GeologyRocks

Hi Dan,

I was hopeless at thin sections. But I can pass what little knowledge I have onto you.

Olivine is very obvious when looking down an actual microscope, rather than pictures. It is often heavily fractured, has an obvious birefringence (very bright, reds and greens) and is often greeny-looking in plane light.

Pyroxene was always tricky. It is often pleochroic (meaning it changed colours in plane light when you rotated the stage). I remember something to do with 120 degree cleavage (or was that hornblendes? :?). Becasue there are many different pyroxenes, there is no easy common denominator.

There are some decent websites out there for thin section work:
http://www.geosci.unc.edu/Petunia/IgMetAtlas/minerals/minerals.html
http://www.humboldt.edu/~jdl1/minerals.list.html#anchor736357

Unfortunately, these sites tend to pick "exciting" examples with exsolution, etc, ratehr than nice simple ones...might help though

Jon

Jon/Dan

Olivine has a very poor cleavage, very good fracture (often with alteration to a reddish coloured mineral called iddingsite along the fracture) and with fairly to very high birefringence. Olivine shapes are generally an or subhedral but fairly well rounded. Pyroxene (both clino- and ortho-pyroxene) can be euhedral, has a characteristic 90 degree cleavage. Orthopyroxene generally has green-pink pleochroism, is never twinned and has low (1st-2nd order) birefringence. Clinopyroxene is not pleochroic, has higher birefringence (at least 2nd order), and can show twinning (common in augite and I think maybe pigeonite) and exsolution lamellae.

The classic textbook by Deer Howie and Zussmann will give more info...

Cheers - hope this helps

Simon

OK, thanks for the replies. Luckily I'm not planning to try to identify anything myself, but should now have a better chance of following the photos in books I'm reading. :oops:

Pyroxenes are pleochroic, have high relief, and a cleavage at 90 degrees. It is also cloloured in plain polarised light.
Olivine is colourless in plain polarised light and has no cleavage,.[/code]

I'm sure I remember learning something last term about Olivine altering along the fracture lines in the larger crystals.

Hey Sarah!

I'm sure you are correct! Olivine in thin section is usually highly fractured. Do you know what it is altering to??!

I think it's serpentine that is usually the product of alteration, but I reckon it depends on the composition of the olivine in question.

Jon

Olivine can alter to iddingsite and talc, as well as serpentine. Iddingsite is the material you commonly see along crqacks and fractures in olivine crystals - alteration to talc and serpentine generally affects the entire crystal, not just along fracture planes (I think!)

I think that we were told Serpentine, but as said by those inifitely more knowledgable than myself above, that probably depends on composition of the olivine.