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Extinction Angle and Pleochroism
IN THIS LECTURE
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Extinction Angle
Sign of Elongation
Categories of Extinction
Extinction in Uniaxial Minerals
Extinction in Biaxial Minerals
Pleochroism in Isotropic Minerals
Pleochroism in Uniaxial Minerals
Pleochroism in Biaxial Minerals
Extinction Angle
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The angle between the length or a prominent cleavage in a mineral and a
vibration direction is a diagnostic property called the extinction angle.
To determine the extinction angle
1. Rotate the stage of the microscope until the length or cleavage of
the mineral is aligned with the north-south cross hair.
2. Record the reading from the stage goniometer at this point (g1)
3. Rotate the stage until the mineral goes extinct (dark)
4. Record the new reading from the goniometer (g2)
The extinction angle is the difference between G1 and G2.
If the extinction angle measured by rotating the stage clockwise is EA,
then the extinction angle measured by rotating the stage anticlockwise
is 90°-EA.
Normally the smaller angle is reported.
Extinction Angle
Extinction Angle
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The extinction angle measured on a specific mineral in thin-section
depends on exactly how the grain happens to be oriented in the sample.
Hence it is necessary to specify the orientation of the mineral grain on
which the measurement is made.
In many cases the diagnostic extinction angle is measured on grains
that display maximum retardation (optic axes horizontal)
In others, extinction angles are specified for certain orientations, such
as resting on a cleavage surface or a cut through the mineral parallel to
a specific crystallographic plane
Sign of Elongation
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Sign of elongation refers to whether the mineral is length fast or
length slow
Length fast means that the fast ray vibrates more or less parallel to
the length of an elongate mineral
Length slow means that the slow ray vibrates more or less parallel to
the length of an elongate mineral.
Length fast is also called negative elongation whereas length slow is also
called positive elongation
Determining Elongation Direction
1. Start with the mineral extinct and with the mineral elongation or trace
of cleavage so that it is less than 45° from the N-S cross hairs. This
generally is the position after rotating to measure the extinction angle
2. Rotate the stage 45° clockwise. This places the vibration direction
closest to the length or prominent cleavage NE-SW.
3. Insert an accessory plate.
4. If the retardations add, the ray whose vibration direction is closet to
the length or cleavage is the slow ray and the mineral is length slow. If
the retardations subtract, the ray whose vibration direction is closest
to the length or cleavage is the fast ray, and the mineral is length fast.
Function of Accessory Plates
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The primary function of the accessory plates is to determine which of
the rays coming through a minerals is the fast ray and which is the slow
ray.
The information is used to determine the sign of elongation as we have
just seen and the optic sign which we will look at later.
In addition the accessory plates may help to distinguish between
different orders of interference colors.
The common accessory plates are gypsum and mica plates.
Accessory plates are carefully prepared so that they produce a known
amount of retardation and the slow ray vibration direction is across the
width of the holder and the fast ray vibration direction is across the
length of the holder.
In most microscopes the accessory plates slide into the optical path in a
slot aligned NW-SE so that the accessory slow ray vibrates NE-SW and
the fast ray vibrates NW-SE.
Accessory Plates
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The gypsum plate produces 537 or 550 nm of retardation and yields a
distinct magenta interference color seen at the transition from first to
second order.
The mica plate produces around 150 nm of retardation and yields a
first-order white interference color.
When looking at minerals under the microscope, if the slow ray
vibration direction is parallel to the slow ray vibration direction of the
accessory plate, then the slow ray relative to the fast ray will be
retarded a distance of Dm + Da giving a total retardation of Dt.
If the mineral produces 250 nm of retardation (first-order white) and
the gypsum plate is used (Da = 550 nm) then the total retardation is
800 nm and the interference color observed will increase to second
order yellow.
Therefore retardations add = slow on slow
If the fast ray vibration direction of the mineral is parallel to the slow
ray vibration direction of the accessory plate the opposite will occur
and hence retardation subtract = slow on fast
Categories of Extinction
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There are four categories of extinction
1. Parallel extinction
2. Inclined extinction
3. Symmetrical extinction
4. No extinction angle
1. Parallel Extinction
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With parallel extinction the mineral is extinct when the cleavage or
length is aligned parallel to one of the cross hairs.
The extinction angle is 0°.
Either the slow ray or fast ray vibration direction is parallel to the
trace of cleavage or length of the mineral.
2. Inclined Extinction
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With inclined extinction the mineral is extinct when the cleavage of
length is aligned parallel to one of the cross hairs.
The extinction angle will be greater than 0°.
Neither vibration direction is aligned parallel to the trace of the
cleavage of the length of the mineral.
If the slow ray vibration direction is closest to the length or trace of
cleavage, the mineral is length slow. If the fast ray is closest the
mineral is length fast.
3. Symmetrical Extinction
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Symmetrical extinction may be observed in minerals that display
either two cleavages or two distinct crystal faces.
If the extinction angles EA1 and EA2 measured from the two
cleavage or crystal faces to the same vibration direction, are the
same, extinction is symmetrical.
4. No Extinction Angle
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Many minerals lack distinct cleavages or do not display an elongation or
crystal faces.
Although they go extinct once every 90° of stage rotation, there is no
cleavage, elongation or crystal face from which to measure an
extinction angle.
In these situations we say that the mineral has no extinction angle.
Other Types of Extinction
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In addition to the before mentioned types of extinction, some minerals
may not go totally extinct at any stage position.
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Usually this is the result of
1. strain in the crystal lattice
2. chemical zoning
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In many deformed rocks, the mineral grains are bent or distorted so
that different parts of the grain go extinct at different times. If the
extinction follows an irregular or wavy pattern it is called undulose
extinction.
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Many minerals grow so that they are compositionally zoned. Because
extinction angle may be controlled by chemical composition in monoclinic
and triclinic minerals, the extinction angle may vary systematically with
composition so that the centre of the grain may display one extinction
angle and the rim another.
Extinction in Uniaxial Minerals
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Tetragonal and many hexagonal minerals are prismatic and either
elongate or stubby parallel to the c-axis.
A sample with the highest birefringence will have its c-axis parallel to
the microscope stage and will display parallel extinction to prismatic
cleavage and inclined or symmetrical extinction to rhombohedral or
pyramidal cleavage.
Extinction is parallel to {001} cleavage in all grain orientations.
Extinction in Biaxial Minerals
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Orthorhombic minerals display parallel or symmetrical extinction in
sections cut parallel to (100), (010) and (001) and inclined extinction in
random orientations.
Grains cut to yield maximum retardation always display parallel or
symmetrical extinction.
Monoclinic minerals display parallel or symmetrical extinction if {010}
happens to be vertical and inclined extinction is most other
orientations.
Triclinic minerals display inclined extinction in most orientations.
Pleochroism
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Pleochroic minerals change color as the stage is rotated when the
sample is observed in plane light.
The color changes because the fast and the slow rays are absorbed
differently as they pass through the mineral and therefore have
different colors.
When the fast ray vibration direction is parallel to the lower polariser,
all light passes as the fast ray, so the mineral displays that color.
When the slow ray vibration direction is parallel to the lower polariser
the minerals displays the color of the slow ray.
If the stage is rotated to allow both the slow and the fast rays to come
through, the perceived color is usually an intermediate between the two
colors.
Isotropic Minerals and Pleochroism
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Isotropic minerals are not pleochroic because they do not experience
double refraction.
In plane light, isotropic minerals display a uniform color as the stage is
rotated.
Anisotropic Minerals and Pleochroism
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Colored uniaxial minerals are usually pleochroic which can be
sufficiently described by identifying the colors of both the ordinary
and extraordinary rays (we’ll get to those terms)
To describe the pleochroism of biaxial minerals it is necessary to
specify three colors
– Light vibrating parallel to X
– Light vibrating parallel to Z
– Light vibrating parallel to Y