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Pete Richards
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Location: Northeast Ohio
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 Posted: Apr 23, 2013 19:03 Post subject: Unusual metamorphic(?) tourmaline |
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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!
| Description: |
Tourmaline
locality unknown
largest crystal is about 8 mm across |
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40722 Time(s) |
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| Description: |
Tourmaline
Detail of largest crystal |
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| Viewed: |
40696 Time(s) |
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Mark Ost
Joined: 18 Mar 2013
Posts: 516
Location: Virginia Beach
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| Posted: Apr 23, 2013 20:57 Post subject: Re: Unusual metamorphic(?) tourmaline |
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| 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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Mark Ost
Joined: 18 Mar 2013
Posts: 516
Location: Virginia Beach
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| Posted: Apr 23, 2013 20:59 Post subject: Re: Unusual metamorphic(?) tourmaline |
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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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Pete Richards
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Joined: 29 Dec 2008
Posts: 845
Location: Northeast Ohio
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| Posted: Apr 23, 2013 21:35 Post subject: Re: Unusual metamorphic(?) tourmaline |
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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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prcantos
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Joined: 17 Apr 2012
Posts: 243
Location: Granada (Spain)
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| Posted: Apr 24, 2013 14:52 Post subject: Re: Unusual metamorphic(?) tourmaline |
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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.
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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...
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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
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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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Pablo Rodríguez Cantos
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Pete Richards
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Joined: 29 Dec 2008
Posts: 845
Location: Northeast Ohio
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| Posted: Apr 24, 2013 19:21 Post subject: Re: Unusual metamorphic(?) tourmaline |
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| 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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prcantos
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Joined: 17 Apr 2012
Posts: 243
Location: Granada (Spain)
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| Posted: Apr 25, 2013 03:17 Post subject: Re: Unusual metamorphic(?) tourmaline |
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| Pete Richards wrote: |
...you seem to be more of a petrologist than many of us...
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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?
| Description: |
Goethite pyritohedra in marble matrix
Sierra de Tocón de Quéntar, Granada, Andalucía, Spain
400X |
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40418 Time(s) |
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Pablo Rodríguez Cantos
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Pete Richards
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Location: Northeast Ohio
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| Posted: Apr 25, 2013 08:55 Post subject: Re: Unusual metamorphic(?) tourmaline |
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| 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.
| Description: |
Tourmaline
unknown locality |
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| Viewed: |
40402 Time(s) |
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Peter Megaw
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Location: Tucson, Arizona
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| Posted: Apr 25, 2013 09:21 Post subject: Re: Unusual metamorphic(?) tourmaline |
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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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prcantos
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| Posted: Apr 25, 2013 12:05 Post subject: Re: Unusual metamorphic(?) tourmaline |
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Ok, of course you had really good reasons! Thank you for your explanations.
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Pete Richards
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| Posted: Apr 25, 2013 16:39 Post subject: Re: Unusual metamorphic(?) tourmaline |
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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....
| Description: |
Tourmaline, better image of best crystal
unknown locality
Crystal is 8 mm across |
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| Viewed: |
40294 Time(s) |
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| Description: |
Crystals about 4 mm
Views of prisms of two crystals, note reaction zone around lower one |
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40273 Time(s) |
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| Description: |
| detail of lower crystal in previous photo |
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40277 Time(s) |
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| Description: |
| Approximate limit of alteration of previous specimen |
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40278 Time(s) |
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| Description: |
| Fibrous mineral in reaction rims |
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40273 Time(s) |
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| Description: |
| Lamellar? mineral in reaction rims: epidote? |
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40303 Time(s) |
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| Description: |
| Simple white rim, more like what a pressure shadow might look like. |
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40253 Time(s) |
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| Description: |
| Chlorite. Interesting that the apparent nucleation surface is not the tourmaline face, but a plane parallel to it across a ?void?. |
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40250 Time(s) |
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| Description: |
| relatively euhedral chlorite crystal group. |
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| Viewed: |
40316 Time(s) |
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Pete Richards
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Joined: 29 Dec 2008
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Location: Northeast Ohio
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| Posted: Apr 25, 2013 16:48 Post subject: Re: Unusual metamorphic(?) tourmaline |
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| 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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Peter Megaw
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Location: Tucson, Arizona
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| Posted: Apr 25, 2013 17:04 Post subject: Re: Unusual metamorphic(?) tourmaline |
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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.
| Description: |
Nifontovite
Rey y Reina Mine, Charcas, San Luis Potosi
4 cm long
Peter Megaw specimen, Jeff Scovil photograph |
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| Viewed: |
40283 Time(s) |
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prcantos
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Location: Granada (Spain)
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| Posted: Apr 26, 2013 03:20 Post subject: Re: Unusual metamorphic(?) tourmaline |
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| 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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Josele
Joined: 10 Apr 2012
Posts: 410
Location: Tarifa, Spain
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| Posted: Apr 26, 2013 16:50 Post subject: Re: Unusual metamorphic(?) tourmaline |
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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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