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Unusual metamorphic(?) tourmaline
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prcantos
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PostPosted: Apr 30, 2013 14:43    Post subject: Re: Unusual metamorphic(?) tourmaline  

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.
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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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PostPosted: Apr 30, 2013 16:31    Post subject: Re: Unusual metamorphic(?) tourmaline  

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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PostPosted: Jul 17, 2013 10:40    Post subject: Re: Unusual metamorphic(?) tourmaline  

Hi, Pete, have you figured out any more information about this specimen yet? Regards.
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PostPosted: Jul 24, 2013 20:55    Post subject: Re: Unusual metamorphic(?) tourmaline  

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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