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I would like to know this rock and the minerals within
  
  Index -> FOR BEGINNERS: What is it? Where is it from?
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RockStudent




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PostPosted: Mar 09, 2017 05:13    Post subject: I would like to know this rock and the minerals within  

Hi all

I would like to understand what kind of rock is this and the first image is polar and the second, is crossed polar.

What I understand so far, correct me if i'm wrong... the high relief grey colour under polar is Garnet, which stays black under crossed polar (isotropic). The light brown under polar is Biotite and it is anisotropic under crossed polar. The light to dull grey under polar is Quartz, which is anisotropic under crossed polar ranging from white to black. There are small bits of dull grey under polar, which is Muscovite and shows pink to slight purple under crossed polar.

Want to know what kind of rock is this?



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Matt_Zukowski
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PostPosted: Mar 09, 2017 06:02    Post subject: Re: I would like to know this rock and the minerals within  

Are you a student? Is this an unknown you were given for an assignment or a test?
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markbeckett




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PostPosted: Mar 09, 2017 06:42    Post subject: Re: I would like to know this rock and the minerals within  

Hi there! If your mineral identifications are correct, ie .much Garnet and Mica (especially biotite), your thin section may well be of a 'Garnet-Mica Schist', a high-grade metamorphic rock. If the 'hand specimen' that this slide was made from shows any planar (or other) alignment of minerals in its texture, then this will confirm. (Do you still have the parent sample? - a petrological name for a rock is often best derived by observing both its large scale *and microscopic features).

This Forum tends to deal with actual Mineral specimens only, and so Petrology is seen as a different field of interest, (although there are many here who are well versed in both).

Hope this is of help to your studies :-) - Mark

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'if you see a strange hill, go and prospect there'...(KFG Hosking)
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RockStudent




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PostPosted: Mar 09, 2017 07:08    Post subject: Re: I would like to know this rock and the minerals within  

markbeckett wrote:
Hi there! If your mineral identifications are correct, ie .much Garnet and Mica (especially biotite), your thin section may well be of a 'Garnet-Mica Schist', a high-grade metamorphic rock. If the 'hand specimen' that this slide was made from shows any planar (or other) alignment of minerals in its texture, then this will confirm. (Do you still have the parent sample? - a petrological name for a rock is often best derived by observing both its large scale *and microscopic features).

This Forum tends to deal with actual Mineral specimens only, and so Petrology is seen as a different field of interest, (although there are many here who are well versed in both).

Hope this is of help to your studies :-) - Mark


Thank you, markbeckett for your interesting insights and thought the same regarding the rock as being a high-grade metamorphic because of the abundance garnet and biotite (mica).

Am I also correct regarding the muscovite as being pink/light purple under crossed polar (anisotropic)?

I'm trying to polish up my mineral identification skill so I can help my son, who has a very keen interest. :-)
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Bob Harman




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PostPosted: Mar 09, 2017 09:39    Post subject: Re: I would like to know this rock and the minerals within  

My response here will be general and not really directed towards anyone in particular. In my opinion, the first response was spot on while the subsequent "helpful" second response was premature.

This forum should definitely be helpful in identification of mineral specimen's for one's own personal collection or something like that, but the forum responses should NOT take the place of student research and study, giving answers to college geology class student assignments.

The original posting was a bit nebulous in that regard so I understood MATT's concern. The thread should then have been left for the original poster to have ample time to further clarify his/her reasons for the posting.

That is my opinion. BOB
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markbeckett




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PostPosted: Mar 09, 2017 10:07    Post subject: Re: I would like to know this rock and the minerals within  

Bob Harman wrote:
My response here will be general and not really directed towards anyone in particular. In my opinion, the first response was spot on while the subsequent "helpful" second response was premature.

This forum should definitely be helpful in identification of mineral specimen's for one's own personal collection or something like that, but the forum responses should NOT take the place of student research and study, giving answers to college geology class student assignments.

The original posting was a bit nebulous in that regard so I understood MATT's concern. The thread should then have been left for the original poster to have ample time to further clarify his/her reasons for the posting.

That is my opinion. BOB


By the way Bob - I wrote my response at the same time as Matt's response, therefore it occurred just after his - me thinking I was first to reply! Surely a helping hand to a beginner is justified - this is part of 'his' research, is it not?
That is my opinion
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RockStudent




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PostPosted: Mar 09, 2017 10:45    Post subject: Re: I would like to know this rock and the minerals within  

Quote:
By the way Bob - I wrote my response at the same time as Matt's response, therefore it occurred just after his - me thinking I was first to reply! Surely a helping hand to a beginner is justified - this is part of 'his' research, is it not?
That is my opinion


Again, thank you markbeckett for your insights and wasn't premature at all. I would do the same without hesitation. Cheers
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markbeckett




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PostPosted: Mar 09, 2017 11:54    Post subject: Re: I would like to know this rock and the minerals within  

Thanks RockStudent! Can I recommend an excellent book - 'Metamorphic Textures and Microstructures' by AJ Barker - ISBN 0-216-92685-8. its a great help when studying *any metamorphic rocks, especially in thin-section, a really worthwhile buy.

Best wishes for your studies
MARK

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Grad - B.Eng. Geol. (1993)

'if you see a strange hill, go and prospect there'...(KFG Hosking)
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Pete Modreski
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PostPosted: Mar 09, 2017 19:01    Post subject: Re: I would like to know this rock and the minerals within  

Dear Rock Student,

I hope you understand the caution that went into the query, "were you a geology student trying to find answers to an assigned question", because it did sort of sound like it could have been that. I also have seen questions like that--from students--come to various organizations that provide "answers to science questions" from the public. It sounds like, no offense was taken (good!).

You had asked about muscovite. In crossed polars, with the flakes perpendicular to the plane of the thin sections, its high birefringence in that orientation is going to produce high-order interference colors, pastel pink and green.

One more comment about your thin section. You may have noticed, to the upper right of the center, there is one fairly large crystal, that appears yellowish both in plane-polarized light and in the crossed polars photo. That is some other mineral that has not been mentioned here yet. I'm not certain about it, but it could be a mineral such as staurolite. This will just give you "something to think about further"!
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