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GEOS 254: INTRODUCTION. Petrological microscopes are used to: (a) identify the minerals present (b) determine the microstructure

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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?)

rock microstructure
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.
what do you need to identify rocks and minerals
What do you need to identify rocks and minerals?
  • 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
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.
remember the rules
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.
close up of plagioclase partly included in pyroxene in ophitic dolerite top is ppl bottom is xp
Close up of plagioclase partly included in pyroxene in ophitic dolerite.Top is PPL,bottom is XP