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Unusual metamorphic(?) tourmaline
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Pete Richards
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PostPosted: Apr 23, 2013 19:03    Post subject: Unusual metamorphic(?) tourmaline  

Last weekend at the Rochester (New York) Symposium I bought a sample of fine-grained greenish rock with many small (<8mm) "andradite garnets" embedded in it. The crystals are disseminated throughout the rock, not just on one surface. This was labelled Andradite from Cornwall, Pennsylvania.

But when I got home to my friendly microscope, it turned out that the crystals are stubby black tourmaline crystals. Many have white pressure shadows of either quartz or calcite (not yet determined). Clearly(?) the tourmaline grew in the enclosing sediment during metamorphism.

The false ID of the mineral also calls the locality into doubt. Communications with the seller don't promise a better ID. I attach images of the rock and the best crystal (colors on lower faces are false - reflection of my fingers - this really is black or very dark brown tourmaline.

Does anybody recognize this material? A locality would be greatly appreciated!

Thanks!



IMG_4042.JPG
 Description:
Tourmaline
locality unknown
largest crystal is about 8 mm across
 Viewed:  23447 Time(s)

IMG_4042.JPG



IMG_4045.JPG
 Description:
Tourmaline
Detail of largest crystal
 Viewed:  23449 Time(s)

IMG_4045.JPG



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PostPosted: Apr 23, 2013 20:57    Post subject: Re: Unusual metamorphic(?) tourmaline  

I thought the matrix was serpentinized at first from the large picture but the micro picture does not look like it. Sure is the wrong shape for Andradite obviously. Do you think it came out of a metamorphed sedimentary basin? Cornwall is in Cambrian rock I think or close to triassic rock (fault basins?).
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PostPosted: Apr 23, 2013 20:59    Post subject: Re: Unusual metamorphic(?) tourmaline  

Looks like a lightly metapmorphed sand stone in the matrix.

Something else I just thought of; if it is black tourmaline it has iron content and iron is found in the triassic units of that area. Don't know if there is a link but a thought anyways.
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PostPosted: Apr 23, 2013 21:35    Post subject: Re: Unusual metamorphic(?) tourmaline  

A little testing with dilute muriatic acid shows that the pressure shadows are calcite, and that the host rock also has some significant calcite content. There are other species present in the pressure shadows, but they are too small to identify....

I will grab a bit of matrix and see to what extent it will dissolve in dilute acid.

While it is possible, I no longer believe this specimen is from Cornwall, any more than it is andradite. Locality is probably in the northeast US, but even that is uncertain!

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PostPosted: Apr 24, 2013 14:52    Post subject: Re: Unusual metamorphic(?) tourmaline  

Hi, Pete. What a nice specimen you bought! I have some comments about it.

Pete Richards wrote:
I will grab a bit of matrix and see to what extent it will dissolve in dilute acid.


Good idea to discriminate siliceous or carbonatic composition. The picture looks like a porphyroblastic slate or phyllite more than a metacarbonate, so this test is useful.

Pete Richards wrote:
Many have white pressure shadows...


How did you determine this? By naked-eye? Can you put some micros of these textures? Maybe it is only secondary calcite on the rock.

Pete Richards wrote:
Unusual metamorphic(?) tourmaline


What do you call "unusual" exactly? Tourmaline is a common accessory in some metapelites: look at this tourmaline-bearing schist.

I will follow this interesting thread.

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PostPosted: Apr 24, 2013 19:21    Post subject: Re: Unusual metamorphic(?) tourmaline  

prcantos wrote:
Hi, Pete. What a nice specimen you bought! I have some comments about it.


Hi Pablo,

Thanks for your entry into this thread, because you seem to be more of a petrologist than many of us.

This specimen seems unusual to me for several reasons: the tourmalines are quite sharply formed and show no signs of invasion by subsequent mineralization processes (but possible engulfing of other [carbonate?] phases), and they are quite stubby.

In addition, the rock, to my eye, is not highly metamorphosed. I would say a phyllite or less. There is carbonate in the rock, but at least in terms of readily soluble (calcite) phases it is not the major component.

Pressure shadows show the typical pattern of presence on some sides of the crystal, and absence on other sides, with some fibrosity. Perhaps I can get some micro pictures. These shadow zones also sometimes hold other mineral species than just calcite. The texture here is definitely different from the bulk texture of the rock.

Probably what surprises me the most is the very sharp quality of the tourmaline crystals (what I would expect from a pegmatite pocket) while they are embedded in a "solid" rock.

Thanks for your interest in this strange specimen! I invite your advice as I try to unravel its secrets!

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PostPosted: Apr 25, 2013 03:17    Post subject: Re: Unusual metamorphic(?) tourmaline  

Pete Richards wrote:

...you seem to be more of a petrologist than many of us...


Well, just an amateur rock scholar, ready to learn a lot whith you!

I agree with you this point: it looks like a low or very low-grade metapelite (slate or phyllite) with minor carbonates.

Meanwhile you try to take some micros of the pressure shadows, I will recognize which was my first idea at looking at the pictures (before reading your text): I thought they were goethite pyritohedra; photo "IMG_4045.JPG" reminds those mixed hexahedron-octahedron (or truncated hexaedron) a lot. Look at these goethite pseudo pyrite specimens in a marble piece from the Alpujárride Complex of Sierra Nevada in my picture. These are so common.

Let me make this question to you: what reasons have you got to think in tourmaline? You have said that they are unusual tourmalines. Have you considered another possibilities such pyritohedra?



547_copia_140.jpg
 Description:
Goethite pyritohedra in marble matrix
Sierra de Tocón de Quéntar, Granada, Andalucía, Spain
400X
 Viewed:  23172 Time(s)

547_copia_140.jpg



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PostPosted: Apr 25, 2013 08:55    Post subject: Re: Unusual metamorphic(?) tourmaline  

prcantos wrote:
...what reasons have you got to think (it is) tourmaline?


There's no doubt about it's being tourmaline. The apparent black color is actually a slightly transparent dark red-brown as seen on internal fractures. The crystals are hemimorphic, with one end more steeply terminated than the other and having more faces. The sides are short prisms, or actually combinations of two or three prisms. Classic tourmaline shape. An approximate SHAPE drawing is below.



tourmaline.jpeg
 Description:
Tourmaline
unknown locality
 Viewed:  23172 Time(s)

tourmaline.jpeg



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PostPosted: Apr 25, 2013 09:21    Post subject: Re: Unusual metamorphic(?) tourmaline  

Pete...I agree completely with your ID of tourmaline here (especially given your usual thoroughness). However, checking through some of the literature what seems most unusual...and probably distinctive...about this rock is the stubbiness of the tourmalines. Virtually everything I have found references to are elongate crystals, to no surprise elongated within the foliation of the schist and often in parallel growth. Google "tourmaline schist" and the first thing that pops up is a link to hundreds of images of tourmaline schists...most elongate. However, just below the picture of a female movie star (?) there is a shot of a rock that looks a lot like yours...unfortunately without locality attribution...but at least you know someone has collected there before and sooner or later someone will show up with the locality

I have sent you a link to one of the more thorough papers on tourmaline wschists from Oxford Journals https://petrology.oxfordjournals.org/content/52/12/2293.full This has some excellent data on the zoning of the tourmalines and their origin.

Much of the literature suggests that tourmalines in schist reflect boron metasomatism from a nearby granitic intrusion...which might help narrow down your search. However, recent work in Mexico by James Lyons indicates that tourmaline may reflect the presence of boron rich evaporites from back arc basins (think Death Valley) otherwise filled with messy fine-grained siliclastic sediments that when metamorphosed create tourmaline-bearing schists.

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PostPosted: Apr 25, 2013 12:05    Post subject: Re: Unusual metamorphic(?) tourmaline  

Ok, of course you had really good reasons! Thank you for your explanations.
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PostPosted: Apr 25, 2013 16:39    Post subject: Re: Unusual metamorphic(?) tourmaline  

Well, some time with a good microscope and photo apparatus offers some new images. I am aiming for a SEM/EDS session to help resolve phases, but these are not just pressure shadows, at least as I understand them. There are mineralization rims around the tourmalines that contain calcite, chlorite, ilmenite?, and epidote?. The level of metamorphism seems very low - a colleague suggested hornfels as an appropriate description. One wonders where all the boron came from to make all the tourmaline....

Images will probably arrive in more than one posting....



30223343.JPG
 Description:
Tourmaline, better image of best crystal
unknown locality
Crystal is 8 mm across
 Viewed:  23052 Time(s)

30223343.JPG



30220913.JPG
 Description:
Crystals about 4 mm
Views of prisms of two crystals, note reaction zone around lower one
 Viewed:  23063 Time(s)

30220913.JPG



30221230.JPG
 Description:
detail of lower crystal in previous photo
 Viewed:  23046 Time(s)

30221230.JPG



30221230b.jpeg
 Description:
Approximate limit of alteration of previous specimen
 Viewed:  23063 Time(s)

30221230b.jpeg



30221451.JPG
 Description:
Fibrous mineral in reaction rims
 Viewed:  23052 Time(s)

30221451.JPG



30221708.JPG
 Description:
Lamellar? mineral in reaction rims: epidote?
 Viewed:  23062 Time(s)

30221708.JPG



30222031.JPG
 Description:
Simple white rim, more like what a pressure shadow might look like.
 Viewed:  23066 Time(s)

30222031.JPG



30222751.JPG
 Description:
Chlorite. Interesting that the apparent nucleation surface is not the tourmaline face, but a plane parallel to it across a ?void?.
 Viewed:  23065 Time(s)

30222751.JPG



Chlorite1.jpg
 Description:
relatively euhedral chlorite crystal group.
 Viewed:  23078 Time(s)

Chlorite1.jpg



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PostPosted: Apr 25, 2013 16:48    Post subject: Re: Unusual metamorphic(?) tourmaline  

Peter Megaw wrote:
...What seems most unusual...and probably distinctive...about this rock is the stubbiness of the tourmalines. Virtually everything I have found references to are elongate crystals, to no surprise elongated within the foliation of the schist and often in parallel growth.


I agree, Peter, and think of several examples such as the dravites from the country rock at Newry, Maine.

However, this material shows no sign of foliation and is definitely not a schist. Whether it is a contact metamorphic such as a hornfels or not, the degree of metamorphism is low and, correspondingly, the degree of reorganization is inadequate to develop lineation features in the host rock, which is about as boring as a dirty siltstone! I may change my mind a bit when I get some thin sections made....

Thanks!

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PostPosted: Apr 25, 2013 17:04    Post subject: Re: Unusual metamorphic(?) tourmaline  

Pete...where you drive the "golden spike"' between hornfels (thermal metamorphism with limited local-scale mass transfer) and skarn (metasomatism) can be an unsolvable academic exercise in many cases. However, in the practical world of skarn exploration we often map the "sparkle line"...where the crystal size of the new species gets large enough for the faces to reflect. You are WAY past the sparkle line here...I would argue you are probably into metasomatism...and things like epidote would reinforce that. The fact that the matrix does not appear reconstitued might also move one awy from simple metamorphism and lean towards metasomatism. My bet is that you will find the matrix is more complciated than you think when you look at it in thin section.

Boron is a common element in metasomatic fluids, which also fits. Boron is also common in evaporites and is soluble enough to get entrained into fluids circulating around intrusions. Places like Charcas and Dalnegorsk are good examples of this..with species like danburite, datolite and nifontovite. The intrusion at Charcas is tourmaline bearing too.



Nifty single web.jpg
 Description:
Nifontovite
Rey y Reina Mine, Charcas, San Luis Potosi
4 cm long
Peter Megaw specimen, Jeff Scovil photograph
 Viewed:  23039 Time(s)

Nifty single web.jpg



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PostPosted: Apr 26, 2013 03:20    Post subject: Re: Unusual metamorphic(?) tourmaline  

Pete Richards wrote:
...However, this material shows no sign of foliation and is definitely not a schist.


Ok, this is a good observation; in fact I thought that the rock was lightly foliated because of some features in the first picture: the flat surface, the irregular ending at the bottom and, the double-level edge at the bottom right.

Pete Richards wrote:
...but these are not just pressure shadows...


I got a bit surprised when you said there were pressure shadows, so that I asked for some pictures. Now that we have the photograph (thanks you for yor wonderful detailed photos!), I agree with you: not pressure shadows, but reaction rims. Pressure shadows don't fit non-foliated structures.

Anyway, the thin section will give you a lot of interesting information, as Peter Megaw suggested:

Peter Megaw wrote:
... My bet is that you will find the matrix is more complicated than you think when you look at it in thin section...


I think this is not a "boring" rock at all, Pete! Please do comment the results here.

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PostPosted: Apr 26, 2013 16:50    Post subject: Re: Unusual metamorphic(?) tourmaline  

Is a rare and interesting piece!
I think the rock could be a high grade hornfels with tourmaline porphyroblast.
Crystals are there and can not grow in a low grade metamorphic rock.
Hornfels, even high grade, do not develop schistosity because were formed under high temperature but low pressure conditions.
Maybe dravite is most probable specie for origin, color and prism perimeter. Also, detected carbonate indicates probable Mg presence.
Porphiroblast are usually garnet, staurolite, biotite, etc, but, in right conditions, there is no reason that could not be turmaline also.
Just an idea...

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PostPosted: Apr 26, 2013 19:50    Post subject: Re: Unusual metamorphic(?) tourmaline  

Initial results from a 2 1/2 hour SEM/EDS session today:

Previously mentioned ilmenite is actually hematite with small Ti content - 1 to 2 atomic %. Chlorite (clinochlore) and epidote are confirmed, at least by elemental components. (EDS analyses of unpolished and uncoated grain mounts cannot be expected to give correct atomic composition!) One of the epidote crystal sprays I looked at had a core that was Ce-rich.

Apatite is also present. Some additional phases are still to be figured out.

These are all in the reaction zone around the tourmaline. I just looked at the interfaces where matrix broke away from a tourmaline crystal, and at the matrix around the edges of these separation surfaces. I have to wonder, why was all this reaction going on? Was it all related to the formation of the tourmaline crystals? If so, how? If not, why is there no evidence of this in the rest of the rock (at least at the same crystal scale)?

There was no tourmaline to examine; this is for a later date.

A quick look at the matrix showed small grains of hematite, and apparent albite and K-spar (probably adularia).

Fun(ny) stuff!

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PostPosted: Apr 26, 2013 23:54    Post subject: Re: Unusual metamorphic(?) tourmaline  

I have spent some time googling various combinations of "reaction rims", "around tourmaline", "low-grade", and metapelite and I have not been able to answer my question which is: Why do the tourmalines look so clean and euhedral, and are large relative to the surrounding grains, when they have reaction rims around them? Maybe this is not a mystery to the petrologists out there, and if so, please comment.

One paper I found:
https://www.researchgate.net/publication/226138690_Tourmaline_in_a_low_grade_clastic_metasedimentary_rock_an_example_of_the_petrogenetic_potential_of_tourmaline
(link normalized by FMF)
talked about how detrital tourmaline in a meta-clastic rock can develop large overgrowths but the overgrowths are asymmetric and look anhedral to me.

Interesting thread.
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PostPosted: Apr 27, 2013 20:55    Post subject: Re: Unusual metamorphic(?) tourmaline  

Matt_Zukowski wrote:
... Why do the tourmalines look so clean and euhedral, and are large relative to the surrounding grains, when they have reaction rims around them? Maybe this is not a mystery to the petrologists out there, and if so, please comment.


I agree that this is a grand mystery, and I don't have more than partial hypotheses. One factor may be that the tourmaline grew during the cooling-down (retrograde) period of metamorphism and therefore was subjected to relatively gentle geochemical challenges. Another is that many of the minerals that are adjacent to the tourmaline are relatively soft and flexible, and so it may have been easy for the tourmaline to shove them aside, but they "greased" the process, keeping the tourmaline from having to shove aside harder minerals that might have interfered with the luster of the faces.

It is certainly a puzzle how one could get such large and abundant tourmaline crystals with relatively little rim volume (I'm not even comfortable calling it a reaction rim any more). It is possible that the bulk composition of the rock approximately matched tourmaline composition, and crystal formation started at random(?) spots in the rock, creating clean tourmaline crystals surrounded by minerals formed from the stuff that did not fit in the tourmaline, but this is highly speculative and would need to be supported by lots of petrological evidence.

Some of the rims have textures that suggest that some of their minerals nucleated on the OUTSIDE of the rim and grew through the rim zone toward the tourmaline. How do you do that?!!!

So - a few ideas, but not much more.

Mark Twain said something like "Geology is a great science! One gets such a wealth of conjecture from so few facts!" We try not to do it that way, but sometimes that's what it feels like!

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PostPosted: Apr 29, 2013 07:35    Post subject: Re: Unusual metamorphic(?) tourmaline  

Hi to everybody!
According to the picture n° 30222031-JPG, the specimen resembles obsidiane "Apache's Tear" from the States (sorry I don't remember anymore the proper location, but I'm sure you all know perfectly what I mean!) on perlite matrix rock.
Have you ever analyzed it?
Greetings from Italy by Riccardo.

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PostPosted: Apr 29, 2013 08:59    Post subject: Re: Unusual metamorphic(?) tourmaline  

Riccardo Modanesi wrote:
Hi to everybody!
According to the picture n° 30222031-JPG, the specimen resembles obsidiane "Apache's Tear".


I see that there is some resemblance, but this is a broken crystal, which is why it does not show nice faces like many of the other images. I'm sure it is tourmaline, not obsidian.

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