Petrological microscopes are used to identify minerals and determine microstructure in rocks. This information is used to classify rock types and infer their formation histories. Microstructure refers to the shape and arrangement of mineral components in rocks. The microstructures of igneous, metamorphic, and sedimentary rocks are diagnostic of rock type. Thin sections of rock samples are analyzed under microscope using plane and crossed polarized light to identify minerals and microstructures.
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Identify Rocks and Minerals Using Microscopes
1. GEOS 254: INTRODUCTION
Petrological microscopes are used to:
(a) identify the minerals present
(b) determine the microstructure
These data are then used to classify the rock
and infer the history of the rock (e.g. has
the rock been deformed? Did the
deformation occur before, during or after
the growth of the minerals?)
2. Rock Microstructure
Microsturucture is the term used to cover the
shape of the components of the rock (minerals,
volcanic glass, fossils). In older books “texture”
is used to mean the same thing.
The microstructure of igneous, metamorphic and
sedimentary rocks are very different and are
generally more diagnostic than the list of
minerals present in defining the rock type.
3. What do you need to identify
rocks and minerals?
1. A “drivers license” for the microscope
2. Access to a list of mineral properties
3. Information on which rock types each
mineral occurs.
4. A knowledge of rock microstructures and
their significance.
4. Sample preparation
Thin sections are slices of rock polished on
one side, then stuck to a glass slide with
araldite and then ground down to 30 microns
or 3 thou of an inch.
So the light is not diffused by the frosted
upper surface, a very thin cover-glass is
glued onto the upper surface. A much more
expensive alternative is to polish the upper
surface and this is the method that must be
used for mineral analysis using the Electron
Microprobe.
5. Remember the Rules
Always start holding the slide up to the light. How
many minerals/ What grainsize/ Is the slide
homogeneous?
Then go to LOW POWER (the slide must be right-
way-up) in plane polarised light (natural colours)
Then go to crossed polars where the “interference
colours” show twinning, zoning etc and generally
distinguish quartz and feldspars having “low”
colours from most grains of olivine and pyroxene
that have “high” colours.
High power is generally only used for “conoscopic
optics” which are tests that can only be applied to
very special grains of any mineral.
6. Details are easy to see in thin
section. Cleavages at 60/120 in
hornblende