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Euheudral equant crystals
  
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Roger Warin




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PostPosted: Dec 27, 2013 08:14    Post subject: Euheudral equant crystals  

Brazilianite from Brazil !
A ray of sunshine for 2014.
Roger.



Brazilianite-4638_R.jpg
 Description:
Brazilianite
Divino, Conselheiro Pena, Minas Gerais
23 mm
ex coll. of R.V. Gaines.
 Viewed:  10716 Time(s)

Brazilianite-4638_R.jpg



Brazilianite-Modele1_R.jpg
 Description:
Brazilianite
Drawing (1)
 Viewed:  10734 Time(s)

Brazilianite-Modele1_R.jpg



Brazilianite-Modele2_R.jpg
 Description:
Brazilianite
Drawing (2)
 Viewed:  10734 Time(s)

Brazilianite-Modele2_R.jpg


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John S. White
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PostPosted: Dec 27, 2013 11:30    Post subject: Re: Euheudral equant crystals  

Being old-fashioned I have a problem with the term euhedral as used in this thread. The term was coined to describe the relative degrees of perfection of crystals of various minerals in igneous or metamorphic rocks, crystals that are tightly interlocked among others. It is relevant in this context in terms of the crystallization sequence of the minerals that comprise igneous and metamorphic rocks. It was never intended to be used to describe more or less perfect crystals that grew in open cavities unimpeded by other crystals. In solid rocks the more euhedral ones grew first and are euhedral because their growth was not impeded by other crystals. Subhedral crystals occurred later and anhedral (no crystal faces) ones simply filled in the spaces around earlier crystals. I cared enough about this concept that I wrote a Let's Get It Right column in Rocks & Minerals on this subject titled "The -hedrals, Euhedral, Subhedral, and Anhedral." (vol. 77, Sept/Oct 2002, pp. 350351).

I fully understand that languages evolve and words can take on meanings that differ dramatically from their original meaning. Perhaps it is too late to reverse this corruption of a very useful term. If so, I find that regretful.

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Roger Warin




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PostPosted: Dec 27, 2013 12:12    Post subject: Re: Euheudral equant crystals  

John,
Thank you very much for this good point. In fact, the euhedral term is widely used in petrochemistry of achondrites, igneous rocks.
However, being too old-fashioned, I studied ancient Greek (and Latin) for 6 years in college. Despite myself, I used the word according to its original meaning, which simply means "well crystallized as in textbooks." Eu means well, and edra means the base or a face.
But in the future, I will consider this language restrictions introduced by petrochemists.
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Don Lum




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PostPosted: Dec 27, 2013 13:46    Post subject: Re: Euheudral equant crystals  

Roger,

That is a gorgeous Brazilianite from Brazil !!!

Thanks for sharing.

Happy New Year !!!

Don

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Roger Warin




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PostPosted: Dec 27, 2013 14:18    Post subject: Re: Euheudral equant crystals  

Hello Don,
No wonder this brazilianite is exceptional, it was chosen by Richard V. Gaines himself. I've never seen so perfect.
Roger.
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PostPosted: Dec 27, 2013 14:31    Post subject: Re: "Euhedral" brazilianite crystal  

It is perfect indeed. And I am excited to learn that it may have come from the Gaines collection as I am the one who handled the sale of that collection after my dear friend Richard Gaines passed away.
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Roger Warin




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PostPosted: Dec 27, 2013 16:48    Post subject: Re: Euheudral equant crystals  

Hi John, all,
As Lavoisier said: “Nothing is lost, nothing is created, everything is transformed”. (about brazilianite).
In response to the relevant John’s remark, I show you a crystal of an euhedral hypersthene. On cooling, this pyroxene dropped inside it another pyroxene by incompatible phases at lower temperatures. This is a poikillitic inclusion. Its corners are rounded by a process of a peritectic resorption.
It is clear that the igneous rock is made with shapeless crystalline grains embedded in a matrix, except some euhedral grains.
This thin section is cut in the meteorite called NWA 4473. This is a diogenite ejected from the asteroid Vesta. A diogenite is an orthopyroxenite (plutonic rock).
Roger.



NWA4473_002678_R.jpg
 Description:
Orthopyroxenite
Asteroid Vesta !
 Viewed:  10534 Time(s)

NWA4473_002678_R.jpg



NWA4473_002680_R.jpg
 Description:
Orthopyroxene
Vesta
with a poikilitic inclusion of another pyroxene
 Viewed:  10555 Time(s)

NWA4473_002680_R.jpg


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Mark Ost




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PostPosted: Dec 27, 2013 17:35    Post subject: Re: Euheudral equant crystals  

I agree also that many terms evolve or are used relative to the target audience. In geology 101, a general introductory course, the term is used to provide a distinction of well developed faces vs samples that do not exhibit the more defined form. It serves the basic student and general audience well by giving them something, within their development level, to hang their hat on. Of course more professional discussions use more exacting language. This forum has the interesting characteristic of having a range of folks from expert mineralogists, to collectors, to general geologists, to the layman so I suppose some latitude is probably a good thing.
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Peter Megaw
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PostPosted: Dec 27, 2013 19:01    Post subject: Re: Euheudral equant crystals  

I'm not sure how euphoric I am about having the euhedral argument raise its head again, especially having been exposed initially to Mark's and Roger's intro-definition as "well formed" without restrictions on context...and later to John's more constrained definition. Eutectic, eutrophic and eutaxitic soon followed..,to some degree with similar refinement as specialists arrogated meanings to their particular intents

John, being the eutheist that he is wants us all to speak as euphemistically (sensu strictu) as possible for which he is to be commended...but perhaps this argument should be considered for euthanasia.

Darn nice brazilianite by the way...euphemistically (sensu latu) speaking

Richard had good taste!

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John S. White
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PostPosted: Dec 27, 2013 21:56    Post subject: Re: Euheudral equant crystals  

Eureka, I suppose I should be euphoric that Peter did not find my eupracticism overly trivial.
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Mark Ost




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PostPosted: Dec 27, 2013 23:42    Post subject: Re: Euheudral equant crystals  

Yes perhaps this argument would be best served by taking a mid block eu- turn and starting over. I can see the eu-tility of eu-tilization of proper eu-se of language.
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Paul Bordovsky




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PostPosted: Dec 28, 2013 12:58    Post subject: Re: Euheudral equant crystals  

Nice job on the eu-logy, Peter.
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Roger Warin




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PostPosted: Dec 30, 2013 01:40    Post subject: Re: Euheudral equant crystals  

Hi,
Olivine crystal.
The general shape is good to the point that despite the rounded faces, you can match the theoretical drawing.
From Kohistan Province, Pakistan.
A Andreas Weert's specimen.
Roger.



06-Peridot5359d_R.jpg
 Description:
Olivine (peridot)
Kohistan Province, Pakistan
6 cm
 Viewed:  10135 Time(s)

06-Peridot5359d_R.jpg



07-Peridot-mod_R.jpg
 Description:
olivine
Kohistan Province, Pakistan
drawing
olivine is the name of a series between forsterite (Mg term) and fayalite (Fe limit)
 Viewed:  10155 Time(s)

07-Peridot-mod_R.jpg


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