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 Post subject: What crystal form is this?
PostPosted: Mon Apr 25, 2011 9:41 pm 
This very roughly 2cm dia spheroidal crystal is approximately 75% complete. It grew on a matrix which is where the missing 25% would be. The surface of the crystal comprises of multiple planes. The number of planes in the whole crystal form is uncertain but can't be less than 24 and could, perhaps, be as many as 28.

The main form-identifying feature is two 'belts' of planes at an estimated 90 deg to each other.Each 'belt' consists of an estimated 10 faces being, alternately, hexagons and tetragons; five of each (were all present) as one counts round each of the belts. Neither the hexagons nor the tetragons are all of the same size. The internal angles of the hexagons all seem to vary but the internal angles of the tetragons are all strictly 90 deg.

A second possible identifying feature is that the surfaces of the hexgonal planes are all finely pitted, eroded and only translucent. However, the tetragonal planes are almost clear and smooth, permitting a clear view through the inside of this colourless transparent stone. Different hardnesses? Or some other effect?

Can someone identify this crystal form?

The stone? SG determination was ruled out because of the attached matrix but it 'hefts', heavier than a slightly smaller Spessartine. With the polaricope filters in the closed position, the stone transmits light throughout a 360 deg rotation. Hmm.... There is one large-ish tetragonal plane conveniently placed and so smooth and near-polished as to encourge me to try for an RI and, at a third attempt with increasing slatherings of RI fluid. I got a weak one. 1.434. Isotropic. Fluorite.

But the crystal form is a new one to me and I can't seem to find it through any of the usual references. Any thoughts?


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 Post subject: Re: What crystal form is this?
PostPosted: Tue Apr 26, 2011 12:25 pm 
A picture being worth a thousand words, let's see if these can help:

1. The crystal. The 'belts' of alternating hexagons and tetragons run N-S and W-E in this picture. The tetragons in both belts vary in both their side lengths. Only the plane facing the camera approaches the dimensions of a square. However, it's possible that another would have grown in the missing part of the crystal, where there is matrix in lieu.
Attachment:
Fluorite 3-08b.JPG
Fluorite 3-08b.JPG [ 46.51 KiB | Viewed 3885 times ]


2. Here at x20 mag, the difference in natural 'finish' between the surfaces of the all hexagonal and the tetragonal planes is well-shown. This difference is present between all hexagonal and tetragonal planes.
Attachment:
Fluorite 3-03b.JPG
Fluorite 3-03b.JPG [ 102.75 KiB | Viewed 3885 times ]


3. As some indication of the natural finish on the tetragonal planes, here's a pic at x65 mag snapped through the large tetragon plane. Subject seems to be a two phase inclusion. No doubt contrast and focus would be better if I cleaned the crystal :oops:
Attachment:
Fluorite 3-04b.JPG
Fluorite 3-04b.JPG [ 51.76 KiB | Viewed 3885 times ]


Any thoughts as to what the crystal form of this piece of Fluorite is, please?


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 Post subject: Re: What crystal form is this?
PostPosted: Tue Apr 26, 2011 5:45 pm 
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Easy one... a cubohedron :P

Image

Kidding aside.. it's a basic combo of a cube and a dodecahedron. I guess the different surface conditions are a direct result of directional hardness. That crystal apparently endured some chemical attack at one stage or another and the cubic faces were able to resist it better than the rhombic ones.


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 Post subject: Re: What crystal form is this?
PostPosted: Tue Apr 26, 2011 6:07 pm 
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re; fluorite
Do you see evidence of cleavage within the specimen or on the fracture surfaces?
Does it fluoresce?

Fluorite's most common xl habit is simply a cube, occasionally with dodecahedrons, tetrahexahedrons and hexoctahedrons modifying the edges of the cube.
Dodecahedral fluorite is very uncommon, but: I found one here.

The SG of spessartite is 4.15
Fluorite: 3.18


Phase inclusions are rather common in fluorite.


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 Post subject: Re: What crystal form is this?
PostPosted: Tue Apr 26, 2011 7:21 pm 
Tim wrote:
Easy one... a cubohedron :P

Image

Kidding aside.. it's a basic combo of a cube and a dodecahedron. I guess the different surface conditions are a direct result of directional hardness. That crystal apparently endured some chemical attack at one stage or another and the cubic faces were able to resist it better than the rhombic ones.


That's it! Or would be in a world where natural crystals were perfect. Thanks Tim. In searching about, I found this in the Mindat entry on Fluorite, which you might find interesting:

' Usually cubes, {001}; less often octahedrons, {111}; rarely dodecahedrons{011}. Also some hexoctahedrons and tetrahexahedrons. Combinations of these forms common, cube corners and edges are often modified. {001} usually smooth and lustrous, {111} rough and dull. Crystals distorted at times by unequal development of faces, as of {013}'. (My emphasis).

However, this is the first Fluorite crystal I have seen which shows this characteristic.


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 Post subject: Re: What crystal form is this?
PostPosted: Wed Apr 27, 2011 1:02 am 
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aha, so you can forget that chemical attack of mine, that's just how those faces develop... :D


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 Post subject: Re: What crystal form is this?
PostPosted: Wed Apr 27, 2011 8:23 am 
Barbra Voltaire wrote:
re; fluorite
Do you see evidence of cleavage within the specimen or on the fracture surfaces?

Pronounced. See below. One can imagine the Creator setting Mankind up to 'invent' the Table Cut and 'portrait stones' :)
Attachment:
Fluorite 3-02b.JPG
Fluorite 3-02b.JPG [ 93.13 KiB | Viewed 3826 times ]


Quote:
Does it fluoresce?

None at all, at either UV LW or SW.

Light transmissions throughout 360 deg stone rotation between crossed polars is anomalous too. I wonder if this is follows from the surface irregularities on the hexagonal planes? This surely must cause light exiting the stone to be refracted substantially at a wide range of angles relative to the axis of the polarising filters. I am unable to arrange for light transmission mainly through an opposed pair of of the smooth(er) tetragons. I also also wonder if this anomaly would persist if the stone is vlewed between crossed polars whilst being rotated in a bath of 2-ethoxyethanol (very close RI to Fluorite). I might try that and report back. OTOH, if you can save me from occupying bandwidth whilst 're-inventing the wheel'.... :wink: :lol:

Quote:
Fluorite's most common xl habit is simply a cube, occasionally with dodecahedrons, tetrahexahedrons and hexoctahedrons modifying the edges of the cube.
Dodecahedral fluorite is very uncommon, but: I found one here.

Congratulations! I trawled the net for several hours and did not find one. I should make more use of Mindat photo resources than I have been doing.

Quote:
Phase inclusions are rather common in fluorite.

Yeah well.... This is the only one of my five Fluorite thumbnail specimens with a surface sufficiently smooth as to permit microphotography through it. I included the pic as a small celebration of this rather than because of any rarity (or beauty) of that inclusion's occurence. I do enjoy seeing things for a (personal) first time though.


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 Post subject: Re: What crystal form is this?
PostPosted: Wed Apr 27, 2011 10:52 am 
Tim wrote:
aha, so you can forget that chemical attack of mine, that's just how those faces develop... :D


Tim,
Mmmm.... I wonder. If I understand the quote from Mindat correctly, it says that the rough face ID is 111, the plane associated with the octohedral crystal form? Yet the only octohedral piece of Fluorite I have as all its planes, well, as clear as crystal :? :| However, its planes do show some planar growth irregularities, similar to that seen also in Rock Crystal sometimes.

Then there is the fact that the only other modified dodechedral Fluorite that Barbra has seen shows the same combination of smooth 001 (cubic) planes and rough 010 (dodecahedral) planes as is present in my specimen. She has probably seen more Fluorite than I have had hot breakfasts.

So, what do you think? Typo at Mindat (i.e. an inadvertent transposition of 111 and 110 in the quoted text) or something else is going on? Should we ask?


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 Post subject: Re: What crystal form is this?
PostPosted: Wed Apr 27, 2011 11:00 am 
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Quote:
Yet the only octohedral piece of Fluorite I have as all its planes, well, as clear as crystal


most octahedral bits of fluorite you'll see are the product of cleaving... that would explain your 'clear' faces...


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 Post subject: Re: What crystal form is this?
PostPosted: Wed Apr 27, 2011 11:06 am 
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'BTW...

Kerensky wrote:
Barbra Voltaire wrote:
re; fluorite
Do you see evidence of cleavage within the specimen or on the fracture surfaces?

Pronounced. See below. One can imagine the Creator setting Mankind up to 'invent' the Table Cut and 'portrait stones' :)
Attachment:
Fluorite 3-02b.JPG


Where exactly do you see that cleavage? (cuz i don't see any... ).


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 Post subject: Re: What crystal form is this?
PostPosted: Wed Apr 27, 2011 11:11 am 
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I'm not seeing cleavage either. :|
I would suggest checking all the fracture surfaces on the xl (at the edges). There should be some rather distinct planar surfaces with mag.

Usually one can find traces of internal cleavages in fluorite as well.

The mineral really wants to be an octahedron, regardless of the shape it is born with.


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 Post subject: Re: What crystal form is this?
PostPosted: Wed Apr 27, 2011 11:21 am 
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In addition (now that I have processed that pizza and can type with two hands): there's no typos on mindat... they never mention that the dodecahedral faces are supposed to be clear, they just mention that the cubic ones are and the octahedral ones aren't. There is no mentioning of the appearance of the dodecahedral faces whatsoever.


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 Post subject: Re: What crystal form is this?
PostPosted: Wed Apr 27, 2011 2:57 pm 
Tim wrote:
'BTW...

Kerensky wrote:
Barbra Voltaire wrote:
re; fluorite
Do you see evidence of cleavage within the specimen or on the fracture surfaces?

Pronounced. See below. One can imagine the Creator setting Mankind up to 'invent' the Table Cut and 'portrait stones' :)
Attachment:
Fluorite 3-02b.JPG


Where exactly do you see that cleavage? (cuz i don't see any... ).


OK... As I understand it, to cut a Table Facet diamond, one of the ancient cutters would cleave the point off an octahedral Diamond crystal, parallel to the crystal's natural girdle, creating a tetragonal table by that process. Is that correct?

On the basis that it is, what I try to show is two things:

1. That the shape in pic 3-02b (and forget all you can't see) is, pretty close, the shape of the top of a Table Faceted diamond - which is a cleaved plane. This does not imply that tetragon plane shown in 3.02b is a cleavage. Rather, my thinking was on the possibility that when seeing such a view of a modified dodecahedron - or of an octahedral diamond cleaved by accident to give a similar 'table', the first cutter emulated this effect through the deliberate cleavage of an octahedron Diamond to create a table. Art imitating nature, as it so often does.

2. Most of right hand edge of the tetragon surface in 3-02 is missing. This reveals part of another, lower tetragonal plane parallel to the surface plane. To me, the inference is clear that this stone has distinct cleavage parallel to the natural tetragonal face. Again, it is by exploiting exactly such planar cleavage that 'portrait stones' would have been split away out of octahedral (or modified dodecahedral) crystals. Is this thinking correct?

Finally, yes, there is what I take to be one cleavage fracture within the crystal than can be seen with a loupe. With reference to the tetragonal plane in pic 3-08, this runs down through the stone approximately perpendicular to that tetragon plane and at an angle to it so that the line of the fracture runs from approximately a litte moe than halfway down the left side of the tetragon to approximately a little more than half-way across the top side. The fracture does not reach any viewable surface of the stone but probably reaches the matrix at the back. The fracture seems to run as straight and true as a die in all three dimensions.


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 Post subject: Re: What crystal form is this?
PostPosted: Wed Apr 27, 2011 3:53 pm 
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Quote:
OK... As I understand it, to cut a Table Facet diamond, one of the ancient cutters would cleave the point off an octahedral Diamond crystal, parallel to the crystal's natural girdle, creating a tetragonal table by that process. Is that correct?


nope

[vimeo]http://vimeo.com/20281170[/vimeo]


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 Post subject: Re: What crystal form is this?
PostPosted: Wed Apr 27, 2011 6:51 pm 
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Very very clear and informative piece of animation. I could not have explained it as well with words only. Nice work Tim!

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