Mineralogy Lab 4 –Introduction to Optical Mineralogy Name _______________ & _______________
Materials: (3) x 10mL graduated cylinders, 2 petri dishes, deionized water, food colouring, disposable
pipettes, light box, white blank paper, protractors, helium fluorescent tube lamp, cardboard diffraction
glasses (Fresnel lenses), mixed flat optics sets of prisms, lenses, Lucite blocks, polarizer lamp base, ring
stand, large thin transparent crystals of: halite, fluorite, calcite, muscovite, gypsum (selenite), ulexite
and glass streak/hardness plate. RI oils, glass slides, covers. Enough for one outfit per group of students.
I. Physical Optics and Polarized light: Light is a vector electromagnetic field. This means it has a
direction of travel and a time varying nature for the strength of the electrical and magnetic components
(always perpendicular to each other). When light encounters physical matter: air or other gases, water
or other fluids (brine, oils) or solids (glass, mineral crystals) one of 3 things occurs . Either: 1- the light is
reflected from the surface (angle of incidence = angle of reflection), 2- the light is refracted towards the
“normal” as it enters the optically denser medium or 3- the light is absorbed by internal vibrations of
electrons in the matter. (see; http://www.physicsclassroom.com/class/light/u12l2c.cfm )The sum of all 3
must equal the incoming energy. Most matter has a stable high energy crystal field that reflects or
bends low energy visible light without splitting it up or affecting its colour. (Read:
http://vallance.chem.ox.ac.uk/pdfs/ReflectionRefraction.pdf). Since matter and the electrons which
hold it together have mass and a uniform rest energy state, the potential field of the light must slow
down and do work causing the electrons and their atoms to vibrate. This slows down the speed of light
to <c, i.e., v < 3 x 108 m/s.
1. Absorbtion is caused by atoms capable of absorbing a particular wavelength of light (colour). Take (3)
10mL graduated cylinders. Fill 2 to the 10 mL mark with deionized water. Put one drop of food colouring
in one of the cylinders and mix it well. Now take 1 mL of that coloured solution and put it into the 3rd
empty cylinder then fill it to the 10 mL mark with deionized water. Compare the colour of the 3 solutions
for these cylinders of constant width. Compare their transmittances to the collimated light beam.
Realizing that the concentrations of food colouring are: 0 drops per 10 mL, 1/10th drop per 10 mL and 1
drop per 10 mL, describe how the concentration of chromophores affects transmittance and
absorbance. Save the dark and pure water cylinder’s contents for the next step.
(10)
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The Beer-Lambert law states that there is a logarithmic dependence between the transmission (or
transmissivity), T, of light through a substance and the product of the absorption coefficient of
the substance, α, and the distance the light travels through the material (i.e., the path length), ℓ.
The absorption coefficient can, in turn, be written as a product of either a molar absorptivity
(extinction coefficient) of the absorber, ε, and the molar concentration c of absorbing species in
the material, or an absorption cross section, σ, and the (number) density N' of absorbers. In some
chemistry applications for liquids these relations are usually written as:
where
and are the intensity (power per unit area) of the incident light and the transmitted light,
respectively; σ is cross section of light absorption by a single particle and N is the density (number per
unit volume) of absorbing particles. This can be quantified with a photo cell or light meter.
The transmission (or transmissivity) is expressed in terms of an absorbance which, for liquids, is
defined as
This implies that the absorbance becomes linear with the concentration (or number density of
absorbers) according to
2. Leaving about 1 mL of the most concentrated coloured solution in your 1st graduated cylinder, take 2
petri dishes and put equal depths of your concentrated food colouring solution in one and pure
deionized water in another. Get a clean graduated cylinder with just deionized water to at least as deep
as the remainder in your coloured cylinder. Shine the collimated light beam through both thin cylinders
and both petri dishes. Comment on how path length and concentration affect transmittance and
absorbance.
(10)
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3. Place the light box flat on a sheet of white paper and use a narrow collimator slit. Place leucite prisms
or lenses in the path of the light beam and observe, and describe how the intensity of the outgoing
beam varies as more matter is placed in the way. You can also examine a cleaved piece of halite and
glass hardness plates. Comment on the different effect on absorbtion and light intensity by the different
solid materials and on the different path lengths within the material. (hint the general expressions of the
Beer-Lambert Law for Transmittance and Absorbtion (above).
(10)
4. Minerals are similar to coloured solutions wherein certain transition metal ions: (Fe, Cu, Ni, Co )2+ or
(Fe, Cr, V, Ti, Ce, La, Y etc.)3 or 4+ are present and capable of absorbing particular wavelenghts of light
depending on their unfilled d- or f- orbitals. Unlike coloured solutions and glass, minerals often have
anisotropic didtribution of these chromophores in their structures which can affect their colour by
position of the crystal relative to the light vibration directions (pleochroism). See the microscope
demonstration of biotite or hornblende from a thin section of granite in plane polarized light for crystals
of different orientations. Biotite structure (see similar muscovite model) has Fe and Mg in the
octahedral sheet. Draw and describe the variation of colour versus position (orientation of cleavages or
crystal elongation direction etc.). How would Fe % variations affect biotite’s colour & pelochroism? (10)
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5. Use the flat light source and diffraction slit to examine refraction. Lay a white sheet of paper under
the exit light beam. Place a lens or prism in the way. Rotate this to find the critical angle whereby the
critically refracted beam no longer emerges from the prism. Draw/trace the light beam direction, prism
position, refracted and emergent light beam labelling each. Repeat this for the case of critical refraction
and measure the angle on you drawing using a protractor. Note intensity variations.
(10)
6. Intercept the light beam with the thin edge of a triangular prism. Rotate this back and forth until you
find the position whereby differential refraction of the colours of light spreads the spectrum the most.
Draw the beam and prism position labelling the correct position of colours. This separation of light into
colours is termed dispersion. Which is more refracted: Red or Violet and why is this so? (read:
http://www.physicsclassroom.com/class/refrn/u14l4a.cfm or an equivalent physical optics site with
illustrations to help.)
(10)
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7. Interpose other prisms or lenses into the spectral beam, noting anything unusual that the second lens
or prism does to the light. Think about how this might apply to light and mineral grains under the
microscope. Draw and illustrate or write down your observations.
(10)
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8. Use the delicate Heluim gas-tube fluorescent lamp and the Fresnel goggles. This variety of lens has
the overall curvature of a much larger lens, but alternate slices have been removed so that each slice is
adjacent and the lens is flatter with the same net curvature. This is like the lenses in old light houses or
ground based aircraft beacons. The fretting or stepping, of the lens rings creates a diffraction edge to
affect the light. Rotate your head and observe the changes. Draw the result including the original white
light line source and its virtual images keeping their relative positions in the correct sense. Lens shaped
grains, and minerals with frosted and circularly polished surfaces, behave much the same way. Draw the
results labelling and explaining the effect and its relevance to lens shaped mineral grains illuminated by
light sources with linear filaments, point sources or diffuse light. Which do you think we prefer for
microscopy and why?
(10)
9. Use the Cylindrical grey light stand with the swing out polarizing filters. Find the extinction position
for the 2 filters and mark them with pieces of opaque tape. Try placing various mineral specimens
(quartz, calcite, muscovite, fluorite, gypsum) on the lower filter and rotate them and note the effects if
any when you view them through the upper filter. Try using superposed sheets of minerals and repeat
the rotation only using the underlying polarizing filter. Draw and describe your findings and note if any
of the minerals have special directions or special effects with respect to polarized light. For minerals
with these special directions, sketch what they are and show how these optical directions relate to the
shapes of the crystals you used.
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II. Refractive Indices: There are a series of mostly isotropic minerals and naturally occurring geological
materials (glass, opal, fluorite, garnet). Most of these materials are optically are isotropic (the same in all
directions, cubic in their crystallography) and have only 1 single refractive index. Delicately use a few
grains of each unknown mineral and try and match its refractive index using the organic liquids. If the oil
is far different in Refractive Index (RI) from the grain the grain appears to be very bold, like it does in air.
This apparent height is called Relief. To do the RI test, we use oblique or low illumination (lower the
light source beneath the stage) and carefully focus at high power magnification (blue objective lens X40
power). When a thin, oil immersed grain is in focus, there is a fine sharp white line at its margin called
the Becke Line. From this position, delicately use the small fine focus knob to back off the focus a turn or
two, observing the apparent motion of the Becke line. The line bends or refracts into the medium with
higher refractive index (oil or mineral). When the oil matches the mineral’s refractive index (RI), the
white becke line will split into colours. You might get so close as to see Red move in and Blue move out.
By using successive oils, do the best you can to bracket each sample’s refractive indices and observe
how its relief changes, then use the tables at the back of Klein and Dutrow (p637-638) or the Perkins
Minerals in Thin Section book (p155-160) to home in on possible identifications. You can also observe
any cleavage traces or angles, fracture types or inclusions to aid in your identity. Are their any samples
not isotropic? If so which one & describe how you know. Are any specimens not minerals?
Mineral Name
Low RI
Estimated RI
High RI
e.g. Leucite/Mineral #
1.500
1.508-1.509, brief, +r
1.516
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