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Pete Richards
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Location: Northeast Ohio
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 Posted: Apr 26, 2013 19:50 Post subject: Re: Unusual metamorphic(?) tourmaline |
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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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Matt_Zukowski
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| Posted: Apr 26, 2013 23:54 Post subject: Re: Unusual metamorphic(?) tourmaline |
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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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Pete Richards
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Location: Northeast Ohio
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| Posted: Apr 27, 2013 20:55 Post subject: Re: Unusual metamorphic(?) tourmaline |
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| 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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Riccardo Modanesi
Joined: 07 Nov 2011
Posts: 631
Location: Milano
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| Posted: Apr 29, 2013 07:35 Post subject: Re: Unusual metamorphic(?) tourmaline |
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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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Hi! I'm a collector of minerals since 1973 and a gemmologist. On Summer I always visit mines and quarries all over Europe looking for minerals! Ok, there is time to tell you much much more! Greetings from Italy by Riccardo. |
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Pete Richards
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Location: Northeast Ohio
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| Posted: Apr 29, 2013 08:59 Post subject: Re: Unusual metamorphic(?) tourmaline |
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| 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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prcantos
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Location: Granada (Spain)
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| Posted: Apr 30, 2013 14:43 Post subject: Re: Unusual metamorphic(?) tourmaline |
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I have been thinking and reading about this topic (porphyroblasts growing) for some days. I have found some information that may be useful.
| Pete Richards wrote: | | ...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. |
In fact, there are many examples of well-formed porphyroblasts in metamorphic rocks, even if some directed (or sheared) stresses are present (e. g. garnet porphyroblasts in regional metamorphisms of pelitic rocks), and in thermal metamorphism rocks (e. g. chiastolites in spotted slates).
In our case we are persuaded to think a low or medium grade (epidote is confirmed) & low P/T metamorphic process, so stresses must have been insignificant. Therefore I think that the main factor controlling the growing of the porphyroblasts is the free energy in the faces of the grains. According to Professor Antonio García Casco (University of Granada, Spain):
Nucleation of new crystals requires an excess of free energy. If nucleation energy is very high, new cyrstalline nucleus (cores) growing is favored; if high, porphyroblasts growing is favored. Porphyroblasts will be euhedral if their faces have free energy enough. (Notes for the practical lessons of "Metamorphic Petrology" subject in the University, cf. https://www.ugr.es/~agcasco/personal/petmet/Seminario02/seminario02.html; translated from Spanish by myself.
(link normalized by FMF)
According to Wulff's theorem, a crystal tends to reach a minimal surface energy (one of the three aditive components of Gibbs energy) compatible with its crystallographical restrictions, and flat faces present less of this energy than irregular surfaces, so sharp shapes are explained enough in these anti-stress conditions.
Furthermore, there is a "crystalloblastic series" that arranges some minerals according to their decreasing surface energy, so that a given mineral is idiomorphic if grows in contact to any of the lower-level species in the list; that is, the first minerals in the list show higher tendency to idiomorphism. Tourmaline is in the second level, the same as garnets, kyanite or staurolite (typical porphyroblastic minerals in metapelites). This is the list:
1) Magnetite, rutile, sphene, pyrite, ilmenite.
2) Sillimanite, kyanite, garnet, staurolite, chloritoid.
3) Andalusite, epidote, zoisite, forsterite, lawsonite.
4) Amphiboles, pyroxenes, wollastonite.
5) Moscovite, biotite, chlorite, talc, prehnite, stilpnomelane.
6) Calcite, dolomite, vesuvianite.
7) Cordierite, feldspars, scapolites.
8) Quartz.
(Translated again by myself from Javier Gómez Jiménez, Notes for "Endogen Petrology II" subject in University of Zaragoza, https://gmg.unizar.es/gmgweb/Asignaturas/EndogenaII/metamorfismo_nuevo/apuntes/apuntes.html
(link normalized by FMF)
I regret I have no textbooks or another references but these notes (excellent notes anyway!), but I recognize I had never gone so far reading and learning on this topic.
I am really impatient to look at the thin section and figure out the mineral assemblage of the matrix.
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Pablo Rodríguez Cantos
Λίθον˛ον απεδοκίμασαν˛οι οικοδομουντες |
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Roger Warin
Joined: 23 Jan 2013
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| Posted: Apr 30, 2013 16:31 Post subject: Re: Unusual metamorphic(?) tourmaline |
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Hello Pete, all,
The observation of meteorites thin sections show different degrees of thermal metamorphism. I leave aside the role of pressure on Earth. The concept of different crystallization sequences appears.
We must not forget that not all minerals crystallize simultaneously. I see tourmaline crystals (or garnet) crystallize first taking at the same time many cations which could not find place in other crystal structures.
What is the nature of the rock matrix in which are embedded tourmaline crystals? I suspect there are many sodium feldspar and quartz.
Roger. |
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prcantos
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Joined: 17 Apr 2012
Posts: 243
Location: Granada (Spain)
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| Posted: Jul 17, 2013 10:40 Post subject: Re: Unusual metamorphic(?) tourmaline |
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Hi, Pete, have you figured out any more information about this specimen yet? Regards.
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Pablo Rodríguez Cantos
Λίθον˛ον απεδοκίμασαν˛οι οικοδομουντες |
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Pete Richards
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Joined: 29 Dec 2008
Posts: 845
Location: Northeast Ohio
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| Posted: Jul 24, 2013 20:55 Post subject: Re: Unusual metamorphic(?) tourmaline |
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Hallo Pablo (and others interested in this topic),
I regret that I have no new information. I have completed the analyses I can do and have not had any further suggestions from other collectors.
Perhaps this specimen is unique! Perhaps the right person has not yet seen it....
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Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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