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[fena kuiperi]

Hi, maybe here someone can help:

Hercule Shen: “We find a special type of Pyrite from Gaoxun Iron Mine, Anhui Prov., China.”
He asked for explanations about the crystal forms, and send some to me.

The octahedron is obvious, but the question is about the modified ribs.
Size of pyrite crystal is about 7 mm.
The steep triangle form may be a Diploid, but with very high Millers, something about {15.14.1}
Is that just unusual or impossible?
These ribs are very irregular formed, maybe modified by several forms? Or just irregular growing patterns?
Faces of the octahedron also have different growing patterns…not the octahedron triangles.

I tried to make foto’s of the crystal, too difficult for me, but 1 in the attechment, I hope for better understanding this pyrite crystal. Don’t know how to post more foto’s in attechment…
My Smorf crystal drawing is also in the attechment.
Thanks a lot for reading all this, and maybe someone can help us out, greetings, Dimf

[Pete Richards]

Crystals of this sort present a definite challenge! The octahedral faces demonstrate that growth occurred in a non-ideal environment, in which the growth layers were often stopped before they extended across the whole face area.

The modifying faces clearly have steps. This means that the Miller indices that describe the overall aspect of these faces are a weighted average of those of the forms that create the tops and edges of the steps. Sometimes it is useful to assign such indices, but it must always be made clear that these are "compromise" or "average" indices, and do not apply precisely to any part of the modifying faces.

To my mind, the test for legitimate Miller indices is that they are based on measurements made on a goniometer, and that the observed face gave one and only one good optical cross figure. The faces in question should also be seen in all appropriate locations on a number of crystals.

An example of faces with high Miller indices that appear to be legitimate is the pyrite crystals from the Duff Quarry in Ohio. The typical habit includes faces that are best described by the indices {22.10.7}. The goniometrtic reflections on good crystals are not ideal but are "pretty good". Crystal drawings show that this form produces the correct morphology, and candidate indices with lower values (e.g. {632}) fail to do so.

A counter example is "octahedral" pyrite from the Barger Quarry in Lexington, Virginia. These slightly bulged octahedra are decorated with scale-like facets, which it turns out all belong to the trapezohedron {211}. Here is an octahedral crystal that has no octahedral facets at all!

[Bob Morgan]

I'll add my two cents worth to this.
The features - pits and striations are in trapezohedral and trisoctahedral orientations. they alternate with each other to make the hexaqonal pit in the center. The striations that align with the faces at the octahedron edges will be trisoctahedral. Test the alignment by taking the crystal out in sunlight and see if they reflect light at exactly the same angle. I suspect they will, and that what appears to be high Miller index faces are only stepped trisoctahedral faces.

[fena kuiperi]

Pete Richards, thank you so much for your explanation!
You are totally right about measuments, only I don’t have the possibility for such a small crystal.
Growing in steps and average Millers, that helps me a lot to understand these faces better.
I tried to photograph with clear reflecting faces, but did not well succeed.

Your example is really amazing and interesting. So I learned a lot today, great, greetings Dimf

a photo of the same pyrite, growing patterns in octahedral faces little better to see

[fena kuiperi]

Bob Morgan, these growing patterns in the octahedral faces were a riddle for me! Thanks for your explanation.
As soon as there will be sun over here, I will give it a try, so no steep triangles that ribs, but stepped trisoctahedral, I like to read every possible explanation, greetings, Dimf

another photo of this pyrite