We use cookies to show content based on your preferences. If you continue to browse you accept their use and installation. More information. >

FMF - Friends of Minerals Forum, discussion and message board
The place to share your mineralogical experiences

FMF English Forum is moderated by John S. White and Peter Megaw
 

Spanish message board






Newest topics and users posts
16 Feb-22:37:59 Re: tucson show 2020 (Bob Kerr)
16 Feb-22:34:27 Re: tucson show 2020 (Bob Kerr)
16 Feb-22:26:26 Re: tucson show 2020 (Bob Kerr)
16 Feb-22:19:04 Re: tucson show 2020 (Bob Kerr)
16 Feb-22:10:39 Re: tucson show 2020 (Bob Kerr)
16 Feb-21:59:07 Re: tucson show 2020 (Bob Kerr)
16 Feb-21:30:02 Re: setting up an fmf homepage (Marvinlewinsky)
16 Feb-20:44:28 Re: setting up an fmf homepage (Steveb)
16 Feb-19:55:37 Setting up an fmf homepage (Marvinlewinsky)
16 Feb-19:41:11 Re: the state of modern geology (Marvinlewinsky)
16 Feb-17:46:51 Re: the state of modern geology (Bob Carnein)
16 Feb-17:46:20 Re: tucson show 2020 (Peter Van Hout)
16 Feb-17:43:25 Re: presentation of new members - reference thread (Don Lum)
16 Feb-17:29:07 Presentation of new member - conor quinlan (tucson, az) (Qqqqconr)
16 Feb-16:27:52 Re: the state of modern geology (Marvinlewinsky)
16 Feb-16:06:42 Re: the state of modern geology (Alfredo)
16 Feb-15:44:19 Re: the state of modern geology (Roger Warin)
16 Feb-15:26:38 The state of modern geology (Marvinlewinsky)
16 Feb-15:17:55 Tucson show 2020 - fmf (Jordi Fabre)
16 Feb-15:11:03 Re: collecting and imagining. (Sante Celiberti)
16 Feb-14:29:15 Re: collection of sante celiberti - a mineralogical trip through italy (Sante Celiberti)
16 Feb-12:22:11 Re: collection of fiebre verde - chivor (#707) (Fiebre Verde)
16 Feb-12:10:04 Re: don lum collection (Don Lum)
16 Feb-11:54:52 Re: collecting and imagining. (Tobi)
16 Feb-11:48:55 Re: don lum collection (Tobi)

For lists of newest topics and postings click here


RSS RSS

View unanswered posts

Why and how to register

Index Index
 FAQFAQ RegisterRegister  Log inLog in
 {Forgotten your password?}Forgotten your password?  

Like
62899


The time now is Feb 16, 2020 23:31

Search for a textSearch for a text   

A general guide for using the Forum with some rules and tips
What type of metamorphic rock is this?
  
  Index -> FOR BEGINNERS: What is it? Where is it from?
Like


View previous topic :: View next topic  
Author Message

RockHunter




Joined: 12 Dec 2019
Posts: 11
Location: British

View user's profile

Send private message

PostPosted: Dec 12, 2019 07:20    Post subject: What type of metamorphic rock is this?  

Hello all,

Just started taken interested in rocks especially metamorphic types. Saw this interesting rock at Cardiff museum.

I'm sure it is a low-grade foliated metamorphic rock because of the flat or elongate shape minerals that are aligned. Looks like this rock was developed under high pressure and temperature?

The green colour makes me think of Greenschist with maybe, Chlorite and Epidote in it.

What kind of rock it is?



image.jpeg
 Locality:
Gwent
 Description:
 Viewed:  1049 Time(s)

image.jpeg


Back to top
Reply to topic Reply with quote
Like
   

Josele




Joined: 10 Apr 2012
Posts: 340
Location: Tarifa, Spain


Access to the FMF Gallery title=

View user's profile

Send private message

PostPosted: Dec 12, 2019 08:14    Post subject: Re: What type of metamorphic rock is this?  

Is strange that in a museum there is not a label identifying the sample.

It looks like gneiss with a quite high proportion of mafic minerals, which would be a HIGH grade metamorphic rock.
Back to top
Reply to topic Reply with quote
Like
   

RockHunter




Joined: 12 Dec 2019
Posts: 11
Location: British

View user's profile

Send private message

PostPosted: Dec 12, 2019 08:52    Post subject: Re: What type of metamorphic rock is this?  

Yes. A lot of the rocks on display are labelled. This one is from a room where kids can pick up a rock and throw under a microscope. There is a box of rocks where you can guess what it is.

Why gneiss? It usually has "gneissic banding", which I don't see in this rock?

Make sense regarding being mafic minerals with the green colour possibly from olivine etc?

Can a high grade metamorphic rock contains porphyroblast, which shows in the picture attached? Looks like mylonite and shear zone?
Back to top
Reply to topic Reply with quote
Like
   

Vinoterapia




Joined: 03 Feb 2009
Posts: 168
Location: Houston, Tx


Access to the FMF Gallery title=

View user's profile

Send private message

PostPosted: Dec 12, 2019 09:30    Post subject: Re: What type of metamorphic rock is this?  

The two pictures (detailed) to the right reminds me of a fine-grained cataclasite, i.e. a rock formed in a fault zone and due to shearing.
Back to top
Reply to topic Reply with quote
Like
   

RockHunter




Joined: 12 Dec 2019
Posts: 11
Location: British

View user's profile

Send private message

PostPosted: Dec 12, 2019 09:42    Post subject: Re: What type of metamorphic rock is this?  

Yes. More like a narrow ductile shear zone with mylonite.

I've added a plane-polarised and crossed polar pics. The centre mineral stays the same colour during 360 degrees rotation. The surrounding colour under crossed polar changed 4 times during 360 degrees rotation.



image2.jpeg
 Description:
 Viewed:  963 Time(s)

image2.jpeg


Back to top
Reply to topic Reply with quote
Like
   

Bob Carnein




Joined: 22 Aug 2013
Posts: 180
Location: Florissant, CO


Access to the FMF Gallery title=

View user's profile

Send private message

PostPosted: Dec 12, 2019 10:23    Post subject: Re: What type of metamorphic rock is this?  

When all of the photos are taken together, this appears to me to be a garnet-mica schist. The porphyroblast in the second group of photos remains dark with rotation under crossed polars, suggesting it's isometric; relatively high relief and "pebbly" appearance suggests garnet. The first group of photos shows a green mineral on the foliation surface, which looks like chlorite; thin section shows micas, probably muscovite and either biotite or chlorite. I'd call it a schist because of uniform distribution of micas throughout and lack of banding, but you'll find that, among petrographers, the boundary between use of the terms "gneiss" and "schist" is a slippery one.
Back to top
Reply to topic Reply with quote
Like
   

RockHunter




Joined: 12 Dec 2019
Posts: 11
Location: British

View user's profile

Send private message

PostPosted: Dec 12, 2019 11:48    Post subject: Re: What type of metamorphic rock is this?  

Thank you your observation and everyone else's.

I agree being on the fence regarding gneiss and schist. What pushed me toward schist was the fact that the rock contains porphyroblasts under a narrow ductile shear zone with mylonite.

So really my early suggestion that the rock is greenschist was kinda close because technically... the word for that is garnet-mica schist.
Back to top
Reply to topic Reply with quote
Like
   

Vinoterapia




Joined: 03 Feb 2009
Posts: 168
Location: Houston, Tx


Access to the FMF Gallery title=

View user's profile

Send private message

PostPosted: Dec 12, 2019 11:56    Post subject: Re: What type of metamorphic rock is this?  

RockHunter, just one question. Are the thin section pictures from your second post actually from the rock in your first post, or are they taken from a different source?
Back to top
Reply to topic Reply with quote
Like
   

RockHunter




Joined: 12 Dec 2019
Posts: 11
Location: British

View user's profile

Send private message

PostPosted: Dec 12, 2019 12:44    Post subject: Re: What type of metamorphic rock is this?  

The thin section pics are part of the whole package to do with the rock in the first post.
Back to top
Reply to topic Reply with quote
Like
   

Kevin Schofield




Joined: 05 Jan 2018
Posts: 110
Location: Beacon NY

View user's profile

Send private message

PostPosted: Dec 12, 2019 12:50    Post subject: Re: What type of metamorphic rock is this?  

RockHunter wrote:
Hello all,

Just started taken interested in rocks especially metamorphic types. Saw this interesting rock at Cardiff museum.

I'm sure it is a low-grade foliated metamorphic rock because of the flat or elongate shape minerals that are aligned. Looks like this rock was developed under high pressure and temperature?

The green colour makes me think of Greenschist with maybe, Chlorite and Epidote in it.

What kind of rock it is?


Hi Rockhunter.

First thing to say is that I think that there are at least three different rocks here, possibly 4, as it is not clear that the photomicrographs are from any of the rocks illustrated in the first set of pictures.

From left to right:

Picture 1 is much more coarse-grained than pictures 2 and 3. It looks to be slightly foliated, and has a lot of green minerals but no well-developed cleavage. Looks as if it might be a serpentinite (low grade metamorphosed ultramafic rock).
Pictures 2 and 3 are a different beast. A fairly equant granular texture, other than the dark "streaks" which may be original mudstone clasts that have been compacted ans stretched, maybe with added metamorphic clay growth. Although there is an aligned fabric, there is no apparent cleavage developed, and a "soapy" look to the broken face. I suspect another low-grade, chlorite-rich schistose rock.
Pictuer 4 does not appear to be any of the above. It appears to retain an original bedding fabric (I see at least three and possibly 4 distinct grain-size -denominated layers), albeit there are indications of flattening or shear. Possibly a metamorphosed sandstone??

On the photomicrographs, I'm with Bob Carnein in as much as it looks like a garnet/mica schist. Presence of garnets suggests to me a higher grade of metamorphism than the greener rocks pictured as rocks.

It is likely that rocks 1 and 2 (pictures 1 to 3) are indeed of the Greenschist Facies, but note that this is a classification of rock-type, (rather than a rock name) indicative of a temperature/pressure regime and mineral suite.
The photomicrographs, as they appear to contain garnet, are indicative of the higher temperature/pressure Amphibolite Facies.

_________________
Veni, Vidi, Emi
Back to top
Reply to topic Reply with quote
Like
   

RockHunter




Joined: 12 Dec 2019
Posts: 11
Location: British

View user's profile

Send private message

PostPosted: Dec 12, 2019 13:00    Post subject: Re: What type of metamorphic rock is this?  

Very interesting, Kevin. Thank you for your insight.

I'm just grasping the basics here and I do know for a fact that all the pictures here are to do with the same rock. The dead give away is the porphyroblast for me as they do occur in narrow ductile shear zone with mylonite compare to wide ductile shear zone with striped gneiss.
Back to top
Reply to topic Reply with quote
Like
   

Bob Carnein




Joined: 22 Aug 2013
Posts: 180
Location: Florissant, CO


Access to the FMF Gallery title=

View user's profile

Send private message

PostPosted: Dec 12, 2019 21:12    Post subject: Re: What type of metamorphic rock is this?  

Here's another source of confusion for beginners. The term "greenschist" refers to a metamorphic facies, i.e. a group of metamorphic rocks that formed under similar temperature and pressure conditions (in this case, low temperature and pressure dynamothermal or regional metamorphism). It really isn't a rock name. A slate or phyllite may result from greenschist-facies metamorphism, as can a marble or a metaquartzite, or, for that matter, a green schist. The end product depends on what you start out with. To name a metamorphic rock correctly, one should name the major minerals present (most common mineral closest to the root name) and then use root names, such as slate, phyllite, schist, gneiss, marble, etc. Thus, the thin section shows a garnet-mica schist with quartz and feldspar (I think).
Back to top
Reply to topic Reply with quote
Like
   

RockHunter




Joined: 12 Dec 2019
Posts: 11
Location: British

View user's profile

Send private message

PostPosted: Dec 13, 2019 13:50    Post subject: Re: What type of metamorphic rock is this?  

Thank you, Bob Carnein.

This is my take... the porphyroblast is garnet (Grt) surrounded by muscovite, biotite and chlorite (mica minerals).

There is another picture, which I forgot to add, has a similar porphyroblast called staurolite instead of garnet. Same thing surrounded staurolite as garnet, but more chlorite mineral.

The type of metamorphic rock is garnet-mica schist and the metamorphic facies is greenschist.

You were saying that this might be low pressure/temperature type? I would have thought that this rock is medium to high pressure/temperature?

Why I think that is the case... there is a chart on where the minerals exist regarding to pressure and temperature, Grt, St and Bt are all medium to high. 240 to 800 MPa and 400 to 600 Celsius.

I'm torn between regional and cataclastic metamorphism.

Added PT chart, which I found from another site...



PT.jpg
 Description:
 Viewed:  736 Time(s)

PT.jpg


Back to top
Reply to topic Reply with quote
Like
   

Kevin Schofield




Joined: 05 Jan 2018
Posts: 110
Location: Beacon NY

View user's profile

Send private message

PostPosted: Dec 13, 2019 15:23    Post subject: Re: What type of metamorphic rock is this?  

RockHunter wrote:
Thank you, Bob Carnein.

This is my take... the porphyroblast is garnet (Grt) surrounded by muscovite, biotite and chlorite (mica minerals).

There is another picture, which I forgot to add, has a similar porphyroblast called staurolite instead of garnet. Same thing surrounded staurolite as garnet, but more chlorite mineral.

The type of metamorphic rock is garnet-mica schist and the metamorphic facies is greenschist.

You were saying that this might be low pressure/temperature type? I would have thought that this rock is medium to high pressure/temperature?


If the rock contains both Garnet and Staurolite co-existing, then you are at the mid- to high end of the Amphibolite meatmorphic facies, in excess of 600 degrees Celcius and 400 MPa. This is almost certainly regional metamorphism not localized cataclasis.
Why I think that is the case... there is a chart on where the minerals exist regarding to pressure and temperature, Grt, St and Bt are all medium to high. 240 to 800 MPa and 400 to 600 Celsius.

I'm torn between regional and cataclastic metamorphism.

Added PT chart, which I found from another site...

_________________
Veni, Vidi, Emi
Back to top
Reply to topic Reply with quote
Like
   

RockHunter




Joined: 12 Dec 2019
Posts: 11
Location: British

View user's profile

Send private message

PostPosted: Dec 13, 2019 15:43    Post subject: Re: What type of metamorphic rock is this?  

I did checked garnet and staurolite's PT ranges...

This is what I found, Kevin... garnet max out at 550 degree C and staurolite max out at 600 degree C.

Therefore, if both co-exist the maximum temperature that the rock has experienced would be 550 degree C.

Like you said, the metamorphic facies would be amphibolite instead of greenschist even though the metamorphic rock name is still garnet-mica schist, right?
Back to top
Reply to topic Reply with quote
Like
   

Bob Carnein




Joined: 22 Aug 2013
Posts: 180
Location: Florissant, CO


Access to the FMF Gallery title=

View user's profile

Send private message

PostPosted: Dec 13, 2019 16:07    Post subject: Re: What type of metamorphic rock is this?  

You're correct that garnet (almandine) indicates the lower end of the amphibolite facies. However, I'm not sure about staurolite in the samples shown (I don't see any). Staurolite won't remain dark when rotated under crossed polars, as almandine does.

Realize, also, that, if the peak temperature decreases slowly, one can get retrograde metamorphism, in which lower-grade index minerals may replace higher-grade ones. Therefore, the highest-T index mineral tells you what the highest temperature was when the rock was metamorphosed. Does that make sense?

Yes, it's still garnet-mica schist (at least from what I can see in the photos).
Back to top
Reply to topic Reply with quote
Like
   

RockHunter




Joined: 12 Dec 2019
Posts: 11
Location: British

View user's profile

Send private message

PostPosted: Dec 13, 2019 16:18    Post subject: Re: What type of metamorphic rock is this?  

Sorry to forgot to add the last photo, which I think will cement the understanding of what it is...

Am I right to name that staurolite? Yes. Under cross-polarised light, it doesn't stay dark.

If garnet and staurolite co-exist therefore; the maximum pressure would be 800 MPa?



image3.jpeg
 Description:
 Viewed:  701 Time(s)

image3.jpeg


Back to top
Reply to topic Reply with quote
Like
   
Display posts from previous:   
   Index -> FOR BEGINNERS: What is it? Where is it from?   All times are GMT - 5 Hours
Page 1 of 1
    

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


All pictures, text, design © Forum FMF 2006-2020


Powered by FMF