Download Geology: Petrological Microscope for Higher

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
eyepiece
focus
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
a thickness of 0.03mm become
transparent.
analyser
lens
rotating
stage
polariser
light
source
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
olivine
muscovite
biotite
amphibole
pyroxene
feldspar
quartz
Read along diagonal to
top for mineral name
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.
calcite
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)
RELIEF
plagioclase
PPL
olivine
Note how the olivine with its high relief stands out from
the surrounding low relief plagioclase
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
FRACTURE
olivine
PPL
The olivine here shows uneven fractures which appear
dark grey in the crystal
amphibole
COLOUR
biotite
PPL
The biotite shows its distinct brown shades under PPL
against the clear colourless quartz and feldspar
biotite
PPL
rotated 90o
PLEOCHROISM
Two views under PPL showing colour change in biotite on
rotating the stage.
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
quartz
amphibole
calcite
white/grey/black
much brighter colours
pearly grey shades
in
of
of
quartz, microcline and
plagioclase
ferro-magnesian minerals
including amphibole,
pyroxene, olivine
calcite