Optical Mineralogy Lab 5
Name __________________________________
Winter 2014 - Lab 5:
Optical properties of common rock-forming minerals; textures in thin section
Camosun College GEOS 250 Introduction to Mineralogy
Goals:
1) recognition of additional common minerals in thin section & comparison to hand specimen.
2) familiarization with interference colors and the Michel-Levy interference color chart
3) use of interference figures to identify minerals in thin section - an introduction
4) identification of textures in thin section
1.Interference colors are observed only under crossed polars. They are a function of birefringence (mineral
structure, composition and orientation as per the difference between the refractive incices in that particular
plane) and the thickness of the thin section. Standard thin sections are 0.03mm (30 microns) thick. A mineral
grain that exhibits the maximum interference color possible will show the maximum birefringence.
To answer this question, set your scope up in orthoscopic mode (light source lowered), crossed polars.
Determine the maximum interference color (e.g.2nd order red compared to 30 micron thickness on the fold
out birefringence chart in Perkins) for the following:
(9 points as 3 each)
a. quartz (in quartzite or a quartzose sandstone)
b. calcite (in marble or sparry limestone)
c. olivine (in dunite or peridotite)
d. According to the birefringence chart, what would the maximum interference color of quartz be in a thin
section 0.04mm (40 microns) thick? (Note: This is more common than one would like, so beware!)
(1)
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2. Observe Quartz in quartzite. Under crossed polars, when the vibration directions of a mineral are parallel to
those of the microscope polarizer and analyzer, total darkness (extinction) occurs. Minerals such as quartz,
tourmaline, apatite, zircon & corundum show extinction parallel & perpendicular to the long axis (c-axis).
Sometimes individual grains of particular minerals do not go totally extinct at once but in an irregular manner
as the stage is rotated. This is called undulatory extinction. Observe and draw the extinction of the quartz
grains in quartzite as you rotate them under crossed polarized light. What causes undulatory extinction?
Before answering this ask yourself if the specimen before you is at the same conditions of temperature,
pressure, strain and fluid content as when it crystallized.
(5)
3. Note the textures of the samples of: quartzite, marble and dunite (peridotite) in #1 above. These are all
metamorphic textures that indicate recrystallization. Forces involved can be shear or compression.
Compressive forces may be uniform in all directions or anisotropic with 2 or 3 different principle directions.
When fluids are present as in air or under water at various elevations/depths forces are “hydrostatic” and equal
in all directions.
Draw this texture and explain what causes it or what we can infer from it?
(5)
4. Centered uniaxial optic axis figures. Examine quartz looking down on the C axis using conoscopic
illumination. (Carefully on high power and on our scopes, bring the sub stage light source up close to the
stage. Finally bring the Bertrand lens into position behind the upper ocular). Make colored drawings showing:
a) The interference figure showing isogyres and regions of equal birefringence/retardation.
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(5)
b) The interference figure with the accessory plate inserted. Label the drawings with the name of the mineral,
optic sign as determined by you, and a scale.
(5)
c. Take care to keep the grain with the centered optic axis figure under the cross hairs. Remove the accessory
plate, flip out the Bertrand Lens. Switch your objective to the low power. Now look at your quartz cut
perpendicular to C in orthoscopic mode, low power, crossed polars (i.e. analyzer inserted). What is the
interference color of the mineral? (Hint: it should be the same for all grains oriented perpendicular to the C
axis. Explain the reason for this particular birefringence colour. Is this the maximum birefringence colour for
this mineral?
(4)
d. Observe the character of the extinction for this grain as you rotate the stage & explain why it differs from
grains with other orientations using the standard notation for omega and epsilon or epsilon prime.
(4)
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5. a) Repeat this procedure for calcite: Draw the interference figure showing isogyres and regions of equal
birefringence/retardation in crossed polarized light
(5)
b) Draw the interference figure with the accessory plate inserted. Label the drawings with the name of the
mineral, optic sign as determined by you, and a scale.
(5)
c) Before losing your calcite grain with the centered optic axis figure, flip out the Bertrand lens and sketch the
crystal you are using being careful to label the orientation of the grain in the extinction position with respect to
its: cleavage directions, twins if present etc. Measure the angle between the cleavage directions and the
extinction position and label this on your sketch. How does this relate to the crystal symmetry positions for
this Trigonal-Rhombohedral mineral? Label any symmetry elements (rotational axes, mirror planes) on the
sketch.
(10)
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6. Examine special mineral thin sections in the white boxes sections containing the following minerals and
note their relief relative to the mounting cement (near quartz for RI), birefringence, presence of cleavages or
twinning, pleochroism and any other distinctive optical properties. Cubic minerals are isotropic and have near
zero brirefringence. Iniaxial minerals have 2 principal refractive indices, one along c and 1 across c. Biaxial
minerals have 3 refractive indices. Look them up in Perkins as you examine them and make a list of the
optical properties that you can observe for each. Also name which group each mineral belongs to for its
chemical type and for its crystallographic structure. E.g. Quartz: is in the Hexagonal system & the silicate
group. Optical properties refers to how it appears: coloured or pleochroic (or not), fractured, cleavages (# and
extinction angle), zonation, inclusions etc.
##-Mineral
29-Quartz
System
Group
Relief
Birefringence
10-Calcite
27-Olivine
22-Leucite
5-Apatite
40-Zircon
17-Garnet
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Optical Properties