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The
Petrological Microscope
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)
A petrological microscope
The petrological microscopediffers 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 a thickness of 0.03mm become transparent.
lens
eyepiece
focus
light source
analyser
polariser
rotating stage
fine focus
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
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.
quar
tz
feld
spar
calcite
oliv
ine
amph
ibol
e
pyro
xene
biot
ite
mus
covi
te
Read along 0.03mm line to the highest order colour seen in the mineral
Read along diagonal to top for mineral name
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).
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)
Note how the olivine with its high relief stands out from the surrounding low relief plagioclase
RELIEF
PPL
plagioclase
olivine
Two sets of cleavage are seen in this amphibole crystal; note the 120o angle between the cleavages
CLEAVAGE
PPL
amphibole
1st set run parallel to
line
2nd set of cleavage
The olivine here shows uneven fractures which appear dark grey in the crystal
FRACTURE
PPL
olivine
The biotite shows its distinct brown shades under PPL against the clear colourless quartz and feldspar
COLOUR
PPL
biotite
amphibole
PLEOCHROISM
Two views under PPL showing colour change in biotite on
rotating the stage.
PPL
biotite
rotated 90o
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)
Interference colours
white/grey/black
in
quartz, microcline and plagioclase
much brighter colours
of
ferro-magnesian minerals including amphibole,
pyroxene, olivine
pearly grey shades
of
calcite
quartz amphibole calcite
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