Refractometry and ReliefRefractometry and ReliefIN 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

What is refractometry?What is refractometry?

• Refractometry involves the determination of the refractive index of minerals.

• The simplest way of doing this is by the immersion method

Immersion MethodImmersion 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

ReliefRelief• 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.

Types of ReliefTypes 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.

Strong ReliefStrong Relief

Mineral grain clearly stands out from mounting materialRI mineral – RI oil > 0.12 RI units

Types of ReliefTypes 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.

Moderate ReliefModerate ReliefRI mineral – RI oil between 0.04 and 0.12 RI units

Types of ReliefTypes 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.

Low ReliefLow ReliefRI mineral – RI oil less than 0.04 RI units

Relief in Anisotropic Relief in Anisotropic MineralsMinerals• 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.

+ve or –ve relief+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 < noil e.g. fluorite 1.433

• To summarise this…..

Variation in ReliefVariation in Relief

Strong +ve

Moderate to strong +ve

Low +ve

Moderate to strong -ve

Low -ve

RI quartz = RI medium

“Invisible”

RI MountingMedium = 1.54

Becke LinesBecke 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.

Becke LinesBecke Lines

Lens EffectLens 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 Snell’s Law

Diverging Lens EffectDiverging Lens Effect

• If nmin < noil , the grain acts

as a diverging lens, and light is concentrated in the immersion oil.

Converging Lens EffectConverging Lens Effect

• If nmin > noil the grain acts as

a converging lens, concentrating light towards the centre of the grain.

Internal Reflection EffectInternal 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.

nnmineral mineral > > nnoiloil

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

nnmineral mineral < < nnoiloil

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

Measuring Becke Line Measuring Becke Line MovementMovementTo 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.

Direction of Becke Line Direction of Becke Line MovementMovement• 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 < noil, the cone diverges upwards and if nmin > 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.

Dispersion and Becke LinesDispersion 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.

Colored Becke LinesColored Becke Lines

Colored Becke LinesColored Becke Lines

Colored Becke LinesColored Becke Lines

n D(oil) >> n

D(min)

n D(oil) > n D(min)

n D(oil) = n D(min)

n D(oil) < n D(min)

n D(oil) << n

D(min)

White line into oil

Red line into mineralBluish-white line into oil

Yellowish orange lineinto mineral, pale blue lineinto oil

Yellowish-white lineinto mineral, blue-violetline into oil

White line into mineral

noil higher for

all wavelengthsnoil = nmin for

orange/red light

noil = noil for

yellow light (589nm)

noil = noil for

blue light

noil lower for

all wavelengths

CONDITION OBSERVATION INTERPRETATION

Index of refraction in thin-Index of refraction in thin-sectionsection

• 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