Ge 114 - Optical Mineralogy: Laboratory Exercise #2
ORTHOSCOPIC EXAMINATION OF MINERALS IN THIN SECTION
Reference: Phillips (1971) Ch. 6 or Nesse (1991) Ch. 5.
Objectives: • To become familiar with interference and extinction, the use of the
accessory plates and quartz wedge, and the determination of mineral
birefringence.
• To learn to describe and interpret correctly the optical and physical
properties of mineral grains in thin section, as seen by orthoscopic
observation.
All questions or procedures in bold type should be answered or recorded in the lab
report.
1. The first slide (3-1, colors) is designed to illustrate interference effects as seen in
orthoscopic light. It is made by 4 superimposed thin sheets of mylar packaging
tape, a crystalline, anisotropic polymer. Examine it under crossed polarizers.
Note: the long slide is best for observing these effects. Colors of the single piece
of tape vary on all the other slides.
Linearly polarized light (vibrating east-west) from the bottom polarizer of the
microscope enters the mylar. The two principal vibration components propagate
through the crystalline polymer with different velocities. They emerge from the mylar
out of phase and recombine to produce elliptically polarized light. The upper
polarizer (the analyzer) transmits the north-south component of the resulting light. If
all wavelengths traveled with the same velocity in the crystal, they would all be out of
phase by the same extent. You would then observe a uniform white to gray color in
the microscope. However, each wavelength will have a different magnitude for the
north-south component in the resulting elliptically polarized light. Therefore,
different colors will be removed to different extents by the upper polarizer and you
will see interference colors. The exact color will depend on the thickness of the mylar
(the more time the light spends in the crystal, the more the two components get out of
phase).
Depending on the slide you have, a single thickness of the tape has a retardation of
either about 620 nm, 320 nm or about 270 nm (± depending on the slide) and
produces a bright blue or pale yellow color depending on thickness. Find this area of
the slide, rotate the microscope stage and observe how the brightness of the sample
varies with the orientation of the slide. You should observe alternating extinctions
90° apart. The long axis of the tape sheet should be parallel to the polarizer when the
tape is at extinction. Rotate the slide to obtain maximum brightness (long axis of the
tape 45° from the vibration direction of the polarizer).
Revised: 09-Oct-2014
Examine the interference color chart (Michel-Levy chart) and try to match the color
you observe with the colors for 320 nm or 270 nm retardation. Note that in observing
interference colors, it is best to have the substage diaphragm closed down as much as
practicable, to give orthoscopic illumination. Next, observe the interference colors
where 2, 3 and 4 layers of the tape overlap. The sheets are oriented so that the
retardations add. Make a table of the number of layers of tape, and their
corresponding interference colors and retardation values. Compare the
observed colors with those predicted from the chart for 2, 3 and 4 times the
retardation of the single layer.
The slow direction of the tape (high index of refraction = slow velocity of light) is
parallel to the long side of the slide. Orient the slide so the slow direction is oriented
parallel to the z' direction on the accessory plate. (z' means the direction of the highest
index of refraction on the accessory plate, a projection of the z direction). Insert the
quarter wave plate and observe how the retardation of the plate adds to the retardation
of the sample. Try this for a couple different thicknesses of the mylar. Also try this
with the 530 nm plate. Then rotate the microscope stage so that the slow direction is
perpendicular to the slow direction of the accessory plate. Now observe how the
retardation of the accessory plate subtracts from that of the mylar. Pay particular
attention to what happens when this is tried with only one thickness of mylar (see
p.50 Nesse). Add two columns to the table you made above: one that reports the
retardation with the slow direction parallel to z’, and one with the slow direction
perpendicular to z’. Finally, observe and record the effect of the quartz wedge
on the 3 thicknesses of mylar in both orientations.
Calculate the birefringence () of the single layer of tape, assuming the tape
thickness is about 47m thick. Use equation 5.3 in Nesse. For future reference,
note that  reported in texts is the maximum birefringence, found from mineral grains
with the highest-order interference color.
2. Examine the slide labeled "Quartz 3-3a,” containing six oriented thin sections cut at
different angles to the c axis. Observe the slide under crossed polarizers and notice
how the interference colors increase as the c-axis is further from perpendicular to the
plane of the slide. Briefly explain why this would happen for quartz (see p.261
Nesse for some help). Use the 1/4 wavelength plate and the 530 nm plate to add and
subtract retardation to the 69 degree (or 65 degree) slice. Compare the interference
colors you observe to the colors you would predict from the interference color
chart. Be careful not to confuse the 6 oriented plates with the long birefringent
spacers towards each long edge of the microscope slide.
The two quartz slides are of different thickness. You will not observe the same
interference color for quartz cut at the same angle to the c axis. Can you explain
why? How would you figure out the thickness of the minerals on each slide?
Revised: 09-Oct-2014
3. A set of seven thin sections of mineral crystals or crystal aggregates is provided for
study. All sections have been ground approximately to the standard thickness of
0.032 mm and are mounted in Canada balsam (n=1.54). For one of the sections,
write a careful description that includes the following properties:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j
k.
homogeneity or heterogeneity of the material.
size of the individual crystal grains.
relief, with estimate of refractive index.
grain or crystal habit (anhedral, subhedral or euhedral; equant, prismatic tabular,
acicular, rhombohedral, etc.).
cleavage and fracture.
internal structures: inclusions, zoning, twinning lamellae, etc.
isotropy or anisotropy: do not overlook weak anisotropy when present in
supposedly cubic minerals.
color and pleochroism (see p.51 Nesse).
maximum birefringence, , using the retardation and thickness in eq. 5.3 Nesse or
an interference color chart.
type of extinction (parallel, non parallel, symmetrical).
sign of elongation (length fast or length slow).
Compare your description with the description of your mineral in Nesse, Phillips
and Griffen, Kerr or Deer, Howie, and Zussman. If cleavage or crystal form
allows you to determine the direction of the c axis in a uniaxial mineral, determine the
optic sign from your data on sign of elongation, and check against the references.
Note: The slides of tremolite and leucite are from Ward’s and should be returned to the
appropriate holder when finished.
Revised: 09-Oct-2014