Refractometry and Relief

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Refractometry and Relief

• IN THIS LECTURE
• What is refractometry
• Immersion Method
• Relief
• Types of Relief
• Relief in Anisotropic Minerals
• ve or ve Relief
• The Becke Line
• The Lens Effect
• Internal Reflection Effect
• Measuring Becke Line Movement
• Dispersion and Becke Lines
• Colored Becke Lines
• Refractometry in Thin Sections

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What is refractometry?

• Refractometry involves the determination of the
  refractive index of minerals.
• The simplest way of doing this is by the
  immersion method

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Immersion Method

• The immersion method relies on having immersion
  oils of known refractive index and comparing the
  unknown mineral to the oil.
• Immersion oil is available with refractive
  indices that cover the range found for most
  minerals.
• Typically the immersion oil will have indices of
  refraction in the range 1.4 to 1.8 in increments
  of 0.002, 0.004 or 0.005.
• Indices of refraction may be measured to
  accuracies of about 0.003 using white light for
  illumination.
• There are two components to comparing the index
  of refraction of the mineral and the immersion
  oil
• Relief
• Becke Line

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Relief

• Relief can be defined as the degree to which a
  mineral grain or grains appear to stand out from
  the mounting material, whether it is an immersion
  oil, Canada balsam or other medium, or another
  mineral.

If the indices of refraction of the oil and
mineral are the same, light passes through the
oil-mineral boundary un-refracted and the mineral
grains do not appear to stand out.
If noil ? nmineral then the light travelling
though the oil-mineral boundary is refracted and
the mineral grain appears to stand out.
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Types of Relief

• When examining minerals you can have
• Strong relief
• mineral stands out strongly from the mounting
  medium, whether the medium is oil, in grain
  mounts, or other minerals in thin section,
• for strong relief the indices of the mineral and
  surrounding medium differ by greater than 0.12 RI
  units.

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Strong Relief
Mineral grain clearly stands out from mounting
material RI mineral RI oil gt 0.12 RI units
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Types of Relief

• When examining minerals you can have
• Strong relief
• mineral stands out strongly from the mounting
  medium, whether the medium is oil, in grain
  mounts, or other minerals in thin section,
• for strong relief the indices of the mineral and
  surrounding medium differ by greater than 0.12 RI
  units.
• Moderate relief
• mineral does not strongly stand out, but is still
  visible,
• indices differ by 0.04 to 0.12 RI units.

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Moderate Relief
RI mineral RI oil between 0.04 and 0.12 RI units
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Types of Relief

• When examining minerals you can have
• Strong relief
• mineral stands out strongly from the mounting
  medium, whether the medium is oil, in grain
  mounts, or other minerals in thin section,
• for strong relief the indices of the mineral and
  surrounding medium differ by greater than 0.12 RI
  units.
• Moderate relief
• mineral does not strongly stand out, but is still
  visible,
• indices differ by 0.04 to 0.12 RI units.
• Low relief
• mineral does not stand out from the mounting
  medium,
• indices differ by or are within 0.04 RI units of
  each other.

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Low Relief
RI mineral RI oil less than 0.04 RI units
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Relief in Anisotropic Minerals

• The relief of anisotropic minerals in both grain
  mounts and thin section may change as the
  microscope stage is rotated in plane light
• This is because the fast and the slow rays have
  different indices of refraction therefore display
  different relief depending on which ray is
  dominant
• This is particularly so of minerals with moderate
  to high birefrigence where the difference between
  the fast and slow rays is more pronounced.

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ve or ve relief

• A mineral may exhibit positive or negative
  relief
• ve relief - index of refraction for the material
  is greater than the index of the oil.e.g. garnet
  1.76
• -ve relief nmin lt noil e.g. fluorite 1.433
• To summarise this..

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Variation in Relief
Strong ve
Moderate to strong -ve
Moderate to strong ve
RI Mounting Medium 1.54
Low -ve
Invisible
Low ve
RI quartz RI medium
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Becke Lines

• To determine whether a mineral has positive or
  negative relief, the Becke Line method is used.
• Becke Lines are a band or rim of light visible
  along the grain boundary in plane light when the
  grain mount is slightly out of focus.
• The Becke line may lie inside or outside the
  mineral grain depending on how the microscope is
  focused
• Becke lines are interpreted to be produced as a
  result of
• the lens effect and/or
• internal reflection effect.

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Becke Lines
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Lens Effect

• Most mineral grains are thinner at their edges
  than in the middle, i.e. they have a lens shape
  and as such they act as a lens.
• Minerals can act either as diverging lenses or
  converging lenses.
• Light is refracted away from the normal on
  passing into a material with a higher index of
  refraction and away from the mineral when passing
  into a material with a lower index of refraction
  as predicted by Snells Law

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Diverging Lens Effect

• If nmin lt noil , the grain acts as a diverging
  lens, and light is concentrated in the immersion
  oil.

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Converging Lens Effect

• If nmin gt noil the grain acts as a converging
  lens, concentrating light towards the centre of
  the grain.

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Internal Reflection Effect

• This hypothesis to explain why Becke Lines form,
  requires that grain edges be vertical, which in a
  normal thin section most grain edges are believed
  to be more or less vertical.
• With the converging light hitting the vertical
  grain boundary, the light is either refracted or
  internally reflected, depending on angles of
  incidence and indices of refraction.
• The combined result of refraction and internal
  reflection concentrates light into a thin band in
  the material of higher refractive index.

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nmineral gt noil
Rays 1 and 4 are refracted into the mineral. Rays
2 and 3 strike the vertical mineral-oil boundary
at greater than the critical angle and are
internally reflected back into the mineral. The
Becke Line is formed by the concentration of
light inside the mineral grain
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nmineral lt noil
Rays 1 and 4 strike the vertical mineral-oil
boundary at greater than the critical angle and
are reflected back into the oil. Rays 2 and 3 are
refracted into the oil. The Becke Line is formed
by the concentration of light outside the mineral
grain
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Measuring Becke Line Movement

• To observe the Becke line
• use medium or high power,
• close aperture diagram,
• for high power flip auxiliary condenser into
  place.
• The direction of movement of the Becke Line is
  determined by lowering the stage with the Becke
  Line always moving into the material with the
  higher refractive index.
• The Becke Line can be considered to form from a
  cone of light that extends upwards from the edge
  of the mineral grain.

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Direction of Becke Line Movement

• Movement of the Becke line as the stage is
  lowered. The becke line may be considered to
  consist of a cone of light that extends upwards
  from the mineral grain.
• If nmin lt noil, the cone diverges upwards and if
  nmin gt noil the cone converges upwards. If the
  stage is lowered, the plane of focus goes from F1
  to F2 and the Becke Line appears to move towards
  the material of the higher refractive index.

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Dispersion and Becke Lines

• The dispersion of immersion oil is greater than
  the dispersion of most minerals, so it is
  possible to produce a match of index of
  refraction for only one wavelength of light.
• If the dispersion curves for mineral and
  immersion oil intersect in the visible spectrum,
  the oil will have higher indices of refraction
  for wavelengths shorter than the match, and the
  mineral will have higher indices of refraction
  for longer wavelengths.
• This results in the formation of two Becke lines,
  one for the shorter wavelengths and one for the
  longer.
• The color of the two Becke lines depends on the
  wavelength at which the dispersion curves cross.

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Colored Becke Lines
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Colored Becke Lines
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Colored Becke Lines
CONDITION
OBSERVATION
INTERPRETATION
noil higher for all wavelengths
n D(oil) gtgt n D(min)
White line into oil
noil nmin for orange/red light
Red line into mineral Bluish-white line into oil
n D(oil) gt n D(min)
Yellowish orange line into mineral, pale blue
line into oil
noil noil for yellow light (589nm)
n D(oil) n D(min)
Yellowish-white line into mineral,
blue-violet line into oil
noil noil for blue light
n D(oil) lt n D(min)
noil lower for all wavelengths
n D(oil) ltlt n D(min)
White line into mineral
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Index of refraction in thin-section

• It is not possible to get an accurate
  determination of the refractive index of a
  mineral in thin section
• Comparisons can be made with
• epoxy or balsam, material (glue) which holds the
  sample to the slide n 1.540
• Quartz
• nw 1.544
• ne 1.553
• Becke lines form at mineral-epoxy,
  mineral-mineral boundaries and are interpreted
  just as with grain mounts, they always move into
  higher RI material when the stage is lowered