Spectro 186-312B, Francis, 2013
LAB 5 - UNIAXIAL MINERALS IN THIN SECTION
A & B. Slides in Group's A and B
1)
Quartz and nepheline are difficult to distinguish in thin section on appearance only. Both
are uniaxial and have low birefringence (maximum interference colours are typically lst
order white to yellow). However, they have different optical signs.
2)
Examine one slide from each of the two groups below under low power in plane light. It
should be apparent (by general appearance) that there are a number of different minerals
present. The first step is to find which mineral is a candidate for being either quartz or
nepheline. First find the minerals that have low birefringence (there are 2 or 3 in the
section) by examination under crossed polars. For each of these minerals, find a grain that
should yield a uniaxial optic axis interference figure. Only one of the minerals in each slide
is uniaxial. The other minerals will not exhibit the characteristic uniaxial interference
figure.
3)
Determine the sign of the uniaxial minerals in your slide and identify whether it is quartz or
nepheline with the aid of a suitable text.
One of Group A (AIC2R, AIC8R, AIC12R, AIC13R) and Group B (AIA31R, AIA45R, AIA2R, AIIA4R)
C. Slide 5.2.n
The coloured mineral in the section is a variety of tourmaline. It exhibits a property called
pleochroism: its colour changes upon rotation of the stage, when viewed in plane light
(analyzer out). This is caused by the differential absorption of certain wave lengths of light
in the two rays passing through such anisotropic minerals.
1)
Compare the birefringence of this variety of tourmaline with that of quartz (ie. does
it exhibit higher or lower interference colours).
2)
Find a grain which will give a centred uniaxial optic axis interference figure and
determine the sign of tourmaline using the appropriate accessory plate.
For pleochroic minerals, obtaining such a grain is simply a matter of finding a grain whose colour in plane
light does not change upon rotation of the microscope stage. Why?
3)
Find a crystal showing the highest interference colours (ie. a grain with its optic axis
in the plane of the microscope stage). Using the procedure of Lab. 4, identify Ne
and Nw in this grain and then by successively rotating each vibrational direction
parallel to that of the microscope's polarizer and then observing it in plane light,
identify the true colour of Ne and Nw.
The pleochroism of this mineral may make the recognition of addition or subtraction of interference colours
with a quartz plate difficult. If you are having difficulty, try one of the following:
i)
Examine the grain margins for a thin spot where the interference colour falls
to low first order. Using the quartz plate, monitor only this portion.
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Spectro 186-312B, Francis, 2013
ii)
4)
Use a mica plate (Delta = 1/4 of quartz plate) which adds or subtracts
retardation equivalent to first order white. This plate is also very useful for
minerals of very high birefringence. In such cases, the quartz plate will only
change the order of the mineral's interference colour which may make
identification difficult. The mica plate avoids this problem.
Look up the tourmaline group minerals in a suitable text. Identify which one of the
following varieties of tourmaline is in the slide: schorl; elbaite; or dravite.
D. Slide 8.2.n
1)
There are two different uniaxial minerals in this slide. One of them has very high
birefringence, which differs greatly from the low birefringence of the 2rd mineral.
These minerals can be distinguished by general appearance in plane light and by
their differing birefringence. Determine the optic sign of each mineral and estimate
their respective birefringence (high, medium, or low -- be careful).
2)
What are the striations exhibited by the high birefringence minerals?
3)
If the slide is approximately 20 microns thick, identify the low birefringence
mineral, and identify the group to which the high birefringence mineral belongs
using an Interference Colour Chart.
E. Slide 8.8.n
The dominant mineral in these slides is olivine with a numerical birefringence of 0.037.
Determine the maximum interference colour (retardation) produced by the olivine, and then
calculate the thickness (d) of the section.
=×d
 = retardation in millimicrons (m = nanometres)
 = birefringence
d = thickness in m ( 2500-3000 = 25-30 microns)
F. Slide 5.5.n
Determine and describe the unusual optical character(s) of the concentrically zoned,
brownish mineral (not the wedge-shaped crystals, which are titanite). Identify the mineral
using an appropriate text. The chemical composition of this mineral should give you a clue
as to the origin of its composite optical character. Comment on this origin. What is the
uniaxial mineral of low birefringence in this slide?
G. Slides L07·63, and L08·118
These slides contain numerous grains of the same rare brownish mineral found in slides 5.5,
except that they are finer grained. Find the uniaxial mineral and describe its occurrence.
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