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 Post subject: hexagonal vs trigonal
PostPosted: Wed Jul 02, 2008 3:15 pm 
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I am working on my colored stones course (GIA) and have a question that has me stumped. How can you tell a trigonal crystal from a hexagonal crystal when you are looking at an actual crystal? I realize that one is 6 fold symmetry and one is 3 fold. But when I have a quartz crystal and a beryl crystal in front of me and compare them, they both look hexagonal. Is there a way to tell from just looking at it? The only thing I can see would be growth lines, but you can't depend on seeing those. Or can you?

I know this probably won't come up on the exam. We deal in rough stones, so I am taking this part deeper than I have to.

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PostPosted: Wed Jul 02, 2008 4:13 pm 
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Like you said, a quartz crystal often ends in a hexagonal pyramid. When you look closely, usually three of the six sides of the pyramid are less developed than the three others. They will seem less wide than the three others, or they may not meet at the point at the top of the pyramid but lower down.

Looking top-down from the point of the pyramid toward the hexagonal prism, you often see like a three-bladed 'propeller' (the three narrow sides being the blades). That would be the hint to it being trigonal as opposed to hexagonal. I'm not sure whether this is 100% foolproof, but others will fill in the gaps I'm sure :)

(All praise to Doos for explaining this to me during the practicals) ;)

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PostPosted: Wed Jul 02, 2008 4:20 pm 
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As a picture tells more than a thousand words...

Image

I found this on the quartz page - looks like an interesting read in itself...

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PostPosted: Tue Jul 08, 2008 12:52 pm 
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Thank you! That does help with some of the crystals :lol:

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PostPosted: Tue Jul 08, 2008 1:08 pm 
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Just be careful with quartz "dauphiné twins"... :wink:

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PostPosted: Sat Jan 31, 2009 8:23 pm 
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I am a noob. Thanks for this thread. It is informative.

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PostPosted: Sun Feb 22, 2009 11:43 am 
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Another tip would be to look at surface markings, this will indicate symetry, for instance in a beryl crsytal you may get hexagonal pit on the hexagonal pinacoid faces and rectangular pits on the prism faces, all six prism faces will show the same markings, in the case of quartz you will often see striations at 90 degress to the C axis on alternate prism faces indicatining trigonal rather than hexagonal symetry.


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PostPosted: Wed Dec 02, 2009 6:36 pm 
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Note that quartz used to be considered as trigonal but is is now considered as hexagonal.
This has been a debate between crystallographers for some time.

By the way, here we use the term rhomboedric instead of trigonal (in a geometrical point of view, a rhomboedric shape is a trigonal shape, but a trigonal shape is not necessarly rhomboedric).

Now, it is often impossible to determine visually which crystal system a crystal belongs to. Due to meriedry, truncation, shape combinations (twins and pseudomorphosis can also complicate the thing) and more simply due to variation of growth conditions affecting the crystal shape, a crystal might look very different from the basic shape representative of its system.
Some argue that when working with big idiomorphic crystals of regular shape (at least one inch long edges) the measuring of angles (using a goniometer) can help determining the crystal system, however one would need a high degree of knowledge in crystallography to do that, moreover an error of a few degree in the measurement could be very misleading.
Anyway, the goniometer method isn't quite reliable: argyrodite long used to be considered by crystalographers as belonging to the cubic system as had shown the careful measuring of angles, but finally X-rays showed that this mineral was actually orthorhombic.
Sometimes the crystal system a mineral belongs to is obvious, but keep in mind that appearances can be extremely misleading.

ps: if you want to immerse yourself more deeply in the wild world of crystallography, find yourself aspirin...lots of.


Last edited by cascaillou on Wed Dec 02, 2009 8:57 pm, edited 4 times in total.

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PostPosted: Wed Dec 02, 2009 8:16 pm 
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A few images taken from the web to illustrate the complexity of determining a crystal system visually

Image
This hexagonal shaped crystal is actually a common aragonite twin, aragonite belongs to orthorhombic system. Very misleading.

Image
This is tetraedrite, which belongs to cubic system (from truncation of a cube you can get a tetrahedron...which looks nothing close to a cube)

Image
Now here is a tetrahedron of chalcopyrite, it looks just like a tetrahedron from the cubic system right? But chalcopyrite belongs to the quadratic system.

Image
When you see an octahedron you might think to cubic system, but this is gismondine, belonging to the monoclinic system.

Image
this quadratic looking crystals are andalusite, which belongs to the orthorhombic system.

Now just think that nature likes to disrupt and distort its own geometry: the above pictures show crystals of very regular shapes, but if the growth space is uneven or if there is a perturbation during growth then you will get uneven shapes, compressed shapes, elongated shapes, etc...making interpretation of shapes even more difficult

ok...I'll stop torturing your mind here :twisted:


Last edited by cascaillou on Thu Dec 03, 2009 3:29 pm, edited 1 time in total.

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PostPosted: Wed Dec 02, 2009 11:26 pm 
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Which system is used, the 6 crystal system with only Hexagonal covering 3 fold and 6 fold rotations, or the 7 crystal system with Hexagonal and Trigonal separated, depends on the source you are using. My understanding is that most American mineralogist now use the 6 crystal system. So I was a little surprised when taking the GIA course to see that they used the 7 crystal system.

To answer the question about how to separate I don’t know if there is an easy way to separate 3 and 6 fold. I think it would take studying the different crystal space classes that fall within each.

For identification I think the best way is to study the different crystal habits for the minerals you are interested in identifying.

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PostPosted: Thu Dec 03, 2009 3:21 pm 
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Quote:
For identification I think the best way is to study the different crystal habits for the minerals you are interested in identifying

Agreed. While trying to elucidate visually the crystal system a crystal belongs to can be very misleading, it is true that knowing about the different habits that a given mineral might show will help a lot identifying rough crystals of this specie.


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PostPosted: Fri Dec 04, 2009 4:27 pm 
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gemmologically speaking I always thought the six crystal systems were an American thing while we brits insisted on seven systems. I'm not sure of this but I think the GIA only adopted the seven crystal systems recently. (Probably in return for convincing the Gem-A to put the blue on the left when using a spectroscope in their new course notes)


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PostPosted: Fri Dec 04, 2009 5:28 pm 
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Frank wrote:
(Probably in return for convincing the Gem-A to put the blue on the left when using a spectroscope in their new course notes)


That is a good thing.... that way the smallest value wavelength is on the left side, just as you'd read the scale in a spectral graph..


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PostPosted: Fri Dec 04, 2009 6:03 pm 
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Probably in return for convincing the Gem-A to put the blue on the left when using a spectroscope in their new course notes

hilarious concern :lol:
I can imagine the round table coucil with mens in black suits grittin their teeth wondering whether it should be on the right or on the left
Now if they can specify whether the blancket should be over or under the beard when at bed, they will have earned my greatest respect. I just can't figure it out.


Last edited by cascaillou on Fri Dec 04, 2009 6:20 pm, edited 3 times in total.

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PostPosted: Fri Dec 04, 2009 6:16 pm 
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shoot....
I might have to go back and flip all my spectrums. :x


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