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GeneralBasal Pinacoid Termination
30th Aug 2010 16:15 UTCJohn Betts
In the past I have described them as having "basal pinacoid termination faces." But wouldn't "pinacoid face" suffice?
30th Aug 2010 16:44 UTCPeter Andresen Expert
Guess it's time to blow some dust of those books...
30th Aug 2010 19:27 UTCJohan Kjellman Expert
cheers
30th Aug 2010 22:37 UTCRob Woodside 🌟 Manager
31st Aug 2010 17:16 UTCJohn Betts
31st Aug 2010 18:07 UTCRob Woodside 🌟 Manager
31st Aug 2010 18:57 UTCReiner Mielke Expert
31st Aug 2010 18:59 UTCRob Woodside 🌟 Manager
31st Aug 2010 19:50 UTCReiner Mielke Expert
How can you tell if you only have one?
31st Aug 2010 20:11 UTCPeter Andresen Expert
31st Aug 2010 20:20 UTCRob Woodside 🌟 Manager
31st Aug 2010 20:25 UTCReiner Mielke Expert
31st Aug 2010 21:02 UTCRob Woodside 🌟 Manager
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
31st Aug 2010 21:25 UTCGerhard Niklasch Expert
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
31st Aug 2010 21:38 UTCRob Woodside 🌟 Manager
"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!!!
31st Aug 2010 22:00 UTCReiner Mielke Expert
31st Aug 2010 22:31 UTCGerhard Niklasch Expert
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
31st Aug 2010 23:00 UTCGerhard Niklasch Expert
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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