GEOL 2800

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GEOL 2800

• Lab. 1

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Use of Lab 317

• Do not bring food or drink into 317.
• You can leave drinks on the bench at the entrance
  for consumption outside of the lab.
• Two other courses plus a second section of GEOL
  2800 will be using these microscopes, from Monday
  to Thursday 2.30 to 5.30 pm. The lab will be
  available Friday afternoon, evenings and
  weekends.
• Room 317 is to be kept locked at all times except
  during lab periods.

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Care of the Microscope

• You will be using either a Nikon or Leitz
  petrographic microscope that will be kept
  permanently on the benches in room 317.
• Although all microscopes work on the same
  principles, there are differences in the
  operation of the various models, and the range of
  objectives, therefore, use only the microscope
  that you have been allocated.
• Do not move the microscopes for any reason.
• Avoid touching the surfaces of objectives,
  oculars, and lower polarizer. Hold the knurled
  ring not the objectives, when changing
  objectives.
• Do not attempt any repairs or maintenance,
  including bulb replacement. Report any problems
  to the TA or Dr Sherriff. Do not attempt to
  clean the objective lenses, if necessary, ask the
  TA. Improper cleaning can damage the lenses.
• Do not interchange components of microscopes.
• At the end of each session
• (1) Remove and store the thin sections
• (2) Turn off the microscope lamp
• (3) Replace the microscope cover
• (4) Clean up your workspace including pencil
  sharpenings
• (5) If you are alone, turn of the lab lights and
  lock the lab door

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Use of the Microscope

• 1. Place thin section 2-23 on the microscope
  stage with the cover slip upwards.
• 2. Always begin focussing with low power before
  going to higher.
• With the high power objectives, there is very
  little distance between the slide and the end of
  the objective moving the objective in the wrong
  direction could break the thin section and damage
  the objective.
• The first step in examining a thin section is to
  obtain an overview of the minerals and textures
  and their distribution under low power.
• 3. The magnification and field of view given by
  each objective are attached to the microscope.
  Each of your drawings must have a line scale
  e.g.1.7 mm
• 4. You should see crosshairs with an E-W scale in
  the right ocular which you can use with the field
  of view to estimate the size of individual
  minerals. This can be focused.
• 4. All parts of the microscope must be well
  centered to produce a maximum light path. For
  each objective, test the centering by placing a
  mineral grain at the cross-hair intersection.
  Rotate the stage through 360o if the objective
  is centred, the grain will remain at the
  cross-hair intersection. If the grain leaves the
  intersection, then the objective requires
  centring.
• Ask the TA to help with centering as you will
  need special tools.

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• 5. Observe the thin section in both plane and
  crossed polarized light. Rotate the stage.
• 6. With the polarizers crossed, insert the tint
  plate. This is a slide in the upper part of the
  microscope. In the Nikon microscopes there are
  three positions 1/4? (mica) plate, a hole, 1
  (mica) plate, a hole, 1? (gypsum) plate.
• Observe the colours and the change of colours as
  the stage is rotated.
• Remove upper polar and put the tint plate in the
  central (hole) position before continuing.

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Exercise 1 Becke Lines Section 2-23,
granodiorite

• The Becke line is due to the difference in the
  amount of light refraction in two adjacent
  minerals or one mineral and the epoxy resin.
• 1. Find a grain of quartz in your section in PPL
  and determine whether the refractive index is
  higher or lower than epoxy or adjacent feldspar.
  You will recognise quartz by clean glassy
  appearance in PPL and undulatory extinction under
  crossed polars.
• 2. In plane polarised light, partly close the
  substage diaphragm, reducing light by 50-75.
• 3. Slightly raise and lower the microscope stage,
  observing the movement of a bright line (Becke
  Line) at boundary of grain. You may need to use a
  higher power objective.
• 4. When decreasing the distance between the
  ocular and the stage, the line moves into the
  material of lower R.I.

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• (i) Draw and label a grain of quartz in XPL
  showing undulatory extinction.1.7 mm
• (ii) Draw the grain of quartz and an adjacent
  mineral grain in PPL showing the Becke Line.
• Make sure that you include a linear scale in this
  and all future drawings. 1.7 mm
• (iii) State whether quartz has a lower or higher
  refractive index than the adjacent mineral?
• (iv) List the refractive index (n) and formula of
  quartz from DHZ.

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Exercise 2 Opaque Minerals (section 2-23,
granodiorite)

• Opaque minerals can be properly identified only
  by using reflected light.
• Find an opaque mineral which does not transmit
  light in plane polarized light. Sketch the
  opaque grain in PPL with a scale.

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Exercise 3 Isotropic Minerals (section 67-12-192)

• Note There are only 7 thin sections so you will
  need to take turns for this exercise.
• Isotropic minerals, as well as glass, epoxy and
  air will always appear dark under cross polars
  regardless of orientation of the grain or the
  microscope stage but will be transparent in plane
  polarized light. Find the isotropic mineral
  garnet in this metamorphic rock.
• (i) Draw one grain of garnet in both PPL and XPL
  with a scale.
• (ii) Find the refractive index (n) and formula of
  almandine garnet in DHZ and list them.

PPL
XPL