Optical Mineralogy

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Optical Mineralogy

• Minerals in Plane Polarised Light

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Interaction of polarised light with
minerals LAB TS-1 Double refraction Relief C
olour and pleochroism Other properties that can
be observed in PPL LAB TS-2 Isotropy and
anisotropy Birefringence and interference Interf
erence colours Use of tint plate
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Polarisation
plane polarised light (single vibration direction)
unpolarised light (all possible vibration
directions)
Nesse, 2000 Fig. 7.4
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Polarisation by selective absorption
plane polarised light (PPL)
polarising film
unpolarised light
Nesse, 2004 Fig. 1.13
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Polarisation in the petrographic microscope
upper polarising filter (analyser)
what happens here??? LAB TS-1
mineral sample (thin section)
plane polarised light (PPL)
lower polarising filter (polariser)
unpolarised light
light source
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Polarisation in the petrographic microscope
what happens here??? LAB TS-2
upper polarising filter (analyser)
what happens here??? LAB TS-1
mineral sample (thin section)
plane polarised light (PPL)
lower polarising filter (polariser)
unpolarised light
light source
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Polarisation in the petrographic microscope
most minerals display double refraction
what happens here??? LAB TS-1
mineral sample (thin section)
plane polarised light (PPL)
lower polarising filter (polariser)
unpolarised light
light source
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Double Refraction
incident light (PPL) split into two
rays vibrating at right angles to each other
property displayed by anisotropic transparent
materials
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Double Refraction
incident light (PPL) passing through an
anisotropic material is split into two rays
vibrating at right angles Why?
wave front
interaction of light with internal crystal
structure
Nesse 1986 Fig. 5.2
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Double Refraction
incident light (PPL) passing through an
anisotropic material is split into two rays
vibrating at right angles Why?
lower V higher n
closely spaced atoms (X)
higher V lower n
widely spaced atoms (Z)
wave front
interaction of light with internal crystal
structure
V (and therefore n) varies with crystallographic
direction
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Double Refraction
incident light (PPL) passing through an
anisotropic material is split into two rays with
different V, vibrating at right angles (single
direction resolved into 2 different directions)
Fast ray the ray with with lower index of
refraction and higher velocity
Slow Ray the ray with higher index of refraction
and lower velocity
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Double Refraction
incident light (PPL) passing through an
anisotropic material is split into two rays with
different V, vibrating at right angles (single
direction resolved into 2 different directions)
nslow nfast birefringence (d)
Fast ray the ray with with lower index of
refraction and higher velocity
Slow Ray the ray with higher index of refraction
and lower velocity
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Double Refraction
ray splitting produces two images
effect of polarising filter
calcite a transparent anisotropic mineral with a
large contrast between V and n in different
directions (high birefringence)
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Relief
relief qualitative term describing relative
refractive index between adjacent
materials evident as degree to which one mineral
stands out or appears rough relative to its
neighbour described as high medium,
low relative n can be determined by Becke
line test
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Relief
low n
low n
high n
high n
light is deflected into material with higher n
high n
high n
low n
low n
mineral in matrix with different n acts as a lens
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Relief
light is deflected into material with higher
n bright line near edge of mineral grain Becke
line
high n
low n
as stage is lowered, Becke line moves into
material with higher n
low n
high n
relative n can be determined by Becke line test
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Relief
low relief
some minerals with large n contrast in
different directions (high birefringence,
d) change relief as stage is rotated (e.g.
calcite)
90o rotation
high relief
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Colour and pleochrosim
Body colour colour of mineral independent of
orientation colour does not change as stage is
rotated
Pleochroism mineral absorbs different l in
different orientations colour changes as stage is
rotated (e.g., biotite, tourmaline)
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Other properties that can be observed in PPL (LAB
TS-1)
Shape Cleavage Opacity / Transparency
opaque materials do not transmit light (even in
thin section)
transparent materials transmit light (given thin
enough section)
isotropic anisotropic
LAB TS-2