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Identity Helpstrange twinning & habit of Durham fluorite

25th May 2007 02:55 UTCTony Peterson Expert

I've just photographed some of the tabular fluorite from the Old Town Quarry and I've noticed some unusual features. The usual (111) twinning of "normal" fluorite is present, but because of the reduced symmetry of the crystals, the twinning involves not two but THREE individuals, meeting in a corner and related by rotation on <111> (see attached photo, Z5).


I'm used to seeing crystals with modified habit, as a result of impurities selectively adhering to one set of faces, slowing their growth, and thereby making them larger. However, this doesn't need to involve a change in symmetry. But here, the flattening of the fluorite cubes (along <001>, I presume) seems to involve a reduction in symmetry (to tetragonal, it would seem). My next post on this thread shows another type of twinning which could only occur if <001> has been differentiated from <100> and <010>.


Can anyone enlighten me? Has a structural determination been made of this tabular fluorite? What is the origin of this change in habit, if not symmetry?


Tony

25th May 2007 03:01 UTCTony Peterson Expert

Here is the second photo; the twinning I speak of here is a little less easy to see. There are stacks of the tablets (stacked along <001>) where individuals are alternately rotated on the <001> axis: by 45 degrees I think; superficially it looks a bit less than 45 but I think that's because there is some translation perpendicular to <001>. So this is a new twin axis of <001>, which is consistent with a tetragonal symmetry.


If I had proper microscopic equipment, rather than just a ring extender for my macro, I could get better shots and I may try to image these stacks parallel to <001>. Comments? Am I seeing things?


Tony

25th May 2007 10:32 UTCSpencer I. Mather

Hi Tony. Are you sure they are twins? They look like parallel growths to me. I have fluorite cubes from Black Dean, Weardale, that look like your specimen and they are definaltely parallel growths. Spencer.

25th May 2007 11:55 UTCPeter Haas

These formations must not be considered as crystals, but as aggregates: cubic individuals varying in size, stacked in all three directions, shifted by a variable multiple of the unit cell length along the three axes, and possibly also rotated by variable angles around the latter. This concept produces the offset faces that are easily seen in your photographs. Most probably, many of the faces that appear perfectly flat in the photos will turn out to be composed of multiple individuals with a low offset at a higher magnification.

25th May 2007 13:19 UTCTony Peterson Expert

So if I understand correctly.....the apparent tabular shape is the result of cubes growing side-by-side along x and y, but not z (or at least, much less along z). I can indeed see "step" growth along x and y, resulting in serrated edges of the tablets. But I am left unsatisfied: this axially specific growth must be the result of a free energy gradient which differentiates z from x and y. Since I see the tablet form oriented in all 3 possible directions, it's not caused by chemical gradients in the environment. It must be something about the fluorite structure. I shall investigate further.


Tony

25th May 2007 13:48 UTCFerdinando Giovine

It is not possible one twinned on 111 of your crystals because for 111 twinned is necessary that the second crystal is inclined. It is possible it is 110 twinned, but necessary study on angles.
 
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