1. Petrological Microscope
The
Petrological Microscope

2. 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)

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
a thickness of 0.03mm become
transparent.
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. mineral may show any colour up to a maximum, reading from the left.
amphibole
pyroxene
muscovite
feldspar
quartz
biotite
olivine
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.

6. 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).

7. 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)

8. 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.

13. 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)

14. Interference colours white/grey/black in
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