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Basal Pinacoid Termination

Posted by John Betts  
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John Betts August 30, 2010 03:15PM
Is the term "Basal Pinacoid" redundant? What is the correct terminology for the C-face of a quartz crystal?

In the past I have described them as having "basal pinacoid termination faces." But wouldn't "pinacoid face" suffice?
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Peter Andresen August 30, 2010 03:44PM
I must admit my crystalographical knowledge is getting rusty, and I'm to lazy to get the textbooks, but my first thought was is it possible for quartz to have basal pinacoides? Isn't the (001) the basal pinacoide?

Guess it's time to blow some dust of those books...
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Johan Kjellman August 30, 2010 06:27PM
you are right, in reference to minerals belonging to crystal systems with a pronounced central axis, e.g. hexagonal and tetragonal, basal is redundant as this is the only pinacoid possible. I am also a bit rusty, but these questions are good as rust-removers, so let's walk out on the thin ice...I am sure that the pinacoid situation is more complex in the orthorombic, monoclinic and triclinic systems and there it would be ok to refer to the basal pinacoid as one out of several possible pinacoids.

cheers
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Rob Woodside August 30, 2010 09:37PM
Turning the xl upside down, the pinacoid becomes basal!!!
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John Betts August 31, 2010 04:16PM
Here are some images of the pinacoid (0001) face on quartz.



Edited 1 time(s). Last edit at 08/31/2010 04:20PM by John Betts.
open | download - 49701det.jpg (45.4 KB)
open | download - 48252det.jpg (57.1 KB)
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Rob Woodside August 31, 2010 05:07PM
Sweet! The first is the least pitted I've seen.B)
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Reiner Mielke August 31, 2010 05:57PM
I didn't know crystals had an up or down!? Probably best to stick to Miller indices.
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Rob Woodside August 31, 2010 05:59PM
{0001} is a pinacoid, {000-1} is the basal pinacoid.X(



Edited 1 time(s). Last edit at 08/31/2010 06:00PM by Rob Woodside.
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Reiner Mielke August 31, 2010 06:50PM
Rob,

How can you tell if you only have one?



Edited 1 time(s). Last edit at 08/31/2010 06:50PM by Reiner Mielke.
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Peter Andresen August 31, 2010 07:11PM
So if the crystal is growing from the roof of the vug it's got a basal pinacoid, if it got one, and if it's growing from the bottom of the vug it's got a pinacoid... (:P)
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Rob Woodside August 31, 2010 07:20PM
Thanks guys. Perhaps we could introduve "Lateral Pinacoid" when the xl grows sideways and of course modified by the adjectives "left" or "right" if it is doubly terminated.B)-
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Reiner Mielke August 31, 2010 07:25PM
If the quartz crystal is left or right handed would the pinacoid be left or right handed as well, or can you have a right handed pinacoid on a left handed crystal?
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Rob Woodside August 31, 2010 08:02PM
This is getting complicated. The 3 fold screw axis in handed quartz prevents the pinacoids from being related by an inversion or a mirror plane. They are related by the two fold axis. So yes a right handed quartz would have right handed pinacoids, etc. So our nomenclature needs revision. Either we could Levinsonise the pinacoid as in "right pinacoid-(l)" for a sideways pinacoid on a left handed quartz, etc. or use an adjectival modifier such as "right levulopinacoid" for the previous example. Perhaps the IMA could rule on this.B)-

Seriously though, this reminds me of something I don't understand about Tourmaline which apparantly has no inversion centre or horizontal mirror plane. Thus it is hemihedral and one often sees Elbaites with a pinacoid at one end and a rhombohedral termination at the other end, i.e. a flat end and a pointy end. However I've seen Yinnietharra Dravite and Brumado Uvite with an apparently obvious inversion centre relating the two rhombohedral terminations!!! What's going on??? Is there some twinning??? Or does it just not show the full symmetry as with a perfect pyrite cube that shows no pyritohedral modifications?:S
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Gerhard Niklasch August 31, 2010 08:25PM
> ...and one often sees Elbaites with a pinacoid at one end...

That would be a rare thing indeed! Tourmalines don't have basal pinacoids. Pedion is the word you're looking for here. :)

> However I've seen Yinnietharra Dravite and Brumado Uvite with an apparently obvious inversion centre relating the two rhombohedral terminations!!!

The polar axis does not rule out inequivalent trigonal pyramids being formed at each end which nonetheless happen to be related by an apparent inversion (without being related by a real symmetry of the crystal lattice). Both are possible forms for the given hemimorphic symmetry.

Quite often you'll find differences in the details though - different modifications by other forms at the two ends, different striations or growth/etching patterns, etc.

Or, indeed, a small pedion at one termination (stereo pair):


but not at the other (same individual, different orientation and lighting):


Cheers, Gerhard
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Rob Woodside August 31, 2010 08:38PM
Thanks so much Gerhard. Yes pedion it is !!!

"The polar axis does not rule out inequivalent trigonal pyramids being formed at each end which nonetheless happen to be related by an apparent inversion (without being related by a real symmetry of the crystal lattice). Both are possible forms for the given hemimorphic symmetry. "

So it is like the pyrite example. Thanks again.

I've seen equant octohedra of sphalerite which are positive and negative tetrahedra in equal development from Naica and Mandan. So sometimes there is more apparent symmetry than the lattice has!!!



Edited 1 time(s). Last edit at 08/31/2010 08:43PM by Rob Woodside.
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Reiner Mielke August 31, 2010 09:00PM
It is a good thing we have Gerhard watching over us and keeping us all on the "straight and narrow". Thanks Gerhard! Your input is always welcome and much appreciated.
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Gerhard Niklasch August 31, 2010 09:31PM
Returning to John's original question:

As soon as you have a 3-fold or 4-fold rotational symmetry, anything that isn't at right angles to the axis will give you more than a pinacoid- you'll get prism and pyramid forms of more than two equivalent faces. If the axis is <001>, and if it's the only such and isn't polar, then the {001} form will be the only one that consists of merely a pinacoid. It's commonly called the basal pinacoid (and that refers to the pair of faces, even if only one of them is present and the other is missing e.g. due to a contact), and in this case the adjective is indeed somewhat redundant. But the usage appears well entrenched.

In the orthorhombic, monoclinic, and triclinic systems you can have pinacoids in many different orientations, and if the habit singles out an axis as the most natural one to regard as "across" the "base", then it makes sense to single out the corresponding pinacoid (parallel to the other two axes) by calling it the basal pinacoid. By convention, one would usually call this axis c or <001> again, so that {001} will again be the basal pinacoid.

See the online well-illustrated Introduction to Crystallography and Mineral Crystal Systems by Mike and Darcy Howard (recently mentioned by DVB in another thread) for an excellent discussion, especially of the conventions in assigning axes, a topic which other sources rarely treat with such care. You'll also find a choice of adjectives to refer to some other pinacoids if the need arises!

Enjoy,
Gerhard
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Gerhard Niklasch August 31, 2010 10:00PM
Many thanks, Reiner and Rob, for your kind words, and for your many contributions (and pointed question, too!) from which I've learnt and keep learning a lot.

I don't think this stuff is supposed to be easy. :S It took our forefathers the best part of a century to figure out the geometry of crystals and come up with the now-traditional terminology which surely isn't the simplest and most natural that one could imagine. It's taken me several decades to get enough of the lot into my head to figure out the easier of my own specimens, and as recently as a couple of years ago, I was blundering along thinking that baryte obviously had to be monoclinic...

Fortunately there are a great many helpful people here and a vast pool of knowledge and experience!
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