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Deducing properties from a thin section

Hi guys, I just kind of had a shower of geological terms thrown at me and I'm a bit confused as to what some of them mean. I've been looking at thin sections down a petrological microscope and have been trying to deduce the main properties of the minerals from looking at them.

By "main properties", generally I mean the fracture, cleavage, relief and twinning. I'm kind of confused as how to differentiate between a fracture and cleavage of a mineral (I guess fracture is more random?). Looking at amphibole thin sections on Google, it's easy to identify the cleavage but I think when you've got more unclear thin sections, it's difficult to tell.

I also need help with basic ways to identify relief and twinning?

Thank you!
Original post by Amefish
Hi guys, I just kind of had a shower of geological terms thrown at me and I'm a bit confused as to what some of them mean. I've been looking at thin sections down a petrological microscope and have been trying to deduce the main properties of the minerals from looking at them.

By "main properties", generally I mean the fracture, cleavage, relief and twinning. I'm kind of confused as how to differentiate between a fracture and cleavage of a mineral (I guess fracture is more random?). Looking at amphibole thin sections on Google, it's easy to identify the cleavage but I think when you've got more unclear thin sections, it's difficult to tell.

I also need help with basic ways to identify relief and twinning?

Thank you!


Hi Amefish :h:

Thin sections are always a bit tricky to interpret, but I'll try to explain each property you mentioned based on my understanding of them :smile:

Fracture - fractures in a mineral look like lots of lines crossing a mineral in different orientations. I'd say fracture is easiest to see when looking at olivine, as it's one of olivine's characteristic features in thin section :yep:

Cleavage - cleavage lines in thin section are much straighter and typically parallel, though some minerals have more than one cleavage plane, which might intersect the other at (for example) 90°, giving a sort of cross-hatch pattern. Try comparing a thin section of kyanite to olivine to see the difference in cleavage and fracture lines :smile:

Relief - relief is basically how light or dark a mineral looks when viewed in plane polarised light. A mineral with low relief (e.g. quartz, plagioclase) looks colourless in PPL, whereas a mineral with high relief (e.g. garnet, biotite) looks darker and has a distinct outline.

Twinning - twinning is where you've got light and dark linear sections in a mineral, which go extinct (turn black) at different rates when you rotate the thin section. Twinning can be simple (with half of the mineral light and the other dark, which swaps as you rotate the stage), or multiple (lots of alternating light and dark sections which change upon rotation). Plagioclase crystals can show both simple and multiple twinning, so it is easiest to identify in plagioclase thin sections.

I hope that makes some sense? :smile:

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