Quartz: A Bull's Eye On Optical Activity: Elise A. Skalwold William A. Bassett
Quartz: A Bull's Eye On Optical Activity: Elise A. Skalwold William A. Bassett
Quartz: A Bull's Eye On Optical Activity: Elise A. Skalwold William A. Bassett
Optical Activity
Elise A. Skalwold
The Mineralogical Society of America William A. Bassett
Title: Quartz: a Bull’s Eye on Optical Activity
Photographer
& Designer: Elise A. Skalwold
William A. Bassett
wab7@cornell.edu
Both Authors:
Department of
Earth & Atmospheric Sciences
Snee Hall, Cornell University
Ithaca, NY 14853
Figure 1. The “bull’s eye” uniaxial optic figure characteristic of quartz is indicative of its optical activity. This is the view seen looking
directly along the optic axis of the crystal when viewed between two crossed polarizing filters in transmitted white light - that is, looking
towards the light source. The N-S, E-W dark areas are the remnants of isogyres which in uniaxial minerals normally form a black cross in
the center, but which in slices thicker than 1 mm are extinguished by quartz’s strong optical activity and low birefringence, thus forming
the bull’s eye appearance. The outer rings of color are isochromes and follow those of Newton’s Rings. The central colors recombine with
increased thickness and ultimately leave only a red changing to green sequence in the center. Rotating the filter farthest from the light
source in a clockwise direction for 180 degrees cycles the bull’s eye through a series of colors and shapes that depend on thickness. For
this 3.0 mm basal slice, the left to right sequence of changing colors indicates that it is likely an optically left-handed quartz; confirmed
with observation using a quarter-wave plate as seen in Figure 5 or by direct measurement with monochromatic light as seen in Figure 11.
Open any textbook on optical mineralogy and one will unknown specimen as being quartz and for orienting
find illustrations of the optic figure produced by all the c-axis/optic axis of pieces of rough in prepara-
uniaxial minerals including quartz, as well as variations tion for fashioning; though its underlying cause is very
caused by such factors as angle of viewing, twinning rarely understood in any depth as it is not needed for
and structural distortions. Less frequently, one will those tasks. In a few cases, the varying color of its cen-
find an illustration of the curious bull’s eye optic figure ter has even been cause for alarm, lest it indicate pos-
seen in relatively thick slices of quartz, so named for sible tampering with its natural condition or even that
its appearance reminiscent of a target (Fig. 1). In min- it indicates a synthetic imposter.
eralogy this is seldom encountered as study with the
petrographic microscope involves thin sections usually A word to the wise
no more than 30 microns (0.03 millimeters); well un- At different periods of time throughout the history of
der the one millimeter thickness above which the nor- optical mineralogy definitions of terms and conven-
mally seen crossed dark isogyres begin to fade in the tions have not always been consistent, either because
center due to quartz’s strong optical activity and low of changing nomenclature as understanding evolved
birefringence, thus forming the bull’s eye appearance. or because of the personal preference of the author.
This fascinating manifestation of optical activity may This can be seen across different types of publications
be overlooked entirely, though the concept may be a old and new and across different specialties – a situa-
notion stored away in the cranial filing cabinet along tion which can lead to great confusion, frustration and
with other required, but rarely used academic trivia even inadvertent misuse of terms. The favored texts
learned in the pursuit of one’s chosen geologic field. of the current authors are Wahlstrom (1969), Frondel
Gemologists and lapidaries are usually quite familiar (1962), and Tutton (1924); the conventions used in the
with the bull’s eye for it is used to instantly identify an present article are described below.
Quartz: a Bull’s Eye on Optical Activity Mineralogical Society of America 3
Figure 3: There are various ways of viewing specimens between
polarizing filters. One is to use a polariscope, such the model at
left. A fixed filter termed the polarizer is at the bottom and a
rotatable filter termed the analyzer is at the top. Transmitted
light is provided by a bulb in the base (or by an external light for
other models). A similar setup may be used in microscopes either
with provided filters or by using a portable polariscope resting on
the stage. The sphere on the end of the glass rod is strain-free
and is used to resolve the optic figure; a 10x loupe works just as
well. A number of test specimens are shown, including 3 mm
and 14.75 mm basal sections and a 10 mm sphere (all on the
stage); faceted purple, yellow and colorless quartz specimens –
the latter two optical-quality quartz were optically oriented by the
author (EAS) and custom cut by Mark Oros. The two milky white
cabochons from Sri Lanka feature multi-star networks; using the
bull’s eye to orient them makes it easy to study their fascinating
intricate networks, especially in spherical versions.
Figure 8b: Diagram “a” represents the quartz/polarizing filter arrangement oriented just as it is inside the mono-
chromater, directly below the round window at the top of the image in Figure 8a. The first stage (lowest in the
diagram) is a good illustration of the simple principle of rotation of plane polarized light within the cylinder of
quartz from polarizing filter to polarizing filter (or polarizer to analyzer).
Diagram “a” shows the left-handed rotation (counter-clockwise) of short wavelength (high-frequency blue end
of the visible spectrum) and diagram “b” shows the long wavelength (low-frequency red end of the spectrum); it
is also blocked from going beyond the first thickest section by the position of the polarizer between the first and
second sections. The second, third, and fourth stages simply take the range of wavelengths from the previous stage
and further stretch it out until the light at the top covers a small range of wavelengths, i.e., is even more mono-
chromatic. As the knob is rotated, the large dial also turns and indicates the wavelength being transmitted. As light
passes through the system, each following stage eliminates half of the light entering it (see Fig. 9c).
The diagram’s original optical handedness labels have been changed in order to be consistent with the point of
view used in a polariscope (Fig. 3); that is, handedness is determined looking into the path of oncoming light (Hurl-
but and Rosenfeld 1952, page 162; diagram courtesy of the Mineralogical Society of America).
(9c)
520 nm
570 nm
650 nm
What is more, since the rotatory power to separate the In the Hurlbut paper, point of view and structural
colors is the same for left and right-handed quartz, the versus optical handedness is not stated. We have taken
choice of handedness for stage 1 and for stages 2, 3, the liberty of relabeling the diagram in Figure 8b to be
and 4 in the monochromator does not matter. Given consistent with the convention which we have taken
that each handedness occurs in roughly equal amounts throughout the present paper, namely, handedness is
in nature (hence any purchased lot will contain roughly determined looking into the path of oncoming light.
equal amounts of each), it is logical to make the thick- Hurlbut’s original diagrams reproduced in Figures 8
est section (53 mm) one half of the total four quartz and 9 have expanded captions intended to illuminate
sections and the remaining three sections of opposite ideas he and Rosenfeld outlined in their American Min-
handedness than the first. The thickness of each suc- eralogist paper.
ISBN: 978-0-939950-00-3