Optical Mineralogy
Lab 13 – Fall, 2015
Uniaxial Interference Figures
1
Conoscopic Observation
• In order to observe an interference figure
the microscope must be used in the
conoscopic mode
• Conoscopic refers to the cone-shaped
illumination obtained when the condenser
lens is near the thin section
• This requires that the following conditions
be met
2
Conoscopic Technique
• A. Analyzer inserted and crossed with
respect to polarizer (CN)
• B. Objective lens with a numerical aperture
(N.A.)  0.65 must be used
• C. The condensing lens must be moved (or
swing-out lens inserted) to focus the light
on a small area
• D. The Bertrand lens must be inserted
3
Choosing a Grain
• Choose a grain that stays in extinction or has
very low colors
 You are trying to locate a grain with its optic
axis perpendicular to the slide
 You want to be looking along the optic axis, or
as close as you can possibly get – this produces a
“centered” optic axis figure
 How close that is depends on the birefringence
of the mineral
4
Choosing a Grain, II
 For quartz, the grain must be almost black at all
times, for olivine, first-order gray will do
 For calcite, any recognizable interference color
will probably work
 Try to be at least in the lower 10% of the
mineral's color range
 Sometimes you just can't do it with a given thin
section, especially if the mineral you're dealing
with has only tiny grains or very few of them
5
Conoscopic vs. Orthoscopic
Observation
• Diagram
compares
the two
types of
viewing
6
Conoscopic Procedure
• Select a grain whose interference you wish to
check
• Make sure the cover slip is facing up
• Move the grain to the center of the stage
• Be sure you are in CN (are the polars crossed?)
• Focus at low power
• Make sure you are not focused on a crack or
impurity in the grain
7
Conoscopic Procedure, II
• Increase to medium power, double check
focus
• Move up to high power and double check
focus
• Be sure to raise or flip in the auxiliary
condenser lens
8
Bertrand Lens
•
•
•
Insert the Bertrand lens
If your scope does not have a Bertrand
lens, remove the eyepiece and look down
the microscope tube
An interference figure should appear –
rotate the stage to see if there is any
change
9
No Interference Figure?
•
•
•
Check that the microscope is in the correct
configuration
Check that the grain on high power is not
focused on a crack or impurity
Also check that the high power objective
is properly centered
10
Uniaxial Minerals
• The optical class uniaxial has minerals from
two mineral systems:
 Tetragonal – A4
 Hexagonal
o Rhombohedral division A3
o Hexagonal division A6
• Each system has a unique high order axis,
as shown – this is the optic axis
11
Quadrant Labels
• The quadrants are labeled
starting in the upper right
and going
counterclockwise
• Roman numerals are used
to designate quadrants
12
Optic Axis
• The optic axis is designated as the
crystallographic “Z” axis
• When a thin section of a mineral is cut
perpendicular to the optic axis, and then
viewed perpendicular to the thin section,
light is traveling along the optic axis
• Light traveling in this direction experiences
a single index of refraction, ω (omega)
13
Optic Axis Figures
The isogyre has 1º color; the area between the
isogyre arms is 1º white, unless isochromes are present
14
Low vs. High Birefringence
• Quartz, low birefringence
Calcite, high birefringence
15
Origin of
Isogyres
Figure 21, page 28, W. W.
Moorhouse, The Study of Rocks in
Thin Section – S marks the slow
ray (for the + case)
• In conoscopic view, ω always vibrates ║ to the z axis
and tangential to the isochromes, whereas ε ́ always
vibrates  to the isochromes
• Whenever one of these vibration directions is parallel
to the polarizer (i.e., E-W), extinction occurs
16
Origin of Isogyres, II
• The two bands of extinction form a centered
cross for an optic axis section
• The point where the isogyres meet is called
the melatope and represents the optic axis
itself
• Melatope comes from Greek words
meaning “dark” and “place”
17
Origin of
Isochromes
•
•
Light which travels along
the optic axis is not split
into two rays, nepsilon' =
nomega, and exits the mineral
to form the melatope
No retardation "between"
rays
18
Origin of
Isochromes, II
•
•
Light following paths 2 & 4
experience moderate retardation
nepsilon' < nomega ~ 550 nm
Light following paths 3 & 5
experience moderate retardation
nepsilon' << nomega ~ 1100 nm because
light makes a larger angle with optic
axis and must take a longer path
through the sample
19
Photomicrograph of HighBirefringence Mineral
• The colored rings are
isochromes
• Calcite – highly
birefringent
20
Accessory Plates
• Accessory plates are plates of anisotropic
minerals ground to a thickness that gives a
particular retardation of light
• When inserted into the light path, they
change the retardation of light coming
through the thin section by a specific
amount and the resultant interference color
helps to identify the mineral
21
o
1
Red Accessory Plate
• This is the compensator you will encounter most
frequently
• The lab microscopes are equipped with one, and
we will use it extensively
• The full wave plate is also called a gypsum plate,
1l plate, 550 nm plate, or 1o red plate (1o rot, in
German) because it is usually made of gypsum
and produces a 550 nm or 1o red retardation
22
Quarter Wave Plate
• This plate is found on your microscopes in lab, but
we do not use it extensively
• As the name implies it produces a retardation of
¼l
• It is also called a mica plate, 150 nm plate, and 1o
gray plate, because it is usually made of
muscovite (glimmer in German) and produces a
retardation of 150 nm, or 1o gray
23
Quartz Wedge
• This is a crystal of quartz cut into a wedge shaped
• Since its thickness varies along the wedge, it can
produce a range of retardations that correspond to
interference colors from 0 (1o black) up to about
3800nm (5o green) - this varies from wedge to
wedge
• The wedge, like all compensators usually has its
slow direction clearly marked, and is inserted into
the microscope tube such that slow direction in the
compensator is at a 45o angle to the polarizing
direction
24
Uniaxial Positive Sign
• In a uniaxial mineral,
the two principle
indices of refraction
are denoted ε (epsilon)
and ω (omega)
• If ε > ω, the mineral is
uniaxial positive
25
Uniaxial Negative Sign
• If ε < ω, the mineral is uniaxial negative
26
Determination of the Optical Sign
• Accessory plates may be used to determine
the optical sign
• Minerals with isochromes are usually
treated differently than minerals without
isochromes
27
Uniaxial Mineral, No Isochromes
• The 1º red (Rot 1) plate is inserted
• On most microscopes, this will be
from the SE
• The slow direction of the accessory
plate (N) should be aligned NE-SW
• A blue color appears in quadrants I
& III, which indicates addition
• A yellow color in quadrants II & IV
indicates subtraction
• This is a uniaxial positive mineral
with low birefringence
28
Uniaxial Positive with 1º Red Plate
• Uniaxial positive
mineral, with 1º red
plate
• Note blue in quadrants
I & III, yellow in
quadrants II & IV
• The isogyres show the
1º red color of the
accessory plate
29
Uniaxial Mineral, No Isochromes
Figure 24b, page 30, W. W. Moorhouse, The Study of Rocks in Thin Section
• A mica or quarter λ plate may be used for minerals with
low to moderate birefringence
• It produces a pair of black dots in quadrants where
subtraction occurs
30
Uniaxial Mineral, with Isochromes
• The isochromes in quadrants
I &III move inward, and
those in quadrants II & IV
move outward
• This is a uniaxial positive
mineral with moderate to
high birefringence
31
Multiple Isochromes
• If the interference figure displays numerous
isochromes, color changes produced with
the 1º red plate become difficult to detect
• In this case the quartz wedge is used
• Inserting the Quartz wedge results in the
movement of the isochromes about the
isogyres
32
Use of the Quartz Wedge
• In quadrants where the colors subtract, the
isochromes move outward as lower order colors
form near the melatope and displace higher order
colors
• In quadrants where the colors add, the isochromes
move inwards, towards the melatope
• The isogyre, on insertion of the accessory adopts
the interference color corresponding to the
retardation of the accessory
33
Uniaxial Mineral, with
Isochromes, using Quartz Wedge
• Left, positive; right, negative
34
Uniaxial Mineral, No Isochromes
• A blue color appears in
quadrants II & IV, which
indicates subtraction
• A yellow color in quadrants
I & III indicates addition
• This is a uniaxial negative
mineral with low
birefringence
35
Uniaxial Mineral, with Isochromes
• The isochromes in quadrants
I &III move outward, and
those in quadrants II & IV
move inward
• This is a uniaxial negative
mineral with moderate to
high birefringence
36
Uniaxial Negative with 1º Red Plate
• Uniaxial negative
mineral, with 1º red
plate
• Note blue in quadrants
II & IV, yellow in
quadrants I & III
• The isogyres show the
1º red color of the
accessory plate
37
Summary of Uniaxial Sign
Determination
• The diagram
summarizes the
determination of
uniaxial signs using a
1o red plate
38
Off-Center Figures
• Finding a grain with the optic
axis oriented exactly
perpendicular to the stage will
sometimes be very difficult
• It would be much more common
to find one wherein the optic
axis is at a slight angle to being
perpendicular to the microscope
stage
39
Off-Center Figure Properties
• Such a grain will exhibit the following
properties:
 It is a grain that shows w refractive index and
an e' refractive index that is close the w
refractive index
 It would also show very low order (1o gray
interference colors between extinction positions
if the analyzer is inserted in orthoscopic mode
40
Off-Center Figure Diagram
• On rotation of the stage, the melatope would rotate in
a circle around the perimeter of the field of view, and
the bars of the isogyres would remain oriented E-W
and N-S
41
Rotation of an Off-center Figure
• Figure 22, page
29, W. W.
Moorhouse, The
Study of Rocks
in Thin Section
42
Off-Center Orientation Diagram
• The melatope lies outside the
field of view
• The vibration direction of the
ordinary ray is tangential to
the isochromes
• The vibration direction of the
extraordinary ray is radial
from the melatope
43
Photomicrographs of Off-Center
Figures
• Thick Quartz – Left 15º off center; Right 30º off center
44
Positive Off-Center Figure
• For an optically positive crystal, all NE and SW
quadrants will turn blue and the NW and SE
quadrants will turn yellow, both colors replacing
the 1ogray color present before insertion of the
compensator
45
Negative Off-Center Figure
• For an optically negative crystal, all NE and
SW quadrants will turn yellow and all NW
and SE quadrants will turn blue, both colors
replacing the 1ogray color present before
insertion of the compensator
46
Flash Figure
• A mineral grain is oriented with it's optic
axis horizontal
• This orientation exhibits the maximum
birefringence, for this mineral in the thin
section, and produces a flash figure
47
Flash Figure II
• The flash figure results because the
vibration directions, of the indicatrix, within
the field of view are nearly parallel to
polarization directions of the microscope
 extraordinary rays vibrate parallel to optic axis
 ordinary rays vibrate perpendicular to optic axis
48
Flash Figure III
• With the grain at extinction, the optic axis is
oriented either EW or NS in the resulting
interference figure
• The interference figure produced occupies most if
not all of the field of view and consists of a very
broad, fuzzy isogyres cross
• Upon rotating the stage, < 5° rotation, the isogyres
will split and move out of the field of view in
opposite quadrants
49
Flash Figure Diagram
• Diagram showing flash figure
orientation, and a flash figure
image
50
Flash Figure after Small Rotation
• The isogyre splits and
quickly leaves the field of
view
• The optic axis lies along the
line connecting the isogyres
51