and in the second case, moderate relief. Relief can thus be used qualitatively to estimate how close the refractive index of an oil is to that of a mineral.
When an exact match is achieved, the grains will be virtually invisible.
Relief can not tell you whether the refractive index of the oil is higher or lower than that of the grain, only qualitatively how different it is from that of the grain.
Save the slide with oil (R.I. = 1.63) for part 4.
The refractive index of any medium varies with the wavelength of light being considered (Fig. 2-2). This property is called
The fact that the variation of R.I. with wavelength differs for different media leads to some interesting and useful results.
FIG. 2-2: Dispersion curves. (a) Color fringe brown (low intensity red) because the crystal index is higher than the liquid only for red. The crystal is low for the standard
D line and other nearby colors. (b) Color fringe yellow because the crystal index is high for yellow, orange and red. The yellow is much more intense to the eye than orange or red. (c) Color fringe orange-red because the crystal index matches the D line.
Prepare mounts as before of
(R.I. = 1.640) in liquids having indices as close as possible to 1.630, 1.640, and 1.650. Since the grains in
6

Spectro 186-312B, Francis, 2013 each mount have almost the same index as the liquids they will be almost invisible and focusing may be difficult. However, if the correct focus for one of the previously used mounts is retained, the task will be easier for these new mounts.
Make oblique illumination tests on all three mounts. For most observers, grains in the oil with an index = grain index (1.64) will show an orange-red border away from the edge of the accessory plate and a blue border adjacent to it (Fig. 2-3). This is due to the fact that red light has a lower index of refraction in the oil than in the mineral, while blue light has a lower refractive index in the mineral than in the oil. The red orange colour is your reference colour for exact matching of index of refraction - remember it for subsequent tests.
FIG. 2-3: Crystal obliquely illuminated by red-orange and blue light.
For the other two mounts, the grains in oil (1.63) will show a yellow border away from the edge of the accessory plate and a shadow adjacent to it - crystal higher than oil. In the mount with oil (1.65), grains will show a dull brownish fringe away from the edge of the accessory plate and a bright blue border adjacent to it - crystal lower than oil.
Test the mounts with the Becke line. For the matching mount, as the stage is lowered, an orange "Becke" line moves into the grain and a bluish "Becke" line moves out into the oil.
The principal advantage of the Becke line test is that small focal length objectives may be employed (high power), thus permitting tests to be made on grains too small for the oblique illumination method.
7

Spectro 186-312B, Francis, 2013
Make three mounts of
(powder # 2-3, R.I. = 1.434) in oils as close as possible to 1.430, 1.435, and 1.440. Upon observing these three slides you will note that the colour fringes observed are much less pronounced than was the case with
. This is because dispersion is a function of the average refractive index. The lower the mean refractive index of a medium, the less the refractive index varies with the wavelength of light. In the case of fluorite, because of its low index, there this little difference between its dispersion curve and that of the matching oil.
Although the above may appear to be a disadvantage for materials of low
R.I., in practice it allows significantly more accuracy in R.I. determinations.
The precision of refractive index determination is about 0.01 for
(R.I. = 1.640) as compared to 0.001 for fluorite (R.I. = 1.434).
Using the central illumination, Becky Line, and oblique illumination tests, determine the index of
:
Start with a liquid of about R.I. = 1.610. If you find that the range above 1.610 is eliminated, take as the next liquid the mean of the range 1.400-1.610, or 1.500.
Continue this selective process, choosing each liquid from the centre of the range remaining from the preceding elimination. The final index is usually found to lie between the indices of two adjacent liquids. After a little practice one should be able to decide that the preferred value lies closer to one liquid than to the other. With liquids spaced at approximately .005 the uncertainty in the determination will not exceed .0025 and may be refined to .001 (.005 liquids obtained by mixing two drops of .01 spaced oils).
As the index of immersion liquids varies appreciably with temperature, the observed index of the grain must be corrected for temperature for work requiring high precision. You do not need to do this in the present exercise, but the procedure is as follows:
Read the temperature of your microscope to the nearest 0.5 degree and enter this value. The temperature at which the liquid was standardised will be found on the bottle label. The rate of change of liquid index with change in temperature (-dn/dt) is read from the accompanying graph (note the negative sign). dn is then known, providing the correction for the estimated index of the grain. Enter this value under n at the top of the form.
8

D.
Spectro 186-312B, Francis, 2013
The vials marked “
?
” contain a mixture of two phases. One is isotropic under crossed-polars and has a light brownish colour in plane-polarized light, while the other is anisotropic under crossed-polars, but is clear and colourless in planepolarized light.
Determine the refractive index of the isotropic phase using the procedure practiced in Section C .
Determine whether the anisotropic phase has a higher or lower refractive index than the isotropic phase by: a) observing Becke lines that appear along the contacts between the two
phases in composite grains. and b) comparing the refractive index of the anisotropic phase with that of the oil
that matches the isotropic phase.
What is the identity of the isotropic phase?
What do you think the sample represents?
Hint: Examine the habits of the two phases and their relationship in composite grains
9

Spectro 186-312B, Francis, 2013
A.
Examine a thin section from Group A under crossed polars (with the analyser inserted) and locate the isotropic mineral. This is accomplished by finding grains that exhibit the characteristic property of isotropic minerals in cross-polarized light: they remain dark upon rotation of the microscope stage.
1) Remove the analyzer and determine its true colour.
2) Does the mineral have high or low relief with respect to other minerals and/or the glue at the edge of the slide?
3) Try to determine whether the refractive index of this mineral is higher or lower than that of either the adjacent clear mineral (n

1.55) or the glue at the edge of the slide (n = 1.54).
HINT: Proceed as if neighbouring crystals or glue were oil, using the central illumination and Becky Line tests.
B. Examine a thin section from Group B and identify the isotropic volcanic glass. Don't be confused by the extremely fine-grained material that is black even in plane polarized light, when the analyzer out. What is the colour of the glass in plane polarized light? Postulate a reason for the differences between the 2 ends of the thin section.
10