Geology: Petrological Microscope for Higher

1
The Petrological Microscope
2

• The use of the
• Petrological Microscope
• The use of the microscope allows us to examine
  rocks in much more detail. For example, it lets
  us -
• examine fine-grained rocks
• examine textures of rocks
• distinguish between minerals that are
  otherwise
• difficult to identify in hand-specimen
  (e.g. the
• feldspars)

3

• A petrological microscope
• The petrological microscope
• differs from an ordinary
• microscope in two ways
• it uses polarised light
• and the stage rotates
• There are two sheets of polaroid
• the one below the stage of the
• microscope is the polariser, the
• other, above the stage, is the
• analyser. The analyser can be
• moved in and out.
• Most rocks cut and ground to

eyepiece
focus
fine focus
analyser
lens
rotating stage
polariser
light source
4

• Preparing thin sections
• Rock specimens are collected in the field, then
  cut into small
• thin slabs. These are glued on to glass slides
  and ground
• down to 0.03mm thickness. At this thickness all
  rocks
• become transparent. Only a few minerals, mainly
  ore
• minerals, remain opaque, i.e. stay black under
  PPL.
• If the sections are too thick, the polarisation
  colours are
• affected. Quartz is used to check thickness for
  this reason
• see the next slide

5
amphibole
pyroxene
muscovite
feldspar
olivine
biotite
quartz
Read along diagonal to top for mineral name
calcite
Read along 0.03mm line to the highest order
colour seen in the mineral

• The colours appear in a series of repeated
  rainbows across the chart and a
• mineral may show any colour up to a maximum,
  reading from the left.

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• Identifying MINERALS in thin section
• When a slide is examined under the microscope, it
  is important to identify any mineral properties
  under plane polarised light (PPL) first (analyser
  out) then proceed to crossed polars (XPL) where
  the two polaroid sheets are at right angles to
  each other (analyser in).

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• Mineral properties under PPL
• colour (natural colour)
• transparency (clear, cloudy or
  opaque)
• relief (high or low)
• crystal or fragment shape
• cleavage
• fracture
• pleochroism (colour
  change when stage is rotated)

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RELIEF
plagioclase
PPL
olivine

• Note how the olivine with its high relief stands
  out from the surrounding low relief plagioclase

9
1st set run parallel to line
CLEAVAGE
amphibole
PPL
2nd set of cleavage

• Two sets of cleavage are seen in this amphibole
  crystal note the 120o angle between the cleavages

10
FRACTURE
olivine
PPL

• The olivine here shows uneven fractures which
  appear dark grey in the crystal

11
amphibole
COLOUR
biotite
PPL

• The biotite shows its distinct brown shades under
  PPL against the clear colourless quartz and
  feldspar

12
biotite
PPL
rotated 90o

• PLEOCHROISM
• Two views under PPL showing colour change in
  biotite on rotating the stage.

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• Mineral properties under XPL
• interference colours
• (under XPL the colours seen are
  not the natural colours of the mineral but
• those caused by the interference
  of two refracted beams of light passing
• through an anisotropic mineral
  they are called interference colours)
• extinction angle
• (as the stage is rotated, each
  anisotropic mineral goes extinct every 90o in
• cases where there is cleavage in
  the mineral it is possible to measure the
• angle of extinction relative to
  the crosswires)
• twinning
• (may be seen in coloured
  minerals under PPL, but most obvious under XPL,
• especially with regard to the
  feldspars)

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• Interference colours

quartz
amphibole
calcite
white/grey/black in quartz, microcline and
plagioclase
much brighter colours of ferro-magnesian
minerals including amphibole, pyroxene, olivine
pearly grey shades of calcite