FMF - Friends of Minerals Forum, discussion and message board

[Bob Carnein]

There's an even better photo on Mindat: It's by Karl Volkman and it's from the Yaogangxian mine, Hunan (Mindat ID64V-CHY). It definitely shows the striation pattern shown by your specimen.

[Pete Richards]

I have to voice my doubts about the identification of this specimen as chalcopyrite. Sante, you did an incomplete experiment! Smart to test your needle against a known chalcopyrite, but you did not test it against a known pyrite as well. Maybe your needle is harder than you thought.

Chalcopyrite is tetragonal. As a consequence, it requires two forms to cooperate to form the "dodecahedron" (in quotes because a true dodecahedron can only occur in the isometric system). While this is possible, it is much less likely than the same shape forming when all the faces belong to the same form. The same two forms could create a prismatic crystal taller than wide or wider than tall.

Furthermore, chalcopyrite is in the same structural family as sphalerite, and this family shares a very different structure than pyrite, one that tends to form tetrahedral and modified tetrahedral crystals. The two forms necessary to create a pseudo-dodecahedron are both uncommon on crystals in this family, especially in the absence other forms.

It is true that the dodecahedron is also an uncommon form on pyrite. It is also true that a pseudo-dodecahedral crystal identified as chalcopyrite from Chenzhou, China is shown on Mindat (Bob just beat me to it!). In my opinion, Sante's crystal is more likely to be pyrite. Pyrite or chalcopyrite, it is an unusual and special crystal.

Greater certainty might be gained by spectroscopic examination of light reflected from one of the exposed surfaces of the crystal, or by Raman spectroscopy, both of which are non-destructive methods, albeit not readily available.

[Bob Morgan]

Sante,

The mystery is gone, so now we can appreciate what a wonderful specimen of chalcopyrite you have. It's shape is stunning for that mineral and so well presented.
Well, I just read Pete's remarks, and getting other confirmation is desirable.
Yes, chalcopyrite is tetragonal, and I'm having a difficult time trying to figure out what faces make the 'pseudo dodecahedral' habit. There is one drawing of this in Goldschmidt which didn't help.
Yet I have specimens from three different localities that have that shape. For me this is another mystery to be pursued.

[Bob Morgan]

Pete's drawings show the final shape with the two forms. The mystery for me is the striations. What is the alternating form that results in striations on those faces? It would have to be where three faces meet to make striations on the three surrounding faces. Perhaps the (221).

[Sante Celiberti]

Hi, Pete.

Thank you for your further and invaluable help in identifying this crystal.
You cannot imagine how much I would prefer it to be a Pyrite. Living in Gavorrano and consequently being a lover of Pyrite, this sample would enrich my collection of its different habits. I bought it because I was told it was Pyrite and its alteration to Goethite seemed to me an indirect confirmation.
As for my test, I was pretty sure of the needle hardness having used it (I mean the same) in several previous tests.
For greater certainty, I repeated the experiment again on an insignificant sample of Gavorrano Pyrite but the steel needle doesn't affect the smooth crystal surface in the least.
If the Chalcopyrite hypothesis can be followed, I would say attempting an interpretation of it.
Thanks again for your time and availability.

A warm greeting.
Sante

[Sante Celiberti]

Hello, Bob.

Thanks for your suggestion.
In the case of my crystal, if it wasn't evident from the photos, the striations are parallel to the bases of the three triangles forming the "trisoctahedron".

Have a fun job...
Sante

[Bob Carnein]

I understand your interest in pyrite, but I think this is a really cool chalcopyrite crystal that would be a proud addition to many collections--it certainly caused a lot of discussion on the forum.

[Sante Celiberti]

[Bob Morgan]

I'm attaching a drawing from shape building off of Pete's drawing. The small triangle is the other hand of the 210 which is the 201 form. To further confirm the chalcopyrite crystallography, these other handed faces appear on only half of the three face junctions of the crystal, which seems to be the case from your photos.
I suspect the striations are mainly alternating slivers of the two forms, 210 & 201 for two of the faces and a similar alternation between the 201 and the 110 forms.
To test that, there should be reflections off the 201 slivers on all three faces at the same angle.
If that proves out, then the crystal probably had 120 layers forming, stacking and spreading with edges of the layers roughly aligning in the pseudo dodecahedral 210 and 110 faces.
This is the way magnetite, which is isometric, develops dodecahedral faces, when its more common habit is octahedral. In that case it is octahedral layers stacking and not spreading completely to result in an octahedron. The layers then aligned into dodecahedral faces built from octahedral layer nucleation and spread.
Well - I can't figure how to attach the drawing. Perhaps someone has an idea.

Bob