Laboratory exercises and
demonstrations with the spindle stage
Mickey E. Gunter
Department of Geology
University of Idaho
Moscow ID 83844-3022 U.S.A.
mgunter@uidaho.edu
Introduction and
background
The
goal of this lab session is to introduce you to the spindle stage and its
possible uses in an undergraduate mineralogy lab. A spindle stage is a one-axis rotation device that mounts on
a polarizing microscope and is used to aid in the measurement of optical properties
of single crystals. At the
undergraduate level, it can be used to identify minerals and to demonstrate the
relationships among grain shape, retardation, and interference figures. A natural extension of these uses is
undergraduate research on the optical properties of minerals. These notes and other references for
the spindle stage are posted at the web site: www.uidaho.edu/~mgunter/opt_min/ss/ss.html.
This
lab session would not be possible without the work of Professor F. Donald
Bloss, and those fortunate ones who have worked with him over the past two
decades to develop, refine, and integrate the methods presented here. Bloss and Light (1973) developed a
student spindle stage, Bloss and Reiss (1973) developed a computer program to
calculate a biaxial mineral's 2V and indicatrix orientation based upon
extinction data, and Louisnathan et al. (1978) refined the double variation
method for precise (+/-0.0001) refractive index determination of minerals. These works culminated in Bloss' MSA
Presidential Address (Bloss, 1978) and a book devoted entirely to the spindle
stage (Bloss, 1981). Several
computer programs were also developed to aid in reducing data collected with
the spindle stage, especially EXCALIBR (Bloss and Reiss, 1973; Bloss, 1979;
Bloss, 1981). With the evolution
of mainframes to microcomputers, EXCALIBR has been modified to work on both
PC's and Mac's (Gunter et al., 1988).
Other programs have been developed to aid in refractive index
calculation based upon the double variation method (Su et al., 1987) and for
routine optical mineralogy calculations (e.g., the relationship between 2V and
the principal refractive indices for biaxial minerals) (Gunter and Schares,
1991). Most recently, Barthelmehs
et al. (1992) rewrote EXCALIBR, making it much easier to use. All of these programs are available
from the web site:
www.uidaho.edu/~mgunter/programs/programs.html.
The
spindle stage has helped research in optical mineralogy from A (i.e.,
andalusites, see Gunter and Bloss, 1982) to Z (i.e., zeolites, see Gunter and
Ribbe, 1992), with many other optical secrets of minerals unraveled in the
middle (e.g., corderites (Armbruster and Bloss, 1982) and feldspars (Su et al.,
1986)). These research projects
did not use the student spindle stages but the more advanced, and expensive,
Supper spindle stage that uses an x-ray goniometer head to mount and hold the
crystal. However, student model
spindle stages can provide almost the accuracy and precision of the Supper
spindle stage. Much more research
could be done, especially at the undergraduate level, using the spindle
stage. Every department has the
necessary equipment (polarizing microscopes) for undergraduates to conduct this
type of research. It is hoped this
lab session will encourage faculty to use the spindle stage in teaching optical
mineralogy, or at least to use it to demonstrate the relationships among grain
shape, retardation, and interference figures.
Lab session
Listed
below are the necessary steps to implement the spindle stage in an
undergraduate lab. Details are
provided on how to build a poster board spindle stage (PBSS). Thomas Armbruster, University of Bern,
Switzerland, is credited with the idea and original design of the PBSS, which
has been slightly modified here.
Student spindle stages are commercially available - for example, the
detent spindle stage of Bloss and Light (1973) (from McCrone Accessories and
Components, 800-622-8122, price = $50) for those who do not wish to build
them. The detent spindle stage is
well made and worth the investment if funds allow. If you purchase it, skip step 1 below.
Regardless
of the type of spindle stage used, an oil cell must be built in which to view
the crystal and determine its refractive index by the immersion method. To build an oil cell, mount a bent
paper clip (or some other type of wire of the correct thickness) to a
petrographic slide. Next, place a
drop or two of immersion liquid in the u-shaped paper clip, and place a glass
cover slip on top.
Glue
a single crystal of a mineral of interest onto the end of needle, and insert
the other end of the needle into the spindle stage. Next, attach the spindle stage and the needle combination to
the stage of a polarizing light microscope. Carefully slide the oil cell into the docking port of the
spindle stage until the crystal is immersed into the liquid. At this point, you are ready to make
optical measurements.
For
a complete description of all aspects of the spindle stage refer, to Bloss
(1981). Also, Nesse (1991) and
Stoiber and Morse (1994) provide
brief descriptions of applied spindle stage use. For those rusty in optical mineralogy, refer to Gunter
(1992) for a short review, or Bloss (1961), Nesse (1991), or Stoiber and Morse
(1994) for thorough treatments.
Please note: As
stated above, my intent is to introduce you to the spindle stage, give some
idea of what it can do, some "hands-on" experience, and, mainly,
provide the resources for you to continue to use the spindle stage in the
future. Remember, there is an
entire book devoted to the subject (Bloss, 1981), and Bloss taught a
semester-long course on its use.
And you will knock a
crystal off the end of needle occasionally, but that's part of the fun!
Lab procedure
The
following is a seven-part, step-by-step procedure. The first line in each step (marked with a "*" and in bold) is the action to be taken. The following text in that step are
pitfalls, words of wisdom, and hints.
1. Build a poster board spindle stage
(PBSS)
materials: poster board, 20-gauge hypodermic
needle (Fisher Scientific), petrographic slide, white glue, straight edge,
compass, small protractor
Follow the instructions in
Appendix A.
2. Build oil cells
materials: petrographic slide, cover slip, large
paper clip, epoxy
Follow the instructions in
Appendix B.
3. Mount crystals
materials: sewing needle (size #12), fingernail
polish, acetone, transparent crystals (0.05 to 0.5 mm), binocular microscope,
glass slide, patience
Mounting crystals is a skill
that comes with time. At first, it
seems very hard, but it gets much easier after you have mounted a few thousand
!
Follow the instructions in
Appendix C.
4. Align the PBSS and oil cell
materials: PBSS, oil cell, sewing needle
* Place a sewing needle
(point first) into the PBSS tubing.
* Move the oil cell
(using the oil port farther back on the slide) into the docking section of the
PBSS and make sure the needle does not hit the slide or the cover slip. If it
does hit, it will need to be adjusted up or down by removing the tubing and
changing the height of the hole (in the tubing holder, Figure 1.4). This should not be a problem because I
did this before the lab. That is
also why I have numbered the oil cells and PBSS's to know which
"fits" well.
* Also, check that the
needle does not go too far and hit the back of the oil mount (i.e., the epoxied
paper clip), or off goes the crystal!
* If the alignment is a
problem, then use the oil port mounted at the slide's edge. This
paper clip is a bit thicker than the other, and it will be harder to view an
interference figure with this oil port.
5. Attach PBSS to microscope and view a
sample
materials: PBSS, polarizing microscope, sewing
needle, Scotch tape (about 20 mm wide), patience
* Insert the needle
without a sample into the PBSS until its end lines up with the pencil marks on
the arms of the PBSS. The needle should be sticking out about
13 mm (see Figure 1.4). The needle
will appear a bit loose in the hypodermic needle. This is intentional.
The loose fit will help keep you from knocking off the crystal. Later, a tighter fit will be important. This can be accomplished by slightly
bending the sewing needle before inserting it, putting a bit of fingernail
polish where the needle slides into the tubing, or carefully crimping the end
of the tubing. The tubing ends can
be exchanged (i.e., the handle taken out and reversed), so one end can be tight
and the other loose fitting.
* Obtain one piece of
tape about 100 mm long and two pieces about 30 mm long.
* Place the long piece of
tape across the PBSS between the tubing holder and the protractor. This
separation was designed for the tape to fit into.
* Place the PBSS on the
microscope stage.
* Using low power,
unpolarized light, center the end of the needle in the middle of the
cross-hairs.
* Press the tape onto the
microscope stage. Also, add the two shorter pieces of tape
to the PBSS arms, being careful to keep them out of the way of the oil cell.
* Insert the oil cell and
make sure the needle does not hit the slide or the cover slip.
* Rotate the microscope
stage and the needle around for awhile to make sure everything is
near-centered.
* Remove the PBSS from
the stage by first removing the oil cell and carefully lifting up the tape. It is
not necessary to remove the PBSS to change samples. You can
use a small pair of needle nose pliers or tweezers to remove the needle and
insert a new one - this is when you get good at it. For now, it is easier just to remove the entire PBSS to
change samples.
Later, we will repeat the
process with a sample. This was
intended to give you some experience without worrying about knocking the
crystal off the needle.
6. Sample exercises
A. determine the refractive
index of a mineral - keep the same crystal and change the liquids
B. find the indicatrix
orientation and 2V of a biaxial mineral from extinction data
Please note: I think
this is the section I would concentrate on in a mineralogy lab. After you do the next section and
demonstrate it to the students, the students should be motivated to build and
use the spindle stage to confirm a mineral's identity by observing its optical
properties. Also, you will develop
the skills needed to assist your students by performing the next section.
7. View grain shape, retardation, and
interference figures
materials: samples A & B provided. Sample A is a uniaxial mineral with its
c-axis perpendicular to the spindle stage axis. Sample B is a biaxial mineral with its optic normal mounted
parallel to the spindle axis.
* Obtain the sample
marked "A" from your glass vial. You will use the PBSS to
observe this grain.
* Fill the oil cell with
1.510 or 1.512 or 1.514 liquid. Use the oil port farther back if
possible.
* Repeat the process from
"Section 5" above to get the grain, PBSS, and oil cell aligned and
affixed on the microscope.
* Using low power and
plane-polarized light, rotate the spindle axis and observe the shape of the
crystal.
* What is the crystal's
shape?
* Switch to
cross-polarized light and rotate the spindle axis to obtain a minimum
retardation.
* Check for minimum
retardation while rotating the microscope stage.
* Repeat the adjustments
on the spindle stage and microscope stage until you find minimum retardation (i.e., the grain shows 1st order gray or lower
retardation as the scope stage is rotated).
* Check the refractive index of the grain against
that of the oil. Is it higher or
lower?
* What index are you
measuring if the crystal is uniaxial? Biaxial?
* Rotate the spindle axis
90 degrees and watch what happens to the retardation.
* Obtain an interference figure for the sample. This
may be tricky. Sometimes the
highest power lens (i.e., the lens with the largest NA) may not be able to get
close enough to mineral without hitting the cover slip. These high NA lenses are usually spring
loaded, so nothing bad will happen.
Two hints are: 1) use the
next highest power lens. It will
image less of a cone of rays and thus show a smaller portion of the
interference figure, but in many cases, especially if the figure is
near-centered, one can determine the type of interference figure, and 2) lower
the stage so the end of the high power lens is 10-20 mm from the oil cell. Switch to a conoscopic illumination
with the high power lens; raise the stage slowly and watch the interference
figure form and fill the field.
Because of a possible collision between the lens and oil cell's cover
slip, I epoxied the cover slip down.
It will not come off the oil cell and make a mess on the microscope.
* What is the optic sign?
* Rotate the spindle axis
90 degrees and watch what happens to the interference figure.
* Replace sample
"A" with sample "B".
* Obtain an interference
figure. Use the same cautions as above.
* What is the approximate
2V and the optic sign?
Please note: I would not recommend having students do this exercise. Instead, I recommend this as a
demonstration for to you do. The
main problem with having students do this is obtaining and mounting crystals in
a preferred orientation, and the inevitable crisis of knocking the crystal
off. I took 5 to 6 minutes to
mount and check each of your samples, but I think this works well for a
demonstration and I wanted you to see it.
My students seem to enjoy this, and it convinces them that the same
mineral will look much different depending upon its orientation. These oriented crystals can also
replace the very expensive oriented thin sections, which are also hard to
obtain.
For
uniaxial minerals, one needs crystals with perfect (001) or (hk0)
cleavage. I have used eudialyte
and scapolite - this is your sample "A". (If anyone knows of any other minerals please tell me,
especially if they are common.)
Given these morphological conditions, the spindle axis can be made
perpendicular to c, the optic axis.
Another method is to use crystals with no cleavage (e.g., quartz) and
view crushed quartz crystals with a binocular microscope set up with
cross-polarized light. The big
quartz crystals that exhibit low retardation more nearly lie on a circular section. They can then be mounted with their
optic axis perpendicular to the needle, and, in turn, the spindle axis.
For
biaxial minerals, one needs a cleavage direction that is perpendicular to the
optic normal. Gypsum is almost
perfect. It has perfect (010)
cleavage with b=Y (i.e., the optic normal is perpendicular to the nice flat
(010) plane). The problem with
gypsum is that you need to mount several crystals (at least I have had to) to
find one with minimal "deformation" to show good interference
figures. The feldspars, especially
K feldspars, are also good candidates.
They all have perfect (010) cleavage. For high sanidine, b=Y, so it should be perfect, but I have
never tried it. Low sanidine has Y
perpendicular to another prominent cleavage, (001). These low sanidines can be mounted, with the aid of
cross-polarized light, with the needle perpendicular (001) while they are lying
on (010) - this is your sample "B". Both orthoclase and microcline share this same optical
orientation and should work as well as low sanidine, but I have not tried them
yet. If anyone knows of biaxial
minerals that fit this condition, please let me know.
In
case we cannot get the interference figures to work, you can view them on my
web site:
www.uidaho.edu/~mgunter/opt_min/ss/ss.html. I placed short quicktime movies on the web site
demonstrating this exercise. There
is a uniaxial mineral, eudialyte, mounted with its c axis perpendicular to the
needle; thus, one can rotate from a centered optic axis figure to a centered
flash figure. There is also a
biaxial mineral, gypsum, mounted with its optic normal parallel to the needle
so one can rotate from a centered acute bisectrix, to a centered optic axis, to
a centered obtuse bisectrix.
There are three movies for each mineral. One movie is made in unpolarized light to show the grain
shape as it is rotated. This is
good for viewing cleavage, grain thickness, pleochroism, etc. The second movie is made in orthoscopic
illumination with polarized light.
In this setup, changes in retardation can be observed; anytime a
circular section is parallel to the microscope stage (synonymous with an optic
axis perpendicular to the stage), retardation is near zero. Retardation is increased to a maximum
when, for the uniaxial mineral, the single optic axis is in the microscope
stage. For the biaxial case,
maximum retardation occurs when the obtuse bisectrix is perpendicular to the
stage. The third movie shows how
the interference figures change as each sample is rotated about the
spindle. It can be instructive to
place all three movies for one crystal on the screen at the same time and
"rotate" each image the same amount to show how they correlate.
References
Armbruster, T., and Bloss, F.D. (1982). Orientation and effects of channel H20 and CO2
in cordierite. American Mineralogist,
67, 284-291.
Bartelmehs, K.L, Bloss, F.D., Downs, R.T., and Birch,
J.B. (1992) Excalibr II. Zeitschrift fuer Kristallographie, 199,
185-196.
Bloss, F.D. (1961) "An introduction to the methods of optical
crystallography." Holt,
Rinehart and Winston, New York.
Bloss, F.D. (1978) The spindle stage:
a turning point in optical mineralogy. American Mineralogist, 63, 433-447.
Bloss, F.D. (1981) "The spindle stage: principles and
practices." Cambridge
University Press, Cambridge.
Bloss, F.D., and Light, J.F. (1973) The detent spindle stage. American Journal of Science, 273-A,
536-538.
Bloss, F.D., and Reiss, D. (1973) Computer determination of 2V and
indicatrix orientation from extinction data. American Mineralogist, 58, 1052-1061.
Gunter, M.E. (1992) Optical Mineralogy.
Encyclopedia of Earth System Science, W.A. Nierenberg, editor, Academic
Press, Inc., San Diego, 3, 467-479.
Gunter, M.E., and Bloss, F.D. (1982) Andalusite-kanonaite series: Lattice and optical parameters. American Mineralogist, 67, 1218-1228.
Gunter, M.E., Bloss, F.D., and Su, S.C. (1988) EXCALIBR revisited. American Mineralogist, 73, 1481-1482.
Gunter, M.E., and Schares, S.M. (1991) Computerized optical mineralogical
calculations. Journal of
Geological Education, 39, #4, 289-290.
Gunter, M.E., and Ribbe, P.H. (1993) Natrolite group zeolites: correlations of optical properties and crystal chemistry. Zeolites, 13, 435-440.
Louisnathan, S.J., Bloss, F.D., and Korda, E.J.
(1978) Measurement of refractive
indices and their dispersion.
American Mineralogist, 63, 394-400.
Stoiber, R.E., and Morse, S.A. (1995) "Crystal identification with the
polarizing microscope."
Chapman and Hall, New York.
Nesse, W.D. (1991). "Introduction to optical mineralogy, 2nd
edition." Oxford University
Press, New York.
Su, S.C., Ribbe, P.H., and Bloss, F.D. (1986). Alkali feldspars: Structural state from
composition and optic axial angle 2V.
American Mineralogist, 71, 1285-1296.
Su, S.C., Bloss, F.D., and Gunter, M.E. (1987) Procedures and computer programs to
refine the double variation method.
American Mineralogist, 72, 1011-1013.
Appendix A: Building
a poster board spindle stage
materials: poster board, 20-gauge hypodermic
needle (Fisher Scientific, these come in 6 and 12 inch lengths, #14-82516E and
14-825-15AB), petrographic slide, white glue, straight edge, compass, small
protractor
* Cut two 50 x 50 mm
squares and one 10 x 50 mm rectangle from poster board as shown below. Part A
will be the PBSS base, part B will be the protractor scale, and part C will
help hold the tubing in place.
Figure
1.1: Starting material sizes for
base (A), protractor (B), and tubing holder (C).
* Mark all three pieces
as below (Figure 1.2), and cut pieces B and C as shown to produce the round
protractor as shown in the Figure 1.3. Part B should first be cut into a
circle and then cut in half along the horizontal line.
Figure
1.2: Marked-up base (A),
protractor (B), and tubing holder (C).
B should first be cut into a circle and then cut horizontally. C should just have the edges trimmed.
* Overlay a petrographic
slide onto the base as shown in A below (Figure 1.3). Use the
slide to mark the base and then cut out the marked area. This is the dock for the oil cell to
fit into.
* After making the cuts
on B above (Figure 1.2), write numbers on it like B below. This is
the template (i.e., protractor) to measure the S angles on the spindle stage.
Figure 1.3 : The base (A) with slide overlain for
marking the oil cell dock. The
protractor (B) cut in a half-circle and numbered.
* Glue the protractor and
tubing holder onto the base as shown below (Figure 1.4).
Figure
1.4: Finished PBSS. Left is the top view showing all the
assembled parts; right is a side view.
* Add the hollow tubing. Cut a
piece of 20-gauge hypodermic needle 55 mm long and make a 90 degree bend in the
middle. (Hypodermic needles can be
cut with a triangular file which does not collapse the needle.) The hole for the tubing in the
protractor's and tubing holder's centers can be made with a needle of similar
diameter. The two holes should
cause the tubing to be parallel to the base so a needle will project from it
and be parallel to the oil cell slide.
Adjustments may need to be made as described in section 4.
Appendix B: Building oil cells
materials: petrographic slide, cover slip, large
paper clip, epoxy
* Bend one side of a
large paper clip into a small "U" about 5 mm long and epoxy it to the
center of a petrographic slide about 13 mm from the end in the slide's center
(Figure 2.1 below). The "U" can be flattened a
bit by placing it on a hard surface and hitting it with a hammer. The thicker the "U," the
easier the alignment of the oil cell - needle - spindle stage combination. However, the thinner the "U,"
the better chance one has to see interference figures. These "U"s are the oil ports
on the oil cell. We have also used
staples and wire to make oil ports.
Figure 2.1: A petrographic slide with two bent
"U"s epoxied on the slide's center.
* Make a second 5 mm
"U" and epoxy it on the end of the cell (Figure 2.1). Less
care needs to be taken for alignment with this thicker "U" at the
slide's edge.
* Cover slips will be
placed on top of the "U" to hold in the immersion liquid. They
can be epoxied onto the "U," but surface tension will hold them
on. If they are epoxied, they stay
on better, but it is harder to clean out the immersion liquid when you need to
change it. When the "U"
is very thin and the cover slip is epoxied on,. air bubbles can form in the oil
port. Bubbles can be cleared by
holding the oil cell vertically and allowing the bubble to rise. If this does not work, stick a small
needle into the bubble while holding the cell vertically. Oil can be removed from the oil port by
sticking a small piece of rolled tissue paper between the cover slip and slide.
Appendix C: Mount crystals
materials: sewing needle (size #12), fingernail
polish, acetone, transparent crystals (0.05 to 0.5 mm), binocular microscope,
glass slide, patience
* Obtain several crushed
mineral grains of interest (0.05 to 0.5 mm). They can be sieved if you
want to remove the fine and the course fractions.
* Place them on a glass
side under a low power binocular microscope.
* Locate a good single
crystal with the microscope. Good means not twinned, homogenous,
etc. You may not be able to tell
if you like the crystal until you see it with the polarizing light microscope.
* Get a needle and dip
its tip into a drop of fingernail polish. Super glue, Duco cement,
or many other glues can be used; fingernail polish has the advantage of being
slow to set, allowing for repositioning of the crystal, and the crystal can be
removed with acetone.
* Bring the needle next
to the crystal and gently touch the crystal with the needle end. The
crystal should stick. If not, add
more fingernail polish and try again.
* Observe the crystal on
the end of the needle. It should be near-centered at the
needle's end. You might want to
move it around a little to get it more centered or in a particular
orientation. The crystal can be
moved by gently pushing it with another small needle.
* Reinforce the
fingernail polish around the crystal/needle contact. This
can be done by dipping another needle in fingernail polish and working it
carefully around the contact; avoid covering the entire crystal with fingernail
polish (mineralogists do not care to measure the refractive index of fingernail
polish).