Thin section characteristics Flashcards
What are the terms to remember when looking at thin section?
Relief
Cleavage
Colour
Pleochroism
Birefringence
Twinning
Extinction
Zoning
Shape
Texture
Alteration
What should you try and do when looking at thin section?
see how many different types of mineral are present (those with similar properties)
What is relief?
PPL- how well you can see the edge of the mineral
High or low (high=easy to see edge)
What will cleavage look like in thin section?
Straight lines (like scratches) going through rock
Can have 2 directions like a corner of a cube
What type of light do we mean when talking about colour in thin section?
PPL
What are some general terms used to describe colour in thin section?
Opaque
Pale colour
Strong colour (potentially pleochroism)
Colourless
What is pleochroism?
change in body colour in PPL when the microscope stage is rotated
One mineral can be different colours when looking through microscope due to orientation
What is birefringence?
In XPL-interference colours
How can birefringence be described?
by the order it can be found in based on chart and if its low or high in order
what colour it is in order
How would you describe the birefringence of plagioclase?
Low first order (greys)
How many orders are there on the birefringence chart?
6 orders
What is lamellar twinning?
multiple or repeated twinning
What is simple twinning?
pretty much crystal split in 2 with each going into extinction at different times
What is extinction of a mineral?
when at specific inclinations in orientation 4 times every 360* no light or little can pass through the crystal structure
What are some types of extinction?
Parallel
Inclined
Indeterminate
Symmetrical
What is parallel extinction?
long direction of the crystal or a prominent cleavage plane is oriented N-S or E-W when the mineral goes extinct
What is inclined extinction?
elongate direction of the mineral or the direction of cleavage is NOT E-W or N-S when the mineral goes extinct (occurs in monoclinic or triclinic minerals)
What is symmetrical extinction?
when the vibration direction of the light bisects the angles between two sets of cleavages
What is zoning in minerals?
looks like exterior shape of mineral inside the mineral itself
How will alteration be observed in thin section?
cloudy or dusty appearance in plane-
polarised light, or indistinct aggregates of fine-grained material in cross-polarised
light
