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
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 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
Relief • Reliefcan 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 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 Relief Mineral grain clearly stands out from mounting material RI mineral – RI oil > 0.12 RI units
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.
Moderate Relief RI mineral – RI oil between 0.04 and 0.12 RI units
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.
Low Relief RI mineral – RI oil less than 0.04 RI units
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.
+ve or –ve relief A mineral may exhibit positive or negative relief: Suppose an Oil has index of refraction n = 1.54 • +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 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
n1 n1 n2 n2 n2>n1 n2<n1 Mineral properties: Index of refraction (R.I. or n) Light is refracted when it passes from one substance to another; refraction is accompanied by a change in velocity the normal to the interface is show as a vertical black line Suppose vair = 1 Then n = 1/vmin • Recall: the ray in the higher-index medium is closer to the normal. • A “normal” is a perpendicular to the boundary between the substances.
Becke Lines • To determine whether a mineral has positive or negative relief, theBecke Linemethod is used. • Becke Linesare 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 linemay 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.
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 Snell’s Law
Diverging Lens Effect • If nmin< noil , the grain acts as a diverging lens, and light is concentrated in the immersion oil.
Converging Lens Effect • If nmin > noil the grain acts as a converging lens, concentrating light towards the centre of the grain.
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.
nmineral > 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
nmineral < 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
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 withthe 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 Movement • Movement of the Becke line as the stage is lowered OR THE TUBE IS RAISED. 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 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 Lines CONDITION OBSERVATION INTERPRETATION noil higher for all wavelengths n D(oil) >> 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) > 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) < n D(min) noil lower for all wavelengths n D(oil) << n D(min) White line into mineral
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 = the tube is raised