I'm looking at some photos of thin sections Here in the hope of learning to identify the major minerals. Plagioclase seems quite easy but I'm not too sure on how pyroxene and olivine are told apart. Pyroxene appears to have a faint brown tint and faint wavy cracks in plane polarized light, but what are the characteristics that distinguish it in crossed polarized? The colours seem to vary wildly, and although an olivine crystal seems to stick more to a single colour and pryoxene mix several together, this doesn't always appear to be the case. :?
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:
Unfortunately, these sites tend to pick "exciting" examples with exsolution, etc, ratehr than nice simple ones...might help though
Geologists are gneiss!!
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
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.
I'm a proud Trowels & Hammers member!
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.
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.