GEOS 254 INTRODUCTION

GEOS 254: INTRODUCTION
Petrological microscopes are used to:

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


(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?)


Rock Microstructure
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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.

What do you need to identify
rocks and minerals?
1.
2.
3.
4.

A “drivers license” for the microscope
Access to a list of mineral properties
Information on which rock types each
mineral occurs.
A knowledge of rock microstructures and
their significance.


Sample preparation
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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.


Remember the Rules
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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 rightway-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.


Details are easy to see in thin
section. Cleavages at 60/120 in
hornblende

Minor alteration of cordierite
around the edge (S-type
granite)


Graphic or runic intergrowth
of quartz and orthoclase


Chromite-rich layer in
Bushveldt Layered Gabbro


Fusilinid fossils in limestone


Quench olivine spikes in slag.
They also occur in komatiites


Olivine in mantle rock (high
temperature metamorphic rock)
is polygonal


Another high temperature
metamorphic rock (near the
melting temp). It is glacier ice!

Quartz-biotite schist with shape
of quartz controlled by the
aligned mica.


Quartz crystals in a vein
showing crystal faces and
growth zones.


Spectacular compositional
zoning in igneous plagioclase


Zoning and multiple twinning
in plagioclase phenocrysts in
tuff.


Sanidine phenocrysts in
trachyte


Simple twin in orthoclase which
also shows perthite exsolution
lamellae


Microcline with perthite

exsolution lamellae


Myrmekite (intergrowth of
plagioclase and quartz) common
in slightly deformed rocks.


Rim of plagioclase around
orthoclase phenocryst (rapakivi)


Ophitic microstructure (dolerite)
plagioclase is included in large
pyroxene crystals.


Close up of
plagioclase
partly
included in
pyroxene in
ophitic
dolerite.
Top is PPL,
bottom is XP


Eclogite (ultra high grade metabasalt) with pyroxene and garnet
(black)