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Identity HelpThin section mineral identification

25th Oct 2014 13:36 UTCJames Sheridan

Hey guys, new here so please be kind!

I am having difficulty in identifying this igneous rock. Olivine is prominent throughout, and I think I can see some plagioclase feldspar. However apparently there is some pyroxenes in this sample. Can anyone see any? If so, would you lean towards a lherzolite or a olivine Gabbro?

Much thanks,

James (amateur!)

25th Oct 2014 15:09 UTCBill Cordua 🌟 Manager

Too much plagioclase for a lherzolite especially photo 3. Pyroxene will show cleavage, and olivine won't. It will also have a bit lower relief. The blue on photo 3 might be a pyroxene - but can't tell for sure from the image. If there is little or no pyroxene, rock could be a troctolite or picrite using the IUGS system. Need a point count!

25th Oct 2014 17:33 UTCJames Sheridan

-- moved topic --

26th Oct 2014 01:05 UTCJason Bennett

James,

Do you have this book available to you? Rocks and Minerals in Thin Section: A Colour Atlas This is particularly useful to help get your eye in at the features and textures described. I used it extensively during my undergraduate degree.

Also, try googling pictures of opx and cpx.

As Bill mentioned, pyroxenes have two cleavage planes at 90deg. This does not mean that you will see both cleavages though, remember that in thin section you have a slice through the minerals in many different orientations. If sliced parallel-ish to the c-axis, you'll get parallel lines for both cleavage traces, but if it is sliced perpendicular-ish to the c-axis, you may see some feint cross-hatching at 90deg (the two cleavage planes). If the crystal goes extinct parallel to these cleavages, then it's opx, and if it doesn't and is inclined to the cleavage then it's cpx.

Hope this helps

Jason

26th Oct 2014 01:56 UTCPaul Brandes 🌟 Manager

Welcome to Mindat, James!

Unfortunately, I'm still on the road at a geologic conference so I'm not much help right now, but I tend to agree with Bill here. I don't see anything that would indicate a lherzolite, especially with all the plag. Who did the thin sections for you, James?

26th Oct 2014 11:20 UTCJames Sheridan

Thanks for all your help guys! I do have the book, and it is a great resource! I'm a 2nd year geology student at Southampton now and it feels like the only mineral I'll ever be able to identify is quartz! I'll need to stop taking photos during labs and actually inspect the minerals thoroughly, as I can't inspect extinctions angles! I've attached a few other photos if anyone can spot any pyroxenes!!!

26th Oct 2014 11:25 UTCJames Sheridan

26th Oct 2014 11:26 UTCJames Sheridan

26th Oct 2014 16:19 UTCCarl (Bob) Carnein 🌟

Hi, James,

I might be helpful to know where this rock came from. Also, always realize that a thin section may or may not be representative of a rock--it depends a lot on how carefully the rock was collected. Geologists always need to be careful, on an outcrop, to collect what's typical, rather than focusing on the odd or unusual (depending, of course, on why the rock sample is collected in the first place). Then, even the direction of the slab in a typical sample can have a big effect on what you see in a thin section.

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