Optical Mineralogy

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Optical Mineralogy

• Interference Figures
• 1. Uniaxial Figures

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Optical Indicatrix and Interference Figures LAB
TS-3 Uniaxial minerals Interference
figures Optic sign Pleochroic scheme LAB
TS-4 Biaxial minerals Interference
figures Optic sign Pleochroic scheme
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• Optical Indicatrix and Interference Figures
• Optical Indicatrix
• Uniaxial Interference Figures
• Biaxial Interference Figures

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Polarisation in the petrographic microscope
upper polarising filter (analyser)
what happens here??? LAB TS-2
sensitive tint plate
what happens here??? LAB TS-1
mineral sample (thin section)
conoscopic light
what happens here??? LAB TS-3,4
condenser lens
plane polarised light (PPL)
lower polarising filter (polariser)
unpolarised light
light source
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Optical Indicatrix
constructed as a sphere or ellipsoid with radii
parallel to the principal vibration directions
and lengths of axes proportional to refractive
index
nmax (slow)
in 2D
nmin nmax
circle isotropic
nmin (fast)
nmin lt nmax
nmin
ellipse anisotropic
nmax
in 3D indicatrix for isotropic mineral is a
sphere (of no further interest) indicatrix for
anisotropic mineral is an ellipsoid 2 cases
uniaxial and biaxial
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Case 1 Uniaxial minerals (hexagonal,
tetragonal a1 a2 c) principal axes ne
// c nw // a e extraordinary ray w
ordinary ray
X Y lt Z
X lt Y Z
Nesse, 2000 Fig. 7.23
Y-Z plane circular section (all planes
perpendicular to X) X optic axis (c-axis
fast) ne lt nw -ve
X-Y plane circular section (all planes
perpendicular to Z) Z optic axis (c-axis
slow) ne gt nw ve
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Case 1 Uniaxial minerals (hexagonal,
tetragonal a1 a2 c) principal axes ne
// c nw // a e extraordinary ray w
ordinary ray
optic axis // plane of section plane of section
contains both nw and ne maximum d
random section contains nw and ne lt
ne intermediate d
optic axis I plane of section plane of section
contains only nw minimum d (extinct!)
Nesse, 2000 Fig. 7.25
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Optic Sign how do we figure this out???
Case 1 Uniaxial minerals Z optic axis
(c-axis slow) ne gt nw ve X optic axis
(c-axis fast) ne lt nw -ve
Case 2 Biaxial minerals ve where Bxa //
Z -ve where Bxa // X
Z
Z
c OA Z
ve
-ve
ve
-ve
c OA X
X
X
a
a
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Optic Sign how do we figure this out???
Requires conoscopic light (condenser lens in
place) interference figures (viewed with
Bertrand lens) use of STP to determine fast and
slow directions Nesse, Ch. 7, p. 139 -143
(uniaxial) p. 143 - 151
(biaxial)
Extinction Angles where optic axis is normal to
plane of thin section mineral will appear extinct
for full stage rotation! applies to both uniaxial
and biaxial minerals how distinguished from
isotropic minerals? (also requires interference
figures stay tuned.....)
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Optical Indicatrix and Symmetry
isometric system a1 a2 a3 all angles
90o indicatrix is a sphere minerals extinct in
XN hexagonal, trigonal, tetragonal systems a1
a2 ( a3) c all angles either 90o or
120o uniaxial indicatrix is ellipsoid X lt Y lt
Z c-axis optic axis e (either X or Z)
parallel extinction orthorhombic system a
b c all angles 90o biaxial indicatrix is
ellipsoid X lt Y lt Z X, Y, Z //
crystallographic axes 2 circular sections I
2 optic axes parallel extinction
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2. Uniaxial Interference Figures (Nesse Ch. 7 p.
139-143) optic axis c crystallographic
axis ne // c nw // a e can be either fast or
slow
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Interference Figures
result interference figure
require conoscopic light
Bertrand lens (on eyepiece tube)
rays focused through centre of sample concentric
interference rings when viewed through Bertrand
lens
condenser lens (sub-stage)
Nesse Fig. 7.36
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Interference Figures
result interference figure
uniaxial optic axis figure
isochrome
OA
melatope
isogyre
OA
number of rings (isochromes) ? birefringence
sample oriented with optic axis normal to plane
of section (in XN, grain appears extinct through
360o rotation)
Nesse Fig. 7.36
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Interference Figures
what it really looks like
uniaxial optic axis figure
isochromes
melatope
isochrome
melatope
isogyre
isogyre
number of rings (isochromes) ? birefringence
cross-hairs
sample oriented with optic axis normal to plane
of section (in Xn, grain appears extinct through
360o rotation)
optic axis figure (OAF) for high d mineral (e.g.,
calcite)
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Interference Figures
uniaxial optic axis figure
isochrome
melatope
isogyre
w
e
number of rings (isochromes) ? birefringence
Nesse Fig. 7.35
sample oriented with optic axis normal to plane
of section (in XN, grain appears extinct through
360o rotation)
e oriented radially w oriented tangentially
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insert tint plate!
Interference Figures Determining Optic Sign
uniaxial optic axis figure
observe colour change in SE-NW quadrants
w
?
e
?
if e slow mineral is ve if e fast mineral is
-ve
Nesse Fig. 7.40
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colours go down (subtraction) w fast e slow
Interference Figures Determining Optic Sign
down
uniaxial optic axis figure
down
ve
colours go up (addition) w slow e fast
w
up
e
up
if e slow mineral is ve if e fast mineral is
-ve
- ve
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Interference Colour Chart
low d optic axis figure
addition grey ? blue
30 mm
subtraction grey ? yellow
what do addition and subtraction look like?
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Interference Colour Chart
low d optic axis figure
high d optic axis figure
addition 2nd order red ? 3rd order red
addition grey ? blue
30 mm
subtraction grey ? yellow
subtraction 2nd order red ? 1st order red
what do addition and subtraction look like?
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colours go down (subtraction) w fast e slow
Interference Figures Determining Optic Sign
uniaxial optic axis figure
Y
ve
colours go up (addition) w slow e fast
Y
SE-NW quadrant if colours go from grey-white to
yellow (subtraction down) mineral is ve (YAY!)
- ve
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colours go down (subtraction) w fast e slow
Interference Figures Determining Optic Sign
uniaxial optic axis figure
B
ve
colours go up (addition) w slow e fast
B
SE-NW quadrant if colours go from grey-white to
blue (addition up) mineral is -ve (BOO!)
- ve
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colours go down (subtraction) w fast e slow
Interference Figures Determining Optic Sign
high d mineral (many isochromes)
no tint plate
ve
colours go up (addition) w slow e fast
low order colours (grey-white) near centre of
figure
- ve
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colours go down (subtraction) w fast e slow
Interference Figures Determining Optic Sign
high d mineral (many isochromes)
no tint plate
ve
colours go up (addition) w slow e fast
tint plate in
(rings move in)
- (rings move out)
- ve
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colours go down (subtraction) w fast e slow
Interference Figures Determining Optic Sign
high d mineral (many isochromes)
no tint plate
ve
colours go up (addition) w slow e fast
tint plate in
(rings move in)
- (rings move out)
- ve
mineral is uniaxial -ve
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Interference Figures
Practical problem(s) 1. How to find a grain
with optic axis normal to plane of thin
section? 2. What if you cant find a suitably
oriented grain?
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Interference Figures
Practical problem(s) 1. How to find a grain
with optic axis normal to plane of thin
section? 2. What if you cant find a suitably
oriented grain?
look for grain that is extinct for full rotation
of stage (opaque? isotropic? hole in slide?
optic axis grain?)
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Interference Figures
Practical problem(s) 1. How to find a grain
with optic axis normal to plane of thin
section? 2. What if you cant find a suitably
oriented grain?
look for grain that is extinct for full rotation
of stage (opaque? isotropic? hole in slide?
optic axis grain?)
look for low d grain with minimum change during
rotation off-centre figure not
ideal, but may be best possible in your section
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slightly off-centre (melatope visible) OK to use
Interference Figures
off-centre uniaxial figure obtained from low
d grain with minimum colour change during
rotation not ideal, but may be best possible
in your section
way off-centre (melatope not visible) best
avoided
Nesse Fig. 7.38
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Interference Figures
Flash Figures both e and w in plane of
section (maximum d) useless for determining
optic sign very similar for both uniaxial and
biaxial
field of view light ? dark very quickly as
stage rotated
Nesse Fig. 7.39
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Uniaxial Minerals Pleochroic Scheme
Nesse, 2000 Fig. 7.30

• In PPL, find grain with minimum colour change as
  stage rotated
• (w in plane of section) observed colour w (
  a)
• In PPL, find grain with maximum colour change as
  stage rotated
• (both w and e in plane of section) w colour
  already determined
• other colour e ( c)
• 3. Can also be determined by finding fast and
  slow rays optic sign

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Optic Sign Summary
Case 1 Uniaxial minerals Z optic axis
(c-axis slow) ne gt nw ve X optic axis
(c-axis fast) ne lt nw -ve
Case 2 Biaxial minerals ve where Bxa //
Z -ve where Bxa // X
Z
Z
c OA Z
ve
-ve
ve
-ve
c OA X
X
Bxa
X
a
a
Bxa
determined from OA figure
determined from Bxa or OA figure